Getting ready to completely rewire my '54 Ford that I'm installing a 302 in. I'm planning on using a new 3G alternator with a lead for the dash warning light. The attached diagram from a Hot Rod article shows to install a 100+ amp Mega fuse in the hot lead from the alternator to the battery. The guy at the electrical shop who I've done business before and has always been spot on looked at me like I was crazy and View attachment 4422998 said not to install the fuse. Either I didn't explain myself correctly or the Hot Rod article is wrong, or the guy selling me the alternator is wrong. I tend to go with the alternator guy. Like he said, a 100 amp fuse on an alternator the puts out 135 amps makes no sense. Anyone have any thoughts? The alternator is a 130 amp single wire 3G with the correct plug in for a dash warning light and I'm using a Rebel 9+3 wiring kit. View attachment 4422998
I would use the Megafuse. I did on my Thunderbird. I did an install on my friends car and used the wire from the alternaator to the solenoid switch from a 99 F150. It had two fourteen gauge fusible links and worked well.
Your alternator guy is correct.... This is one of the most misunderstood items when doing aftermarket wiring. Overcurrent protection is there to protect the wire, not the device. There's two possible scenarios where you may need protection; one, against catastrophic short circuit, and two, overload. The first one, short circuit, will occur only when the wire shorts to ground. This can be addressed by routing the wire so it's not exposed to physical damage. In the days before monster-sized alternators, this is how it was done by the OEMs; no fuses needed. If you really feel the need for protection, then the fuse should be sized to 125% of the maximum alternator output, or 1.25 x 130 = 162.5 amps. Rounded up to 175, the nearest size. Fuses blow at more-or-less 100% of their rating, you need the extra 25% to avoid nuisance tripping. Overload is much trickier, and this is where most 'fusing suggestions' fall apart. For correct overload protection, the fuse rating CANNOT be more than the actual wire ampacity rating (INCLUDING WIRE CONNECTORS), which for the sizes in your diagram are 65 amps for #6 and 75 for #4. A 175 amp fuse will be 270% of the #6 rating, and 230% of the #4. In effect, you have no overload protection. The OEMs get semi-around this by using fusible links instead of fuses. A link will withstand short-term overloads inversely to the amount, i.e. a small overload will have to last a long time before the link will open, and larger overloads will open quicker. The problem here is there's no standardized ratings/data on either the maximum amps or the time slope of how much/how long before they open for fusible links. The OEMs use these for the simple reason of saving money. By reducing the wire size and using links for protection, they save the cost of the larger wire; no small amount when building millions of harnesses a year. They have one big advantage over us; they can test/select links, having full data on both the links and how their harness operates. Keep in mind that these large alternators exist because modern cars have much heavier electrical loads compared to older cars. 'Back in the day', a 30 amp generator was common, and a 70 amp alternator was a 'heavy duty' model. Very few 'HAMB friendly' cars are going to have electrical systems that require a big output for 'normal' use. About the only time you'll see anything approaching full output from one of these large alternators is immediately after starting, and even then only for a short time. If you're concerned about your load, I'd suggest doing a total load calculation then sizing the alternator/fuse panel wiring accordingly. Most guys install these fuses/links because some 'expert' who is likely is no more knowledgeable than they are tells them to (while making a buck selling them the parts). I'll also point out that alternators don't 'short out'; they fail to 'open', either reducing or totally losing output. If your branch circuits are fused correctly, protecting your alternator/fuse panel wiring against mechanical damage/shorts to ground will be more than adequate.
While I totally agree with Crazy Steve, I need to state that you cannot provide TOO much protection . His figures are spot on for the fuselage link and or circuit breaker! Even thought you lay out the most elaborate circuit system, you need to figure in” shit happens”! Anyone how has worked on a Lucas’s system knows what I mean! During my career, I over built and over protected every system I wired! With success, I,may add. Just saying...... Bones
I'll have to disagree to a point. Every time you break the wire and have another connection, you've introduced another failure point. In all the years I worked in the trade, I'll estimate that probably 80% of all wiring failures (and remember, we're trying to protect the wiring here) I saw were due to failed connections, with most of the rest either mechanical damage or incorrect overcurrent/overload protection. Every time you add a fuse, that's four more possible failure points; two for wire connections to the fuse holder, and two for the fuse contacts. The alternator-to-battery and battery-to-fuse-panel wiring are the two places on a typical vehicle harness that is all-but-impossible to protect because the load can vary so widely. Driving down the road on a nice day, with no other electrical items running? Total load in either wiring will probably be less than 10 amps. Heavy traffic on a hot day, got the AC on and your electric fans kick in? You might see 50-60 amps. Car has sat for a time, the battery is down and it's hard to start? You might see full output, 100 amps or more on the alternator wire until the battery starts to get charged up. The OEMs and virtually all of the aftermarket harness suppliers use wire sizes for these that will cover the 'average' load, although IMO they both skimp on them at best. Install a fuse big enough to cover the maximum load without nuisance tripping, and you're hopelessly oversized for overload protection given the wire size. Use fusible links, and you'll be guessing as to sizing. Too small, you'll be replacing them (and they're not convenient to replace). Too big, and you'll let out the smoke.
Thanks all for the responses. I went ahead and calculated my amp draw with a guide provided by my alternator guy and even with an electric fan (which I'm not planning on using) the max amp draw calculates out to needing a 105 amp alternator. That said I'll use a 105 amp alternator and use a #6 wire from the alternator to the battery. If down the road I do add an electric fan and/or any other heavy use accessories I'll up grade to a 135 amp alternator.
Steve , I agree on keeping the electrical system as simple as possible and eliminating as many contact/ connections as possible. I just learned from my experiences of mixing water and electricity and vibrations the protection of fuses/ circuit breakers outweighed the problems of poor connections. But, I alway made sure of clean tight and corrosion protected connections. The OEMs fused this circuit....sometimes......and sometimes they didn’t! Lol Bones
The OEMs started using fusible links for overload protection somewhere in the '70s. I've never seen a fuse installed on either the alternator output or fuse panel feed from the factory, for the reasons given above.
Go to the boneyard and pull the mega fuse and the heavy lead from a ford tarus it will give you plenty of wire and fords choice of fuse.
Buss makes them up to 350 amp. That what I used when I was outfitting new trucks with high draw electrical options. Bones
I ran a 3G/4ga/175a combo in my OT ride for years with nary an issue. I'm also running the same combo in my avatar roadster. Your diagram is exactly how I ran my setup. Made my own 4ga wire and pirated the fuse and fuse holder from Pick-N-Pull. You can find them on the backside of the transverse motors in '90s Ford Contours. The 'yards are filled with 'em!
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