Need an Adult. Alternator Wiring Question. Non-OEM System

-

Just Send It

There goes the neighborhood!
FABO Gold Member
Joined
Sep 9, 2020
Messages
639
Reaction score
1,855
Location
Oregon
Hey Guys,

I'm an idiot. I need someone to hold my hand with a very easy wiring question regarding fusing an alternator to a firewall bulkhead. It's literally the last thing I need to do on my car before it hits the road.

Key Details:

- 72' Scamp. The entire wiring harness was replaced with a modern EZ kit. Nothing is original.

- I have a 1 wire Powermaster alternator that generates a max 100amps.

- Battery is mounted in the trunk. 2AWG wire runs though the cab to a bulkhead at the firewall. In the engine compartment, this bulkhead feeds the starter relay and the starter. Bulkhead is circled in red in the photo below.

-Behind the firewall is a relay board that powers my accessories. Everything on this board has its own relay and is individually fused. This relay board is powered by the cab-side of the bulkhead bolt.

20211218-134959.jpg


- I have an inline MAXI fuse from the starter relay to the EZ fuseblock. This is circled in blue in the photo below

20220808-132211.jpg


- I will be using a 4-post master disconnect switch in the rear. Not sure if this is relevant, but wanted to mention it.

20220808-132336.jpg


So here's my question...I need to run a 2.5ft charging wire from the alternator to the bulkhead bolt that goes to the battery. Does this 2.5ft wire need to be fused? If yes, should the fuse placement be somewhere in the engine bay or in the trunk near the battery and switch?

Thanks in advance
Jim

20220808-132211.png
 
Last edited:
After almost burning my car to the ground because of a shorted alternator wire, I would definitely recommend using a fuse. I would say the closer to the battery the better. Reason being because when my alternator wire shorted out, it burned the whole wire from battery to the short, burning the rest of the harness it was routed next to with it.
28BBFCF6-4FF6-40DF-9CEB-65E30E4F9AFA.jpeg
 
Understood. Since I've got a 2AWG wire running from the rear battery to the firewall bulkhead, would a 125amp breaker be sufficient?

The distance is only 2.5ft from the alternator to the firewall. From the firewall rearward, it's the big main wire.

Just not sure what could handle that load with that large of a wire. Especially during starting.
 
Understood. Since I've got a 2AWG wire running from the rear battery to the firewall bulkhead, would a 125amp breaker be sufficient?

The distance is only 2.5ft from the alternator to the firewall. From the firewall rearward, it's the big main wire.

Just not sure what could handle that load with that large of a wire. Especially during starting.
The starter would definitely pop that fuse, and a fuse big enough for the starter could still start a fire if there was a short. I meant to say as close to the bulkhead bolt as possible. So it’s only the alternator wire that’s getting the fuse, not the battery cable. Sorry.
 
The starter would definitely pop that fuse, and a fuse big enough for the starter could still start a fire if there was a short. I meant to say as close to the bulkhead bolt as possible. So it’s only the alternator wire that’s getting the fuse, not the battery cable. Sorry.

Perfect. Thank you!
 
You can fish the big positive cable through some hose as much as possible to give you extra protection against shorts to the frame.
 
Just a point to consider.

A fuse, any fuse, an system, for any purpose is designed to protect the wiring from melting. That is all it is there to do.

The size of the fuse should be based on the wire you are trying to protect,

Also wire has a temperature rating, so you need to buy wire that will work at the expected temps. In other words a wire that can handle 100 amps at 75 deg ambient air temp might only be able to handle 75 amps at 100 deg. (Numbers for example only I pulled them out of my butt.)


Also there are slow blow fuses and circuit breakers, that can handle more amps then rated for a short period of time like while cranking.
 
-
Back
Top