Overcurrent Protection. This is another sometimes poorly understood aspect of wiring; what size fuse do I use? Too big, and you can be one of those cars with smoke billowing out from under the dash. Too small, and you'll be buying fuses in bulk. Getting the fuse sizes right is critical for a safe and reliable system. The first thing to understand is the fuse is there to protect the wire. If a component fails, the fuse isn't going to 'save' it; at best, the fuse may limit the damage to it, but what you're really trying to do is keep from damaging the wire and possibly a large piece of the harness. So rule number one is never install a fuse with a rating higher than the wire ampacity. Failure to observe this is a leading cause of wiring fires. So it would appear to be simple; you've got a 20A load, so use 20A wire and a 20A fuse and you should be good to go, right? Wrong!!! Fuses (and circuit beakers) cannot be continuously ran at 100% capacity; you have to derate them. They are designed to trip at 100% of rating, so unless you want to be plagued with nuisance tripping, you need to go a bit larger than the actual connected load. The accepted rule is any overcurrent device connected load should be limited to 80% of its rating. So a 20A fuse is actually only good for a 16A or smaller load. Stop and think about the implication of that for a moment..... What this means is your 20A wire/circuit is really only good for 16 amps; if you need a full 20 amps, you'll have to go up a size on the wire and the fuse. So in terms of actual connected load, all the wire ampacity ratings I gave before need to be derated to 80%. So does this mean that if you match the fuse to the wire, and the fuse is big enough for the load after derating, that this gives proper protection? Not necessarily; it's possible to over-fuse the connected device and have a fire hazard. An example could be a 8 amp device. Too big to use #16 wire (after derating it's 8 amps max fuse size down to 6.4 amps), when you step up to #14 with it's max size 15A fuse, the current available to the device is over 180% of what's actually needed. This could cause catastrophic failure and fire in the device. So what to do? The accepted practice is to figure your fuse size at 125% of the connected load. So a 16A load is 16 X 1.25 = 20. Our 8 amp load above is 8 X 1.25 = 10, so a ten amp fuse would be perfect. You will run into circuits where the 'calculated' fuse size will end up 'between' standard fuse sizes, so it's acceptable to increase to the next size up, but the fuse rating still cannot exceed the wire ampacity rating. So the 'general rule' is to size your fuse to 125% of the connected load, as long as it doesn't go over the wire ampacity rating. The exact same rules apply to circuit breakers if you use them. Now, there can be some special exceptions to all this. One is 'diversity' and I'll get into that during 'system design'. Another is motor loads, which present some special issues of their own. There's a phenomenon known as 'inrush current' (basically present in all circuits) that on motor circuits in particular can be an issue (and I'll note here that a solenoid as used for door latches/locks/etc can be considered as a 'motor'). A motor that draws 10 amp while running can see 'instantaneous' current of up to 500% of it's running current when starting. Sometimes using the 125% calculation doesn't quite give you enough 'headroom' on the fuse size and you'll get nuisance tripping. Bumping the fuse size up to 150% of load will generally clear the problem, but you still need to match the fuse size to the wire. Generally, if you're buying these parts from a vendor they'll be able to give you the required amp rating of the fuse, but if all you have is the amps required, use the 150% number. This will be more of an issue if using junkyard parts, best bet here is find out what size fuse the OEM used and go with that, or actually hook the part up and check amps under load. Another issue with motors is they can have very different amps required depending on the conditions in which they're used; amp ratings don't tell the whole story. Motors are actually rated in horsepower, and that's a critical difference when figuring actual amps needed. Here's why.... A motor is doing work, whether it's turning a fan, moving a window, or raising an antennae. You need X amount of HP to perform that work. But what happen if there's a drag or bind? Now you need more power to do this work... so what happens to motor amps? Horsepower is defined as 1 HP = 750 watts (rounded off). So let's say the motor is rated at 1/3 HP, but only 1/4 HP is needed to do the work. So the 'rated' current at the full 1/3 HP would be 20.8 amps (750/3 = 250 watts. Watts is volts times amps, so divide 250 by 12 volts). But if only 1/4 HP is needed, actual current will be only 15.6. But if a drag is present and the load needs 1/2 HP for the same 'work', now amps go up to 31.2. Don't know the HP? If you know the 'rated amps' or actual amps, simply multiply that by 12 to get watts/HP. I'll note here that if you do have an issue and the amps needed are more than expected, don't simply increase the circuit size; find out why it's higher. That motor can overheat and present a fire hazard. Another issue is voltage drop; as current goes up, voltage 'lost' in the circuit increases. Let's look at that 1/4 HP load again. At 12 volts, you need 15.6 amps to produce 1/4 HP. Drop the voltage to 11 volts, and now you're up to 17 amps. Drop another volt, and now it's 18.7. I'm bringing all this up so everyone will realize how important it is that motors be installed so they can operate as-designed, and also the importance of adequate circuit size. I'll get into this more in system design. I'll let everyone chew on this for a bit, and if there's any questions about this or ampacity, now's the time to ask them.
