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Technical Main fuse size and wire gauge?

Discussion in 'Traditional Hot Rods' started by AGELE55, Aug 3, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    My 55 Chevy wiring was cobbled together over the years, mostly 30 years ago, when I was young enough to know what I was doing...lol.
    I have virtually everything in the car running off a 10 gauge (maybe even a 12?) totally unfused wire that I jumped off the starter large BAT terminal and ran to the ignition switch and fuse panel (scavenged from an old MOPAR) for the “always hot” circuits. The “key on” circuits jump from the ignition switch also. So essentially EVERYTHING is traveling along this wire.
    Upon deep reflection and soul searching, I’ve decided that the younger me may have been a bit over confident in my knowledge of automotive wiring... although, you gotta give credit to that little wire for hanging tough all these years. So now I’m thinking of retiring the little wire for an upgrade.

    ...and finally..the question:
    Assuming I pick directly off the battery and use a maxi fuse, what size fuse? I actually found a schematic I put together back in the day, thinking some day it would come in handy. It’ll answer your questions about what circuits are in the car. 59024542-0E3E-4EA7-830F-6FE390CC6ACC.jpeg
     
  2. solidaxle
    Joined: Jan 6, 2011
    Posts: 571

    solidaxle
    Member
    from Upstate,NY

    Having the wiring diagram is essential. That's a great start. It's better to have larger then smaller wire, but 10awg would be a little heavy for some of the loads.
    There's a lot of info on this site. Here's a link /www.jalopyjournal.com/forum/threads/wiring-101.843579/
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 50,228

    squirrel
    Member

    30 or 40 amps is ok for 10 gauge wire in a car....if it's 12 gauge, then 20 or 25 amp.

    I just went through this with my Chevy II, and I ended up using a 30 amp self resetting breaker for the main feed. If I were to rewire the car, I might change things just a little bit.

    I've also used a #14 fusible link on that wire that gets power from the starter battery connection, the fusible link connects to a #10 wire that feeds everything else. just make sure that all the circuits connected to it have their own fuse, so there is no wire smaller diameter than #10 between the fuse and the fusible link.
     
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  4. solidaxle
    Joined: Jan 6, 2011
    Posts: 571

    solidaxle
    Member
    from Upstate,NY

    To answer your question. The fuse needs to be sized per your wire gauge. You need to find the size of the main wire and fuse accordingly. You may find may it to be to small and will have to change to a larger wire.
    Since you are assessing the the wiring, I would consider a couple of things.
    Is the wire old and brittle.
    Is the wiring installed in a neat and workman like manner.
    Are there grommets in the firewall and other barriers to protect the wiring from sharp edges.
    Are the connections good. Connections are the source of most problems, being loose screws or corrosion.
    I always use the smallest amperage fuse I can in a circuit.
     
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  5. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Thanks Squirrel. I just double checked it and its a 10. I'll wire in a 30 amp and see how it goes, and keep a 40 in the glovebox if it doesn't go ...;)
     
    squirrel likes this.
  6. Well, you might want to read through this.... https://www.jalopyjournal.com/forum/threads/wiring-101.843579/
    ... but the short answer is it's all but impossible to meaningfully install a fuse on the main power supply. The OEMs recognized this and didn't bother with a fuse on main feeds, depending on the individual circuit fuses to protect the wiring and careful routing of the main feed wire to protect it from physical damage that could cause shorts. Remember that the fuse is there to protect the wire, not the downstream devices.

    The problem is the main feed load isn't fixed. While individual circuits have specific loads that can be fused to that load, the load on the main feed changes depending on what's on at any given time. You have both continuous loads and intermittent loads. Generally the main feed wire will be sized to safely carry the continuous loads plus a bit extra, ignoring the intermittent loads (like the horn, lighter, brake lights, turn signals, power seats/windows) as they're usually not on long enough to cause excessive heating in the wire.

