Wire Gauge Question

Hello all!

I’m getting ready to rewire my new alternator and I found this diagram. I’m just curious if someone can confirm the wire sizes of these wires. Any help appreciated

 

On my Jeep I used a 10SI GM 1 wire alternator, I ran an 8 gage to the battery, and the alternator Ground, the rest you can us a 14 gage wire, like the rest of the wiring harness has. I used an 8 gage because I have a high output 10 SI, 110 amp, and wanted to make sure I had a large enough wire for it. Good Luck

 

A good procedure for things like this is to refer to how the factory did it.

Jeep uses 10 ga for the 10SI battery connection, with factory sized alternators. They also send the sense wire (plug connection 2) right back to the BATT connection. There is no need to make an additional long run of that wire to the battery; connecting at the BATT terminal is electrically identical. Jeep uses 14 ga for the short connection of "2" to "BATT." You do not need an actual ground wire - grounding through the brackets is fine.

For the ignition switch connection (plug connection 1), you can use whatever you have of small-gauge primary wire (14, 16, 18). Jeep uses a 24 ga solid nichrome resistance wire in place of the bulb and wire in your diagram. You need either a diode, a bulb or a resistance in that wire to block back-feeding from the alternator to the ignition switch when you attempt (!) to shut off the engine.

 

Thank you for this as I also had the question on a Alt light I want to install but was also confused reading about a diode need. So the dummy light (non led) would be enough?

 

NP! Yes, the incandescent bulb has enough resistance to prevent back-feeding. The alternator works by using part of its own current generation to create the magnetic field, and generate more current. Terminal 1 bootstraps the alternator to make current at startup. It only needs a trickle of current to bootstrap, but it's enough to light the bulb. Once the alternator starts generating, terminal one is held at full voltage by the alternator, no current flows and the light goes out.

The incandescent bulb could burn out, and then the alternator won't bootstrap. Supposedly these alternators will bootstrap themselves from the residual magnetism in the field iron, but the engine must spin fast for this to happen.

 

great stuff. Just so I follow completely ...Bootstrap? haha! No really define please and thank you.

 

Huh. Comes from "lifting yourself by your bootstraps" I believe. Self-initialize, sp. using existing processes or resources to generate new processes or resources. Implies positive feedback.

That's a character from Pirates of the Carribbean?

 

Got it...and yes. Bootstrap Bill

 

Can this also be the voltmeter? Or does that wire in differently?

 

Voltmeter is different from an ammeter. Connections 1 and 2 are the same, and function the same. However, instead of sending the charging current to the ammeter in the dash and back to the battery, charging current goes directly to the battery. You can connect the voltmeter to any switched power source in the dash. Look at the diagram for the '76 CJ with a 258. That has a voltmeter and a Delco 10SI alternator.

 

For the alternator to battery wire, use an appropriate gauge for the size of the alternator and the length of the run. I'm using a 145 amp alternator (standard for new Corvettes), and the run is about 6 feet long by the time I go around the engine. I'm using a 2 gauge wire as I didn't want it to be undersized. Smaller alternator and shorter run would use less. There is some discrepancy among the available charts.

Here's a marine industry chart.

 

JMO - there's nothing wrong with using huge wire to connect from the alternator to the battery. Whether its excessive or not, I kinda suspect it is. And I sorta don't trust tables like the above, since it says nothing about how the wire is routed or enclosed, and how the authors came to those conclusions. I like to use Excel for questions like this:



You can look online and find the resistance of various sizes of wire. Here it's shown in thousandths of an Ohm per meter. The previous poster's length of wire is about 6 ft, so I used 2 meters. Power in watts is current squared times the resistance. Finding the voltage drop is easy as long as you know the power and current: power/current = voltage drop along the wire.

When the alternatior is charging at its full current, that 6' length of #2 wire is radiating about 17 watts. The drop in voltage between the alternator and the battery is about 1/10 of a volt. 17 watts may seem like a lot when you compare to a 15 watt bulb like you'd use in your oven or refrigerator. That bulb's heat is concentrated in one spot. Consider that a flashlight bulb is about 1 watt, and you could have 17 flashlights arranged along a 6' length... not much danger of heating. Personally I would be fine with 4 ga in this application.

Jeep uses 10 ga in the '70s and '80s Jeeps with the Delco alternator. Say we drop the charging current to 60 amps and recalculate:



You can see how the I^2 dependency affects the dissipation. Roughly half the current throws off about one fourth the power. And in the Jeeps I've seen, the alternator is on the same side of the engine bay as the battery, and the wire is say 3' long. That's another 50% reduction in power and voltage drop, though the temperature rise of the wire does not change.

[Another reason to get rid of that ammeter in the dash and its stupid-long charge wire!]