This was going to be the first installment of system design, until I realized I hadn't really addressed voltage drop. Now believe it or not, I'm trying to keep this as non-technical as possible. But some of this just can't be helped if you want even a basic understanding of how the system works. So the next thing I'm going to talk about is voltage Drop. Probably one of the least-understood factors of wiring design for the layman. I touched on it briefly while discussing motor circuits, but this needs more explanation. The first thing to know is any circuit will have voltage drop. How much depends on a number of things, and what's an 'acceptable' amount? Now, how much can be a judgment call; an engineer with all electrical data on all parts of the circuit can make the call on how much drop can be tolerated before circuit performance drops below what whatever minimum he's decided on. But as a general rule, the less voltage drop the better. So you've followed along as I've gone through wire ampacity and the some of the reasons for why I'm recommending these values. Voltage drop is directly related to this, and the 'harder' you 'run' the wire, the more drop you'll have. So let's revisit ampacity a bit more to see how it effects voltage. There's many different 'automotive wire ampacity' charts out there. Ones that are commonly seen show ampacities based on wire size and length. Now, the wire length really doesn't change the current-carrying capabilities of the wire, but what's happening is as the length goes up, so does the voltage drop if amps stay the same and delivered voltage at the end of the wire gets too low to properly operate the circuit. So reducing the amps lowers the drop. What most charts don't tell you is just how much drop there is when using these values. Two members (31Vicky and AZBent) sent me links to alternative charts, with two different methods of calculating ampacity. One was a GM manual, designed for installers adding circuits to existing vehicles. Now, they furnish a chart that assumes a voltage drop of up to 10% which I consider very excessive and I'll explain why further on. But let's go ahead and show just what these charts will give for wire size. So in the first chart, you look up the maximum connected load in amps, then go down that column until you find the shortest length that's equal to or more than your circuit length and that line will be your wire size. So let's assume a 20A load and 25 feet of wire. 14 is too small, as max length for that size is 21 feet and voltage drop will be more than 10%; number 12 is 33 feet, so that's the size you need. Now you go to the temperature correction chart for the wire type, and look in the column for the maximum temp that the circuit or any part of it will be operating in. Go down the column until you find an amp rating that's equal to or more than what you need for wire size. In this case I'll use the chart for the 135C wire and assume a 100C temp; now, 16 wire is rated for amps, but not for the voltage drop. Number 12 will work, as it meets both the voltage drop and amp requirements. Note that you could end up needing a larger wire if the temps is different, and that you need to perform these calculations for every circuit, maybe each part of the circuit. And if you mess up and have to extend a circuit or get a load wrong, it may suddenly be undersized. The other link I got was to a FAA manual. Very similar to the GM method, but much more conservative. Voltage drop is limited to 1%, not ten, and amp ratings allowed were lower across the board, leading to wire sizes as big or bigger than what I'm recommending. So why the big differences? Remember what I said about basic design at the start? That a big part of proper design is managing the heat generated during operation. For the FAA, safety is paramount, cost is a secondary consideration, and preventing fires almost outweighs anything else. But the OEM manufacturers who are building thousands if not millions of harnesses every year, by cutting safety margins to the bone and running every bit of the system as close to it's maximum rating as possible, they can reduce wire sizes enough to save substantial amounts of money. It's all about the bucks. Now why is voltage drop so important? Personally, I want to see no more than 5% voltage drop anywhere, and less is better. Let's look at a couple of things to better understand this. The first is voltage drops add up. In other words, if you do a circuit calc between the battery and a terminal and the drop is 1 volt, everything downstream from that terminal will be short that volt. Do a calc from the terminal to a switch, if you lose another volt now you're down 2 at the switch. From the switch to the device, you could lose yet another. Lose too much, and now you have a motor that's drawing more current than you figured and nuisance tripping fuses, or lights that aren't as bright as you thought they should be. If you've ever installed a relay to 'brighten' your headlights, this is the cause; too much voltage drop. Another thing to think about; where's that voltage go? It just doesn't disappear into thin air... it's converted to heat. The bigger the drop and/or the higher the current, the more heat you see. How much? One way voltage drop is figured is by using the formula VD (voltage drop) = I (current) times R (resistance). Watts is voltage times current, so if you have a 10% drop to 10.8V at 20 amps in a specific wire (length doesn't matter), simply multiply the 'lost' volts times the current for the watts; in this case 1.2 X 20 = 24 watts. While that doesn't sound like much, remember that a brake light only uses 27 watts and they get plenty hot. This also consumes amps; if you started at 12 volts, it takes 2 amps to produce those 24 watts, also 'lost'. There's only two ways to reduce the voltage drop in a wire; reduce the amps, or use a bigger wire. Keeping voltage drop to a minimum will reduce overall heat and make the system more efficient. I'll add that in reading the GM manual, I was struck by certain things that will give you a window into their thinking when designing a OEM system. The first thing is there was no 'bundling derate' chart. Part of both the NEC and the FAA 'codes', this was nowhere to be seen. Another thing they noted was to take into account 'inrush current' when sizing switches and relays. This information is usually only available to the engineers and can vary depending on the device. Again, they have all that information for every part, and I have no doubt that they do take 'bundling' into account as well as the factors I've brought up, but having more knowledge they can 'cut corners' closer than we can. The one thing that both methods have in common with what I'm saying is calculating overcurrent protection; exactly the same formula, so you can use what I've said with complete confidence.