    Wire sizes are rated in amps. Any fuse that's installed should never exceed the wire rating. Unfortunately, fuses are stupid; all they look for is current in excess of their rating and they'll open. So if you have a #10 wire as your main feed, the max fuse rating would be 30 amps (the wire rating). But if switching on one of your intermittent loads causes the load to exceed 30 amps, you're now dead in the water as the fuse will blow. Installing a larger fuse to eliminate this tripping now means the wire can be overloaded beyond it's capacity before the fuse trips, effectively the same as if no fuse has been installed and that's how fires occur.

    If you feel you must fuse the main feed, then size the feed wire for all loads (including intermittent) then fuse to the wire size. Generally you'll need at least a #8, possibly bigger. I'd do a #6 on your car, that will allow a 50 amp fuse.
     
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  7. Gasser_Dave
    Joined: Aug 18, 2013
    Posts: 111

    Gasser_Dave
    Member
    from St. Louis

    I think there are going to be a lot of answers to this question, but for my main fuse, I added up the total draw if everything were powered on at the same time and fused it at that. Rarely will you have everything on at the same time. Then the fuses in the fuse box can handle each accessory separately. Or, if for some reason you have everything on, over fuse it by just a smidge. Remember, fuses are supposed to protect the wire. I used o gauge from battery to starter then from starter to fuse box I used 8 gauge. Hope that is right!
     
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  8. Jokester
    Joined: Jan 29, 2005
    Posts: 547

    Jokester
    Member

    ...and finally..the question:
    Assuming I pick directly off the battery and use a maxi fuse, what size fuse? I actually found a schematic I put together back in the day, thinking some day it would come in handy. It’ll answer your questions about what circuits are in the car. View attachment 5133496 [/QUOTE]

    You might consider using a circuit breaker instead of a fuse. Fuses are like bullets. You can only use them once. Also, always carry a spare.

    .bjb
     
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  9. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,185

    Mr48chev
    Member

    Looking up my 71 GMC 3/4 ton the recommended fusible link is a 14 gauge.
    If I was going to run a main feed circuit breaker, fusible link or fuse I want it close to the battery as you are actually using it to blow in case the main feed shorts out or the fuse block shorts out.
     
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  10. nosford
    Joined: Feb 7, 2011
    Posts: 589

    nosford
    Member

    Many new cars use Maxi Fuses for the primary circuit protection and they are incorporated into the Power Distribution Center but you can find an in line fuse holder that will work. These fuses look like a giant normal plastic fuse and go up to 80 amps or possibly more. I used them when I wired my F100, I used two fuse blocks, one hot all the time and one hot only with the key on. The power wire going to each fuse block has a 60 amp Maxi fuse so two main power fuses. Now I will open a big OT can of worms, there is something that can be used for smaller loads that the OEM manufactures have used for years called PTC circuit protection devices. These are solid state circuit breakers that will open the circuit if there is an overcurrent event (a short) and will automatically reset as soon as the overcurrent goes away. They are rated for 100,000 cycles (or more) and are designed to protect each individual circuit, they wire in series just like a fuse holder would. I will also attach a link to a chart that shows a bunch of these and some specs for them from a wholesale distributor, they are somewhat hard to buy unless you are a business. An example of how they would be wired would be a light circuit for park and tail lights. There would be a main fuse for the power from the light switch (say 20 amps) and one of these for each individual light (5 amps each) two in the front and two in the rear. If the right rear tail light shorted the PTC for that light would open but the other three lights would continue to work as normal. If there was a short in the main power feed from the switch then the fuse would blow and all the lights would go out. Last time I bought some of these it was from a company called Mouser Electronics and they were around 0.75 each but that was several years ago. The bigger they are the more current they will carry and the more they cost, common sizes go from milliamps to 15 amps or so. Something to think about, Mark https://www.digikey.com/en/products...7KAbRABYAOANhAF0BfAmcqUZSdbfI08AVgDsYAJz0GEoA
    Link to Maxi Fuse holder, 60A, 6ga wire
    https://myautovaluestore.com/bussmann/maxi-fuse-holder-bus-hhx
     
  11. Guys get fixated on fuses and forget why they're really there. Not everything needs a fuse. They're generally there to protect the wire from device failure. Your harness is supposed to be installed in such a way that minimizes or eliminates the possibility of physical damage to the wire, the exceptions being devices/wiring that may be subject to physical damage in an accident like horns and lighting. Feeder wiring doesn't have specific devices connected, should be installed in the 'center' of the vehicle where physical damage would only occur in a catastrophic accident (where you'll have far more other critical issues), and the specific devices connect to the fuse panel and are protected there.

