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Wire capacity

2old2fast

Active Member
Here's a ? For you electrical gurus , the charts I've seen show 30 amp DC max for 10 GA. Wire . I've never ever seen anything larger than 10 GA. Alternator to battery , yet many of those alternator are rated at 100 amps or better . What gives ??
 
It's not just the wire size, but also the length of the wire. A 10 gauge wire can carry 200 amps
at 10 feet and 36 amps at 25 feet. There is a specific resistance per foot of wire.

You may have never seen larger than a 10 gauge, but that does not mean it's not used. I have a
4 gauge wire from the alternator to the battery. My alternator is rated at 150 amps. Look at some
cars with a high powered audio system, if done right then they will have a larger than 10 gauge
wire.

The less voltage drop across the wire, the more that gets to the battery and charges it.

WireGaugeToAmpTable01.jpg
 
It's not just the wire size, but also the length of the wire. A 10 gauge wire can carry 200 amps
at 10 feet and 36 amps at 25 feet. There is a specific resistance per foot of wire.

You may have never seen larger than a 10 gauge, but that does not mean it's not used. I have a
T4 gauge wire from the alternator to the battery. My alternator is rated at 150 amps. Look at some
cars with a high powered audio system, if done right then they will have a larger than 10 gauge
wire.

The less voltage drop across the wire, the more that gets to the battery and charges it.

View attachment 15709
OK This chart explains a lot , the ones I saw started at 15 feet.
 
Another thing that can cause confusion about charging and wire gage, is that even though it's maybe a 140 A alternator, it can't put anywhere near that into the battery . . .

It has to do with the voltage difference between the alt output and the battery, and all the series resistances, including the battery.

Alt output is regulated to about 15 1/2 volts, if the battery is at 12.6 or so, (which is normal for a serviceable battery) there isn't much difference, so you won't see a lot of current flow.

If the battery is at lower voltage, down to about 10V, it will draw more current because of the greater difference.

Battery internal resistance also changes with state of charge.

You wouldn't want to be charging a battery on a regular basis with more current than you can easily carry in a moderate length of 10G wire.
 

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