Ok, I've gone on about ampacity, overcurrent protection, and voltage drop and I'm sure some out there are thoroughly confused, what with all these charts and dire warnings about what not to do. Have faith, a little further on I'll give up a 'trick' that will vastly simplify figuring safe wire sizes for even the most 'electrically challenged' as long as you can come up with certain bits of information. But I went into all this so everyone will at least have a basic idea as to all the forces at work in an electrical system. With that said, let's move on to... System Design, Part 1. So you've decided to tackle this, where do you start? I talked about the 'logic' of the system in the first installment (switched vs unswitched, etc), but now we need to break this down more. Each individual circuit will fall into one or more of four categories and here's the types and differences: 1. Branch circuits. These supply power to a single device or system. This will be most of your harness (but not all) and includes most of what everyone thinks of when you say 'wiring'. 2. Feeder circuits. These supply power to multiple branch circuits and/or have combined loads on them. These can include the wiring from your battery to a fuse panel or a switch, or the power wire out of your generator/alternator to the battery. These can also be a power wire supplying a circuit that has multiple devices/loads connected to it. 3. Continuous loads. These are all the circuits that if you turn them on, can be on for an extended period or will be on anytime the vehicle is running; most lights, ignition, radio, heater, and so forth. 4. Intermittent loads. This will generally be power accessories like power windows, locks, seats, etc but can include any load that will be usually on for short periods of time (like under one minute). A feeder circuit will almost always be a continuous load, but a branch circuit could be either. The reason for splitting the branch circuits into two types is so we can apply a concept known as 'diversity' when figuring feeder loads. This is based on the idea that not everything will be operating at once. For an example of this, if you've ever wondered why all the breakers in your panel in your home add up to more than the 200 amp main breaker size, that's diversity; nobody plugs something into every outlet, turns on every light, and runs every appliance all at once (well most don't... LOL). There's actually a formula for this in the electrical industry, but unfortunately it's not really applicable to automotive.... So the very first thing you have to do is figure just what circuits you need. I'll try to post a 'checklist' for this at some point in this thread but my scanner is down right now. List each circuit, and don't assume anything. Basic car, maybe all you need is ignition, start circuit, headlights, brake lights and taillights. But if you want more than bare bones, you need to add circuits. Things like dash lights, radio, etc all need to be added as separate loads; don't figure 'they're small and don't matter'. Small loads add up to a large load at some point; you can bet your last nickel that the OEM engineers do this. So a 'typical' rod or custom will likely have ignition/start, headlights, taillights, brake lights, dash lights, license plate light (include this with the taillights), interior lights, radio, turn signals, heater blower, and maybe an electric cooling fan. If you decide to get fancy, you may be adding power windows, power seat(s), AC, door and/or lock solenoids, emergency flashers, additional stereo gear like extra amplifiers, maybe a compressor if you have airbags. If you're a smoker and want a lighter, add that; even if you're not a smoker, you may want a lighter socket for use as a power point for phones, GPS or portable compressor. Most cars came standard (or had included as a cheap option) with lighters starting in the early fifties, they can be handy. And remember, even if you may not be installing all these things right away, if you think you might in the future go ahead and add them. This will insure that your basic 'core' wiring will be big enough for the added load. Now, that's not to say that these things couldn't be added later; after all, it was common practice with the OEMs to have a 'basic' harness, then if the vehicle was ordered with extra electrical items, they used specific 'add-on' harnesses to power these items. To some degree, they still do this. But adding like this means more wires in more places and may not be very easy on a finished car. Not to mention it will probably be cheaper in the long run if you do this out of the gate. So, got your list? Next thing is to figure the load in amps on each circuit. Ignition amps may be tough to come up with as these circuits are rarely fused. I'd figure a 12 amp load for most, if you're running some sort of high-powered electronic ignition check with the manufacturer and use his number. For the starting circuit, you can ignore that one; it's small in any case (basically the solenoid coil current) and will rarely be on for more than seconds. Lighting? That's simple; add up the wattage of each lamp on the circuit, divide by 12 (or 6 if you're still running a 6 volt system) and that's the load. Typical amp values at 12 volts (per each lamp) are 5A for headlights, brake/turn 2.25A, tail/park .7A, dash .3A. Double these numbers for 6 volts. Best bet is to do a little research and find the actual watt rating for each type lamp you're using. If you're using high-output Halogen headlights with a non-standard wattage (AKA 'off-road'), use the watt numbers for that lamp. Many will have the watts marked on the lamp, but a quick web search will turn up the watts for any common lamp you're likely to use for any purpose. If you have indicator lights, those should be included in the circuit they indicate. I will comment on the use of LED lamps. These use much less power (typically only a few tenths of an amp) so you can run pretty small wire to these. But I'd caution against doing so for exterior lighting, as the photometrics of these in some 'vintage' taillights can be poor and if you decide to switch to 'standard' lamps at some point, you'll have a badly undersized circuit. LEDs are generally fine if used for dash or indicator lights, or license plate lights. But I was somewhat surprised to find the 'map' lights in my '13 Mustang were conventional lamps; these and the HID headlamps are the only non-LED lamps on the whole car. Poor photometrics is the reason I'm sure, so you might think about that if you want interior lighting. I'll also note that if you use LEDs for dash lights, you'll need a special LED dimmer if you want the ability to dim them; standard dimmers won't work. I'll talk about a few more lighting issues before moving on. First, dash lights are usually a separately fused circuit. The big reason for this is if you do have a fault in the circuit, it won't take out any of your exterior lighting. But they're usually fed from the taillight circuit, as that eliminates a separate terminal on the light switch. The advantage to doing it this way is if your dash lights go out, this may indicate that your taillights are out too; a safety item. The second item is there is some 'overlap' in lighting loads. For most vehicles, the brake/turn/emergency lights share lamps, depending on which circuit is in operation. Now, when figuring load on each branch circuit (and each one must be figured separately), you have to include the amps for each lamp in order to correctly size both the wire and the fuse size for that circuit. When we get to figuring 'feeder' loads, I'll explain on how to 'consolidate' these loads so you don't count them twice or three times.