    Every time you break a wire, whether it's for a fuse or junction point, you're introducing another failure point.
     
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  12. The problem with those is they'll clear a short or device failure but you may not know it. You could end up with no rear lighting and never know it unless you check that every time you drive.
     
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  13. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,258

    jimmy six
    ALLIANCE MEMBER

    Agele55. Don’t get too worked up about your past at all. My stock 56 feeds a #10 to the ignition jumped to the light switch from the regulator with no fuses. They came after that. If your upgrading we all make improvements.
     
    Last edited: Aug 3, 2021
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  14. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Lol... yep, my 55 was pretty much the same. I guess I figured I should improve a bit on the 66 year old technology.
     
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  15. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    I was thinking along the same lines ; Upgrade the starter to fuse box and the alternator to battery to #8.
     
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  16. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,258

    jimmy six
    ALLIANCE MEMBER

    Funny but after I installed AC turned on everything on high, fan, radio, lights, and stepped on the brakes for the lights. It only drew 29 amps and awg #10 can handle it. So far I’ve never driven at night with the AC on and funny it freezes us out on medium so we run it on low. I also installed a 100amp alternator because it the typical size today from Powermaster.
     
  17. 64Belvedere
    Joined: Sep 20, 2007
    Posts: 49

    64Belvedere
    Member
    from Alabama

    I used a marine fuse when I installed a Powermaster alternator on my '64. I used a Blue Sea fuse block that I mounted directly to the alternator power terminal, and then connected the wiring to the fuse. They sell different capacity fuses for the mounting block just like the Maxi Fuse. For me, it just seemed like a clean install option. I've had no issues with it, but I'd be the first to admit that my installation may not be technically correct...
     

    Attached Files:

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  18. 1971BB427
    Joined: Mar 6, 2010
    Posts: 7,090

    1971BB427
    Member
    from Oregon

    The wire ampacities for various AWG sizes are rated based on the wires being inside a raceway. So a #10 wire inside a raceway is rated 30 amps, but it's never in a raceway in a car, so it can indeed carry higher current in free air, as all conductors can. The free air rating of a #10 awg wire is 40 amps.
     
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  19. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,594

    Elcohaulic
    Member

    At the battery mount two small fuse blocks good for 60 amps.

    Run one straight from the battery, radio, running and * head lights..

    Run another right next to it, this one will be switch from the ignition switches "on" position, this will power, ignition *, gauges, wipers, heater blower and what ever else you would like..

    * for the ignition and headlights use a resetting breaker..
     
  20. Blues4U
    Joined: Oct 1, 2015
    Posts: 6,090

    Blues4U
    Member
    from So Cal

    Man, I wish the AC in my 47 worked that good. On a hot day it barely keeps up on full blast.
     
  21. Blues4U
    Joined: Oct 1, 2015
    Posts: 6,090

    Blues4U
    Member
    from So Cal

    After all these years I don't know why you'd change it. Adding a Maxi fuse is adding a potential failure point. Depending on where the battery is located, using the starter as the source can provide a shorter path to the fuse block/ignition switch vs the battery. As long as the wire is protected from damage I'd leave well enough alone. But I'm a minimalist and like to keep things simple.
     
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  22. That 'free air' rating is predicated on it being suspended singly in air or laying unbundled in an open cable tray along with a ambient temperature of about 80 degrees. Bundle it with any other wires, tuck it into a confined space with little or no air circulation to keep it out of the way, and the most likely scenario, expose it to underhood heat that can greatly exceed 80 degrees and you're derating back down.

