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Concerns About My Wire Harness' Fuses

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I'm in the process of installing a pre-made aftermarket wire harness in my 1977 Firebird. The harness is made by American Auto Wire and includes just about everything to switch the old 70's electrical system to a more modern setup with more individual circuits, relays and fuses. It's a very nice product with good quality wires that are slightly larger than the awg they claim to be and excellent crimps throughout. The only thing I don't like is that it's not wrapped in any way along any of its length, and since it's already mostly assembled, the only way I can wrap it is to wind tape around it (which isn't even provided). But anyways, that's not my issue here.

My issue is with the power supply kit that they provide. This kit runs a 6 awg cable from the starter positive lug, which then goes into a fuse block with 2 large fuses. From there, one fuse connects to a 6 awg wire that goes to the alternator feed lug, and the other fuse feeds a 10 awg wire that powers the main fuse box in the car. I've attached a diagram from their installation instructions.

Here's the thing. Both of those fuses are 175 amps. That means that there's a 175 amp fuse protecting a 6 awg wire (which is ok), and another 175 amp fuse protecting a 10 awg wire (!!!). That doesn't seem adequate to me.

I brought up this concern to AAW tech support, and their reply is that due to the fuse block living in the engine bay, the industry standard is to de-rate it with an assumption of 125c air temperature. So the 175 amp fuse is really rated to 110 amps. And they claim the 10 awg wire they're using (which is admittedly slightly larger than 10 awg) is rated for 103 amps continuous current.

But doesn't the wire get de-rated, too? I checked, and their assertion that their wire could handle 103 amps continuously is correct... at 25c ambient temperature. If the fuse gets de-rated as if it was in a 125c environment, then shouldn't the 10 awg wire also get de-rated the same way? In that case, its continuous current carrying capacity drops to less than 60 amps. Seems to me like there's no way that the wire is resting at 25c if it's attached to a fuse that's burning up at 125c.

To be clear, this 10 awg wire could be really long or really short, depending on where I plan to install the fuse block. But the kit comes pre-assembled with 12 feet of it. I'm planning to use 7 feet of it. In my mind, that's a potential 7 foot-long line of fire if that wire ever grounds itself on a control arm or the exhaust manifold or one of the many other obstacles it'll be encountering on its way to the firewall.

Am I wrong in being concerned? I'm seriously considering trying an experiment in my driveway to see which melts first when I short out the circuit on a battery, the wire or the fuse.

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It sounds like your power feed wire is going to act like a fusible link for your fuses.

It does certainly seem to be a mis-match.

Plus some higher output alternators can put out a lot of current if the demand it there - such as a discharged battery (charged batteries take negligible current as the vehicle should be running off the alt') and/or high vehicle demand. The way the wiring is I could see the alt' fuse blowing without too much trouble as it's going to be suppling both.

I'm installing a 150 amp alternator. I think if the "de-rating" thing was an issue (ie. if I was installing the fuse block in a high heat area), then you're right that I'd be likely to get some nuisance blows, especially immediately after the engine is started. But I'm planning to install the fuse block at the front of the engine bay, where the battery used to be (which I'm moving to the trunk). It'll get plenty of airflow there and should maintain its full rating. It's a slow-blow fuse, so it can tolerate full amperage for extended periods.

Which, of course, just makes the 10 awg wire even more prone. It'll be getting run past the exhaust manifold to the firewall, where things get real hot, and therefore any potential de-rating will affect it much more than it would the fuse block.

So I'm going to eat my words here.

I tried the experiment. Ran a 2 awg cable from the battery positive to one end of the fuse, then ran a piece of the provided 10 awg wire from the other end of the fuse to the negative battery terminal (with a 500 amp battery switch in the middle). I even recorded the whole thing on video with a long pre-amble, fully expecting the experiment to result in a flaming wire and expecting Internet notoriety when I published the video on YouTube. But when I turned on the battery switch, the 175 amp fuse popped in a fraction of a second and the 10 awg wire barely got warm. I'm very surprised and very impressed. This kit is everything AAW promised it would be: better, safer and more versatile. No notoriety for me today, I guess.

Now to go and wrap it...

Before you get too confident, the bigger problem is going to be heat build up and failure through that mode.

Have a quick review of the maximum expected current the wiring may experience, and use that as a guide for load testing it.

On that, you still have at least two options - upgrade the wiring which, as it isn't enclosed shouldn't be too difficult, and/or use lower amperage fuse(s).

I wasn't worried about that. A good 10 awg wire can safely provide all the amps most cars will ever need without getting strained. This is only powering the in-car fuse box, it's not being used to charge the battery or turn the starter. My only concern was with the fuse protection in case of accidental short-circuiting, and that's proven adequate in my test.