System Design, Part 1 continued. Ok, I've talked about ignition, starting, and lighting, now for the 'optional' stuff if you're going beyond bare bones. If you're reusing existing accessories, simply see what size the factory fused them at and figure the load at 80% of the fuse size if possible. I say 'if possible' because sometimes the OEM engineers shamelessly combine circuits on one fuse and it can be very hard to figure out just what the actual load is. So unless you're maintaining the exact same circuitry, you may have to measure or calculate the individual loads. But if the fuse is for only one item or you do keep the OEM circuitry, this will give an accurate enough number to use. Some 'typical' load values (note that these may or may not be accurate for your parts) that should get you close can be lighter - 16A, radio - 7A, wipers - 6A, horns - 12A, heater - 8A, heater/AC - 24A, power seat - 16A, power windows - 16A. But to be honest, I'd personally verify each load using an ammeter to avoid sizing problems before using these numbers; you can find variations between makes/models for 'similar' parts. But many times, you won't be using original parts. If you're buying in the aftermarket, get the required amps for each device from the manufacturer if at all possible. Again, if all else fails you can physically measure the current on the device to get the load. The one caution I'll put up here is if you're measuring a motor-driven device, make sure it's under the mechanical load it will be under as-installed. No-loads amps can be considerably smaller and may prove to be too little once installed. If you're using boneyard or 'donor car' parts, try using the same methods as you'd use with existing parts, only source your information off the donor. One common mistake to avoid is trying to size the circuit wiring off the wire size that you may find on a 'pigtail' coming out of the device. Manufacturers are notorious for using undersized wire for pigtails out of motors and other items, working off the idea that this short piece won't degrade the circuit enough to cause harm. They are right about that, but that doesn't mean it's the right size for wire coming to it. Got all your branch circuits listed and loads assigned to each? Now we need to start dividing them into types.... Start by dividing them into switched and unswitched circuits and use two lists. The unswitched circuits will be most of the 'safety' and convenience circuits; pretty much all lighting except for turn signals, horns, lighter/power point, convenience lighting, power locks/entry, alarm systems, radio 'memory', clock and anything else you want to work with the key off. Switched circuits will be ignition/start, turn signals, heater/AC, power seats and windows, radio, wipers and anything else you want to work only when the key is on. Now take each list and identify each circuit as either a continuous load or an intermittent load. Continuous loads are basically any load that would be on all the time while driving the vehicle down the road under expected 'normal' conditions; ignition/start, head/tail/brake/dash lights, heater/AC, radio, wipers. Intermittent loads are those that will only be on for short time at any given time or may be used instead of another load. Horns, turn signals (unless you're one of those who like driving for 20 miles with the left turn on...LOL), power seats/windows, convenience lighting, emergency flashers, lighter to name most. Now, if you add all these loads up you can get a scarily large number, far more than what your charging system can put out on it's best day. But in the next installment, I'll start explaining 'diversity' and we'll whittle that number down...
Doesn't this undersized "weak link in the chain" cause a sort of fusible link? Why would it be any safer becasuse it's a short piece than if the whole run was undersized?
How do you determine wire quality? Are there certain trusted mfgs that are sort of the standard for the industry? Is wire dated? I ask this because PVC degrades over time and I once bought a roll that was severly sun bleached on one side. It had evidently been on a display rack that got sunshine through the store window. It felt very dry and brittle. How about the same questions for connectors?
Crazy Steve, I have just completed reading this thread since the last time we talked, very good. The FAA chart that is listed on there website is not the one that I like. But, as an aircraft mechanic, that is the one I have to use if I am doing something that would require it. The chart that I told you was out of date does take into consideration for wires in bundles, single, and intermittent use (starters etc). I do think that some people will receive a greater understanding of what is required and how electrical systems operate. You are very good at explaining the problems and potential problems, thank you. Clik, you are right in that PVC does deteriorate. Doing inspections on planes and the insulation is cracked, is reason for repair. But remember that one of the biggest problems with wire insulation deteriorating is heat. That is why Crazy Steve talked about wiring at the start of this thread. Wiring is almost never out in the open where it can deteriorate from the sun, so the problem (of fading) you talked about is almost unheard of. Mark
I owned a 1973 triumph bonniville and it was my only transport for a year about 5 years ago, you got good at diagnosing electrical problems or you got stranded! big thanks to the british motorcycle industry for helping me understand wiring !! and thankyou steve for spelling it out for us, real handy thread. CHEERS.