    The big issue for us with running wire at it's maximum rating is voltage drop. The more current you apply to any given wire, the greater the drop. I'll note here that voltage drop is independent of the system voltage; you'll have the same drop whether the system is 12 volts or 120. Losing a .5 volt in a 120V system won't be noticed at .4%, but a 4% drop at 12V can be. They're also additive. Start at 12V and lose 4% between point A and point B you're now down to 11.52V. B to C and now it's 11.06V. C to D it's 10.62V... and you have dim lights. Increased ambient heat will also increase voltage drop.

    If you're an OEM and building a million harnesses a year, running the bare minimum size wire will save you money. But I'd rather be conservative and spend a few more $$ for larger wire in places it's needed and not have these problems.
     
  23. squirrel
    Joined: Sep 23, 2004
    Posts: 50,228

    squirrel
    Member

    That's all true.

    But you'll still be OK running a 40 amp fuse/breaker with 10 gauge wiring on a car. Even if it's not optimum.

    Making sure the wires are all safely routed, etc. is still important.
     
  24. You're right, but I don't like it because it reduces your margins for error. You and I have a thorough understanding of all this and know where and how much we can safely 'cheat'. It's the guys who don't have this knowledge that get into trouble...
     
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  25. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    So a super easy swap for the 55 would be to change out the #10 alternator to battery wire for a #8. It shows evidence of being hot at some point. It has a plastic dress up sleeve on it and near the battery it’s tied in with a spade connector, which kind of, sort of, melted the dress sleeve.
    I can also easily swap the starter lug to fuse box for a #8 also....just cuz I can.
    Or am I just pissing in the wind and should say the heck with it?
     
  26. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,351

    Boneyard51
    Member

    What system are you running in your car? I thinking of adding A/C to my wagon and need the best A/C, due to it’s size and Oklahoma heat! Thanks in advance ,










    Bones
     
  27. squirrel
    Joined: Sep 23, 2004
    Posts: 50,228

    squirrel
    Member

    Yup.

    Although when we play with robots, which wiring resembles that on modified cars more than anything else, the rules allow us to use 14 gauge wire protected by a 40 amp self resetting breaker....and the wires never smoke.

    I still try to stick to the house wiring standards for fuse/wire sizes on cars, though. Even though most old cars were originally built with smaller wire. Then again, I also try to avoid using high current stuff like electric fans, etc. that are pretty popular nowadays.
     
  28. You don't say what size alternator you're running. If you have one of those 100+ amp units, I'd upgrade to at least a #4. 95% of the time you won't need the large size, but it's that 5% that will melt stuff... LOL. The upgrade from the starter to the fuse box would be good too, a #8 there should be fine unless the car is loaded with electrical accessories. Bigger wire is always good, the usual issue is connecting it to some devices.
     
  29. I assume that the limiting factor that allows that is battery run time. I know from some wiring methods used on larger motors that a large overcurrent of short duration is more acceptable than a smaller one continuously. When you get into industrial motors they'll have multiple protection devices; quick-acting fuses or a breaker for large overcurrents, overload protection that will tolerate high currents for several minutes or so but will trip at much lower levels, generally just a bit more than the rated full-load current if it goes on too long, and sometimes phase-loss protection on three phase motors. Most (if not all) solid-state motor starters now contain these features.

    Electrical fires rarely start from a catastrophic event. It's usually a long-term overload or poor connection that isn't quite enough to trigger the overcurrent protection but builds enough heat over time that things finally combust, sometimes helped along by a too-large overcurrent device. The fuses/circuit breakers we use on our cars aren't 'smart' enough to tell the differences. Fusible links should be an answer, but the lack of information available for sizing make them a crapshoot IMO so I avoid them. Not easy to find or replace either, another black mark.
     
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  30. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    I’m running a GM CS-130 alternator rated at 100 amps, but I’m thinking I’ll never draw anywhere near that. No power seats, fans, power locks, or stereo that can be heard in the next town over... just the basics.
     

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