Seeing how the melting point of copper is nearly 2000 degrees F, it makes a lousy fusible link. Why is it 'safer'? Voltage drop has to do with the length of the wire, although that's not the only factor; long wire, more drop (and heat produced), short wire less. In a case of small leads out of a device, let me show an example. Assume a 12V motor that draws 10 amps and it has 16 gauge leads coming out of it and both are 6 inches long (total 1 foot). The drop in this length will be .04 volts, about .3%. This will produce .4 watts of heat. Now extend those wires to 20 feet. Voltage drop will increase to .8 volts (6.6%) and heat will go up to 8 watts. Increase the length by 20, increase voltage drop and heat by the same. Let's try a larger wire size to the motor, up one size to 14. Now you have 19 feet of 14 and one foot of 16. The drop in the 14 will be .48 volts, add the .04 volts for the leads for a total drop of .52 volts, or 4.3% . Watts lost in heat drop to 5.2. But there's more to it that just this if a motor is involved and I'll go into more detail when I get to sizing circuit wire. Wire quality.... The first thing to remember is in terms of electrical properties, copper wire is copper wire, regardless of who makes it. As long as sizes match, they will be the same electrically. It's insulation is where the differences lie. Now most uses for wire require that certain standards have to be followed and the wire is built and tested to those standards. Not so in aftermarket automotive. This is a small market, life safety isn't involved generally, so nobody is watching. The OEM users purchase wire built to their specs (and likely test it), but for the aftermarket this doesn't exist, so you're at the mercy of the supplier and whatever they tell you. If you want wire built to a known standard, you'll have to go with wire that's rated for FAA or NEC use, although the FAA wire will be expensive and NEC-rated wire generally isn't sold in small quantities. But some of the larger manufacturers do sell into all markets, the two that come to mind are Belden and Southwire. When buying wire, if you ask for a wire that's rated to 90C at least, that will be good enough for our purposes. This rating is almost the 'standard' in NEC wire these days in sizes below #6 so it won't be hard to find. Connectors/terminals? Same thing, the better quality manufacturers will have ratings on their stuff or if it's sourced from the electrical industry it will meet NEC standards. Quality costs money, it's up to you.... Wire isn't 'dated' either; modern wire installed correctly will outlast the vehicle. But UV-rated wire is rare (it does exist, but isn't suitable for automotive use) as AZ pointed out; wire is very rarely installed open to the elements.
This is a great thread and I know I'm not the only one learning some new stuff. Thanks for putting such effort into such a detailed thread. Your examples really help to make sense out of the topics.
A friend of mine made some jumper cables out of big diameter welding wire and I questioned the logic stating that he still only had contact at the ends of the little teeth that are on the clamps that grip the terminal. Was I right or wrong?
The little teeth will melt their way into the lead battery terminals and make good contact....if that's what you really want to do.
System Design Part 2. Diversity. Ok, you've got all your loads broken out by now and have them identified as to what 'type' they are. You have four categories; unswitched continuous, unswitched intermittent, switched continuous, and switched intermittent. The first thing to do is group the unswitched loads together, then do the same with the switched loads. I'll get into the reasons why a bit later in another post (and some may have already figured it out), but for now this is what you need to do. Now if you just add these loads up, you can get some pretty big numbers if you have a lot of loads. Like I said before, maybe more than your charging system is capable of. Examples seem to be the easiest way to get this across, so lets do one. I'm going to use a late-50s, early 60s full-size car with AC and power accessories, and this car has four headlights and tail/brake lights. So let's start with the unswitched loads... 1. Headlights. You've got four, and on high beam the 'typical' amps each is 5A for a total of 20A. 2. Taillights. Again, you've got four and at .7 amps each (don't forget the license plate light at the same), that's 3.5A. 3. Brake lights. Four at 2.25 each, 9A. 4. Dash lights. I'll figure only four at .3A each, 1.2A. 5. Emergency flashers. You're flashing the four brake lights at 2.25A each, two dash indicator lights at .3A each, and the front turn lights at 2.25 each for a total of 14.1A. 6. Interior lights. Let's say a overhead light and two footwell lights for a total of 1.8A. 7. Back-up lights. Two at 2.25A, 4.5A. 8. Lighter. 16A if you actually have a lighter, if you only use it for a power-point, actual load of the device you're going to plug in. I'd recommend the higher amount as that give you more options.. 9. Horns. Two at 6A each, 12A. If you add all these up, you get a total of 82.1 amps, a big number. But not all of these are continuous, so you don't need 81 amps all the time. Your continuous loads are only items 1, 2, and 4 as a general rule, so the continuous load is only 35.5 amps, less than half. Now you can't just ignore the intermittent loads; you do need to factor those in to prevent excessive low voltage to the continuous loads when you do use an intermittent load. Generally speaking, if you use 80% of the largest intermittent load, this will give you adequate 'headroom' without significantly affecting continuous loads. The largest load is the lighter at 16A, so 80% of that is 12.8, so total unswitched load is 48.3 amps. When I get to actual wire size selection for feeder circuits, I'll talk about this more. I will specifically talk about the emergency flashers. In most cases, these will only be on if the car is stopped with the headlights off. If you want to be able to drive the car with headlights and flashers on, you'll need to move these to the 'continuous' category and increase the load. Moving on the switched circuits, it's the same process. So.... 1. Ignition, 12A. 2. Wipers, 6A. 3. Turn signals, 7A. 4. Heater/AC, 24A. 5. Gauges, 4A. 6. Radio, 7A. 7. Power seat, 16A. 8. Power windows, 16A. Again, total load adds up to a big 92 amps, but your continuous load is lower. Items 1, 2, 4, 5, 6 are continuous and total 53 amps, a bit more than half. Again, you want to add a percentage of the largest intermittent load. So if you add up just the 'basic' continuous loads (35.5 and 53), you get the number 88.5 amps for total continuous load. Why is this important? It's important for many reasons, but this number represents how many amps your charging system needs to put out just to stay even, much less have any 'extra'. If you're driving down a road at night in this car and it's raining and sweltering hot, you've got the AC on to be comfortable, listening to tunes to cheer you up, and have the high beams on to cut through the gloom, if the charging system isn't up to it you may arrive with a dead battery or not arrive at all... For sizing the charging system, the intermittent loads won't count; you can draw off the battery and then the charging system can 'replace' that power as you drive. One thing I'll note here is I've used 'typical' values for a lot of these loads. Yours will very likely be different, and having the exact loads may lower some of these numbers significantly. At this point, some may be saying 'Damn! This to too complicated for me!' Hang in here, I'm going to try to post a 'checklist' that will help considerably. But this does give you insight into the process that the OEM engineers have to go through to produce a system that functions safely and well, all at a cost-effective price. I'm much less concerned about price, and lacking most of the engineering data they have I'm trying to give guidelines that will allow for that lack. In electrical, too small is bad, too big is good.... If there's questions on this aspect of system design, now's the time to ask...
So, when you mentioned that lead in wires are often a smaller gauge wire it got me thinking about them. I really never paid too much attention to it butbMost of them are also tinned copper wire that I've seen and the insulation is usually different. Not quite sure which it is but it's different. Also here's a great source. http://www.wirebarn.com/Wire-Selection-Guide-_ep_29.html brief explanation of wire insulation differences ( for the guy that asked) Packs of multi colored wire sold by the foot with the different automotive specialty insulations . A pretty neat calculator to size the wire you need. Plug in your amp load, length of run, % voltage drop and it gives you a lot of info. http://www.wirebarn.com/Wire-Calculator_ep_41.html Great thread man !
Great links! And even better, these guys sell small lengths which can be the hardest thing for the hobbyist to come up with if you want multiple colors. The reason most 'leads' are tinned is because they're nearly always soldered inside the parts they come out of, and that makes soldering them easier. The reason these are smaller is simply cost; if you're building a million motors a year, the price difference between a million feet of one size and another can be a big chunk of money! The voltage drop calculator is the 'trick' I'll supply later. There's a bunch of them out there on the web, I'm going to use a slightly different one that should prove to be a bit easier to use and gives more useful info.
I have a question. How come most aftermarket wiring harness's state that if you are running an alternator greater than 100 amp you need to run a wire from the starter to the alternator positive lug? I assume it is trying to bypass something but do not understand what and how or is this some sleazy way to avoid problems? Thanks
If the battery gets run down, a high power alternator can charge it back up in a hurry....that means you'll get a lot of current in the wires connecting the alternator to the battery. More current than the normal 10 gauge wire can handle safely.
You should connect the output from the charging system at the closest point to the battery you can in any case. At the battery itself is rarely used because this exposes the wire to corrosive fumes which can damage it. The next closest place is usually the starter (actually, the starter solenoid), so that's where it goes.
electricity will follow the path of least resistance- the wire to the starter should be large enough to handle the entire load. The electricity does not usually split between two wires, though both would be live.
System Design, Part 3. Layout. Ok, you now have all your branch circuit loads, segregated into 'types'. You also know how much of a demand this will put on your charging system. Now you need to determine just what goes where, i.e. where are you going to actually install all the components. This is extremely important as circuit lengths will affect wire sizes, sometime drastically. At this point you'll also need to start selecting the 'other' components you need for a functioning system; fuse panel or panels, switches, and possibly relays. So where do you start? Up until now, we've just been gathering data, now you actually have to go look at the vehicle. First, decide where you want your fuse panel or panels. You can install these virtually anywhere, but keep in mind distance from the power supply (battery) can change wire size, you want the circuit paths to be as short as possible. As an example, let's say you put the panel under the dash. The load on the panel is 40 amps and it will take 8 feet of wire to get to the power point. You'll need #10 wire to keep voltage drop to an acceptable level. Go another 6 feet, now it's #8, another 4 feet and now it's #6. Same thing goes for your branch circuits; the longer the path, the more voltage drop you get. You also want to mount the panels where they're accessible and not exposed to environmental conditions they're not rated for, including heat. Generally speaking, mount them in the coolest place you can. So what if your battery is mounted in the trunk or under the floor? You will be running a large, high-amp cable to your starter solenoid, so this connection at the starter is a perfect place for your power point for the rest of the electrical system. The closer to 'center' of the system the panels are, the better. There can be exceptions for individual circuits, but this should be the general rule. Remember, the panel or panels will be carrying the combined branch circuit loads, so the feeder wires to these are high-amp wires. Got your panel location? Now we move on to start figuring out the branch circuit lengths. With the possible exception of the ignition/start circuit, every other circuit will come from the panel or panels unless you don't mind fuses being installed all over the vehicle, so that's the point of origin for your branch circuits. Now most of your branch circuit component locations will be dictated by function; lights, power accessories, etc. Some you'll have to decide. Once you have that decided, now you need to decide wire routing. You'll have wiring going in multiple directions from the panels, picking the best route should take some time. You're trying to protect the wire from excessive heat and exposure to damage. Avoid routing the wire next to high-heat sources, or use a heat shield if it's unavoidable. Heat is the number one enemy of electrical systems. When 'bundling' wires, try to keep a 'mix' of continuous load and intermittent load wires and avoid bundling multiple high-amp-load wires together; several small bundles are preferable to one large one. If you do have a problem down the road, this will limit damage as well as making tracing easier. If you're using that corrugated split plastic tube for your harness, this is much less of an issue but I would still recommend splitting your bundles up. A very important thing to remember when routing the wires is you may need to apply 'derate' factors when bundling. A key thing to remember is you only need to count current-carrying wires. In other words, a continuous load wire will count, but an intermittent load wire doesn't. How far apart do they need to be to be considered not bundled? Basically, a free space of a half inch should be adequate in most cases. And if wires get grouped together for a short distance (like going through a grommet), you can usually ignore that. One thing needs to be pointed out; whether your building your harness from scratch or buying one from a vendor, you'll still need to figure out your wire routing. Ok, so back to the branch circuits. Starting at the fuse panel, the first stop for most circuits will be the switch that controls it. Some items may have a single point of connection (radio, lighter to name two) but most will have a control switch. Write down that length. From the switch, go to the component. Now if it's a single item, write down that length. But what if there's more than one like headlights or taillights? Best bet is to figure to the furthest part, although there are exceptions that I'll cover later. Repeat this for each branch circuit, writing the lengths down for each one. Now, it's time to talk about something that hasn't been mentioned up to now; ground wires. Up until now, we've only talked about half of the circuits, the positive half. But the ground or negative half of the circuit is every bit as important, sometimes more so. Every circuit needs a ground path back to the battery. In many cases, the component will be internally grounded and attach to a metal part of the vehicle, so you don't need to specifically install a ground wire. For these, as long as the metal parts used for the ground path are all connected together ('bonded') and connected to the negative battery terminal, you're good. But in some cases you'll need to add a ground wire; a headlight is a common example. You need to add the length of any ground wire to the total circuit length. This wire is part of the circuit and will be carrying the same load, so it must also be sized the same. So what about those of you that have non-metal parts or cars? You're running ground wires to every component that can't be grounded to it's mounting point. And you have to add that length to total circuit length, which will almost always mean you'll need larger wire for the same load compared to a steel car. In these cases, a common mistake is to run a small wire to multiple components and ground all of them to that. Not good.... This can cause problems, as anybody who has ever had trailer lights act weird from a poor ground can attest. Now, you can combine grounds for multiple components, but you need to size the wire to the combined total of those loads. As an example, if grounding a headlight and a turn signal, add each load together and size the wire to that. Failure to do this can result in a headlight that 'blinks' along with the turn signal. Any questions on this, let's hear 'em.
System Design, Part 4. Wire Size Selection. Some of you will saying 'It's about time!' by now, but this is a critical step and can mean the difference between a system that works flawlessly, to one that has 'peculiarities', to one that burns your car to the ground. And this is the point where you have to decide how much you want to 'compromise' the system for various reasons that I'll go into. There are compromises in any system, make no mistake... So at this point you should have all the branch circuits wanted, their loads (and by multiplying each load by 125% or 150% if it's a motor load, the maximum fuse size), and their length. The last piece of the puzzle is how much voltage drop will be 'acceptable'. Generally speaking, the larger the drop that's allowed, the smaller the wire you can use. Now larger wire obviously cost more than smaller wire, may be harder to terminate in some cases, and uses more space. Again, if you're building millions of harnesses a year cost of all this is a major factor. This is something even the aftermarket vendors think about. But realistically, this won't add much over $100 to the overall cost of a single harness, so how badly do you want to save money? So how much voltage drop is acceptable? There's no simple answer to that unfortunately, but let's back up and revisit the factors that you need to think about. I talked about ampacity and voltage drop as two separate subjects, but these are interrelated, very much so. Copper wire, any copper wire, will have X amount of voltage drop at X amperes in X size per foot. The only way to change that is to make the wire out of a different metal. So here's a couple of voltage drop calculators; note that neither asks about the wire insulation.... http://www.wirebarn.com/Wire-Calculator_ep_41.html There's three things I don't like about this one. One, when you put in a value for % drop, if it goes over even by a little bit it moves you up to the next size wire. Two, it only allows you to select 1, 2, 5 or 10 percent. Three, to find the dropped voltage, you have to subtract the 'end' volts from the start volts. I do like the length input though, as this calculator uses actual length. I will recommend that you read the 'notes' at the bottom of the page. http://www.calculator.net/voltage-d...nce=10&distanceunit=feet&eres=23&x=55&y=16 I like this one better. Reads out the actual drop in volts, end volts, and actual percentage. You can also input actual voltage, and it accepts fractional amounts. The one thing that's 'wrong' is on the length, it assumes if the input is 20 feet, that's one way and doubles this number for wire length ('set of conductors'). So for our use, you'll need to divide your circuit lengths in half before plugging them in, unless of course you have a fiberglass car and need the ground wire going out. So lets play with the second one a bit.... Let's assume a #12 wire, 20 feet long, with a 17 amp load, and we want voltage drop at or below 3%. We're starting at 13 volts at the fuse panel. Plug these values in, and you .54 volt drop, for a % of 4.15. Too much drop, go up one size on the wire to #10, now you're at .34 volt drop and 2.62%. Perfect! Now, just for fun, go back to the #12 size and start increasing just the amps. As amps go up, so does the drop and the percentage. Get high enough, and the drop can consume almost all the voltage; at 400 amps, all that's left for the load is .29 volts. And remember when I said voltage drop causes heat? That 20' of wire is using over 5000 watts (watts = amps x volts), more than most wall heaters put out! Now, it may not get that high; the wire will probably melt first! This brings home the fact that voltage drop does two things that are detrimental to the system, it introduces heat, and wastes power. But almost no one uses 1% drop as a max (except the FAA), so what's a good number? Well, it depends.... Besides the heat and power loss, you also need to know the tolerances of the connected loads. Most devices that use power will accept a 'range' of inputted power. On our '12V' systems, actual voltage will be more, as much as 14.5 volts under certain conditions, but usually in the 13.5 range. You won't see more than that unless your charging system malfunctions or your car gets hit by lighting... LOL. But what's the lower limit? For 'standard' lights, low voltage just means they get dim. Electronics? That's less clear-cut; excessive low voltage can damage these. Motors? Less voltage, they draw more current (and get hotter) and if it gets too low, can damage them. For the OEM engineers, they know just how low they can go but we don't. Now GM allows up to 10% for added circuits, but that manual didn't give any info about the original design. I'm going to recommend that you limit voltage drop on branch circuits to 5%, and 'feeder' circuits to 2%. Why the difference between the two? Here's why. If you go back to my 'example car', I came up with 88.5 amps of continuous load. This was the 'worse case' you could have when driving the car normally. The first thing to remember is this is the branch circuits. Now at this load, the charging system is working hard probably and output voltage may be lower, so lets use 13 volts as the voltage available at the battery. 5% of 13 is .65 volts dropped, so to find the watts 'lost' you multiply .65 x 88.5 = 58 watts (rounded). It will take about 4.5 amps of additional power to produce that heat, so now your load is 88.5 + 4.5 = 93 amps. But we haven't added the 'feeder' drop yet, the wire from the battery to the fuse panel. This will have it's own drop, and if we use 5% again, there's another 4.5 amps added to the total load. That's 9 amps doing nothing but producing heat and robbed from your charging system. But wait; now your branch circuit calcs are off; you no longer have 13 volts, but only 12.35 volts at the start of the circuit, out at the end of the branch circuits you could be down to only 12 volts or less. If you use a higher voltage drop %, the numbers get worse; at 10% on each type, that's 18 amps 'lost'. Admittedly, this is a bit of a worst-case. If you're wiring a 'bare bones' car with no bigger loads and enough charging system capacity, you can use a higher percentage and get away with it. I would try to keep drops to lights well under 10% if for no other reason to make sure they're bright. But even my recommendation will still give you a 7% drop by the time you get to the device at the end of the branch circuit. So armed with this calculator, you have all the needed information to plug in to find the drop on each circuit. You know the load, the length (don't forget to divide in two), and the fuse size. The fuse size will tell you which size wire to select (remember those wire amp ratings I listed; time to use them) and in most cases will spit out a voltage drop low enough to work. You don't need to hit your target number exactly either; if you can get an average of your target number between all circuits, that will usually be close enough. I will say that try to keep any motor circuits as close as you can; excessive low voltage there will increase circuit amps. Now, this isn't the last word on this; there are some circuits that can still use some 'figgerin', and I'll talk about those in the next installment. But you're armed with the tool you need to find wire size, so go forth and multiply! (Pun intended... LOL)
Current flow on each conductor would follow ohm's law, so if you had two identical cables with identical resistance, in theory, equal current would flow on both. If one cable had less resistance than the other, then proportionally more current would flow on that conductor. In this case, each conductor would share the load based on the formula Current = Volts/Resistance. It's not all or nothing.
So why is one big wire and one little wire carrying the load OK on DC and not on AC? I once asked my electrician why we couldn't double up some small ga wire temporarily rather than wait 'til the next day to get larger wire and get our compressor on line. The answer he gave was that one wire might carry more load than the other and overheat. I was in the water business thinking pressure and volume, so his answer didn't jive to my way of thinking, but I've never seen what I suggested, elec wise/ac, done either.
What he told you is correct... And you shouldn't do it either place, AC or DC. The larger wire wire will end up carrying nearly all the current, at least until it heats up and has the same resistance as the smaller wire. There are exceptions to this rule, but all wires have to be the same size and length.
So the advice that Leaky got from harness mfgs was wrong: Leaky: " I have a question. How come most aftermarket wiring harness's state that if you are running an alternator greater than 100 amp you need to run a wire from the starter to the alternator positive lug? I assume it is trying to bypass something but do not understand what and how or is this some sleazy way to avoid problems?"
His post is poorly worded.... You should have a wire from the 'bat' terminal of an alternator to probably the starter solenoid already, so if you run another you're not 'bypassing' anything. If what he (or somebody else) was trying to get across is the existing wire may be too small, running another wire will give more current-carrying capability BUT is a very poor idea for the reasons given above. You should never run two feed wires between two points on a car unless both wires are exactly the same size and length and each is capable of carrying the full load by itself.
The Jalopy Journal
