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Old 05-06-2015, 10:08 PM   #1
clemsonteg
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Alternator wire size

I've been reading a lot the last few days about upgrading the stock alternator in my 70 c10 to a 12si. The only question I have is about upgrading the wire size of the wire which goes to the battery terminal on the alternator. Should this be upgraded? I see that Mad Electrical uses an 8 gauge wire in their kit. Should the wire size only be increased from the alternator to the junction within the harness or should it be increased from the alternator to the junction and from the junction across the core support to the battery?
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Old 05-06-2015, 10:21 PM   #2
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Re: Alternator wire size

I upgraded to a CS144 in my truck and at the moment I am still running all stock wire except from alternator to the battery directly which is a 8g wire.

Some people say you should use 4g but I find its too big and cumbersome.
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Old 05-08-2015, 11:11 AM   #3
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Re: Alternator wire size

The highest rated stock 12si is 94 amp. Depending on distance of wire 8 gauge would be sufficient and I would go all the way to the battery. Think of your wiring as a water hose, if you start off with a 3/4 inch hose and couple it to a 1/2 inch hose you can only flow as much as the smaller diameter will carry, so the larger one does no good once it is restricted.
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Old 05-08-2015, 11:12 AM   #4
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Re: Alternator wire size

8g is fine unless you have a huge electrical system demand with winches and offroad lights etc.
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Old 05-08-2015, 11:25 AM   #5
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Re: Alternator wire size

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8g is fine unless you have a huge electrical system demand with winches and offroad lights etc.
IF you are running winches they would be tied direct to battery. If you are running a lot of accesories you would then upgrade the system further this was saying stock truck with possible 94 amp alternator.
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Old 05-08-2015, 12:48 PM   #6
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Re: Alternator wire size

I always recommend going to an 8 gauge wire just in case the amp draw on the alternator gets high. The engineers designed the wire sizes to cover most of the amp draws in the 67/72 trucks based on factory loads. When we started adding cooling fans and big stereo amps, air bag compressors etc. the draw goes way up and the alternator outputs and the wire sizes need to be increased as well to handle the increased loads.

If you google the wire size charts you'll find that a 10 gauge wire is pretty adequate and 8 gauge is almost always plenty.

the water hose analogy is a good one but remember the 3/4 hose feeds more than one smaller hose(wire) so the resistance will decrease with the larger hose.

As far as the 12 gauge wire across the top of the radiator, It is designed with length and resistance in mind and it only has two purposes. To act as a shunt for the battery gauge and to feed battery power to the truck whenever the alternator isn't functioning. 12 gauge will charge the battery just fine and it will carry ignition power and small loads if the alternator fails. If you don't care about the battery gauge accuracy then it can be upgraded. I am using a voltmeter in my truck so I went to a 10 gauge wire.

Here is a little fact that most guys don't consider. On the factory trucks all the battery and alternator power goes to a four way junction and all the loads to the cab are carried by one 12 gauge red wire to the firewall block. If you put a huge wire off the alternator or battery you still have one little 12 gauge wire carrying all that amperage.

the best thing to do is to upgrade that wire and use a different junction to feed the high draw loads with separate wires from that junction.

Here's an example.

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Old 05-08-2015, 03:09 PM   #7
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Re: Alternator wire size

Easiest way is to just use the factory junction from a 73-up truck for use on 67-72 trucks and upgrade the charging system accordingly.
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Old 05-08-2015, 10:26 PM   #8
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Re: Alternator wire size

Thanks for the replies. The other reason I was considering upgrading the wire from the 4 way junction to the battery is that I have the my compressor for my airbags and my vintage air setup wired to a junction near the battery. There is a decent load on that side of the truck, so I thought it may be worthwhile to upgrade the wire running across the core support.

If I understand what you are saying Vet, what I should do is run from the 4 way junction on the drivers side to a new junction on the passenger side for my air compressor and AC.
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Old 05-09-2015, 01:43 AM   #9
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Re: Alternator wire size

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Originally Posted by clemsonteg View Post
Thanks for the replies. The other reason I was considering upgrading the wire from the 4 way junction to the battery is that I have the my compressor for my airbags and my vintage air setup wired to a junction near the battery. There is a decent load on that side of the truck, so I thought it may be worthwhile to upgrade the wire running across the core support.

If I understand what you are saying Vet, what I should do is run from the 4 way junction on the drivers side to a new junction on the passenger side for my air compressor and AC.
You can do that or if you have the extra terminals on the drivers side junction for just two more loads you could use them to feed the AC and the compressor. Just remember that you want the main power source for them to be the alternator. You won't need to feed them with the wire across the radiator. You'll want to control them with high amp relays controlled by smaller wires in the cab. Make sure you use fusible links in the feed wires though. I can post up a diagram if you need it.VV
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Old 05-09-2015, 08:52 AM   #10
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Re: Alternator wire size

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You can do that or if you have the extra terminals on the drivers side junction for just two more loads you could use them to feed the AC and the compressor. Just remember that you want the main power source for them to be the alternator. You won't need to feed them with the wire across the radiator. You'll want to control them with high amp relays controlled by smaller wires in the cab. Make sure you use fusible links in the feed wires though. I can post up a diagram if you need it.VV
Thanks for the additional advice. I definitely have relays already that use ignition sources from inside the cab as the trigger and the main power is fed from the alternator through the relay.

The only reason I suggested to run a larger wire across the radiator to a new junction on the passenger side is because the compressor and AC main power lines already terminate near the battery.

I have a fuse built into the air compressor power line which is 20% higher than the expected amp draw of the compressor and the AC has a built in circuit breaker. Should I still use fusible links in those cases? I do have a fusible link between the battery and the current junction on the passenger side.
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Old 05-09-2015, 02:48 PM   #11
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Re: Alternator wire size

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Originally Posted by clemsonteg View Post
Thanks for the additional advice. I definitely have relays already that use ignition sources from inside the cab as the trigger and the main power is fed from the alternator through the relay.

The only reason I suggested to run a larger wire across the radiator to a new junction on the passenger side is because the compressor and AC main power lines already terminate near the battery.

I have a fuse built into the air compressor power line which is 20% higher than the expected amp draw of the compressor and the AC has a built in circuit breaker. Should I still use fusible links in those cases? I do have a fusible link between the battery and the current junction on the passenger side.
Sounds like you've got it covered.
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Old 05-09-2015, 05:15 PM   #12
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Re: Alternator wire size

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Sounds like you've got it covered.
I hope so. If I use an 8 gauge wire on the battery terminal of the alternator, should I also install a 12 gauge fusible link between the alternator and the 4 way junction? Sort of like the diagram you posted previously?
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Old 05-09-2015, 05:31 PM   #13
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Re: Alternator wire size

You don't need a "master control" fusible link as it were....you just need to use fusible links from the remote junctions IF you are using a spade type mini or maxi style fuse inline anywhere. I'd run a large 8ga or bigger lead from the alt charge post to any junction-then from there fuse all of your power leads whether its a fusible link or an inline fuse....then route to individual relays.
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Old 05-09-2015, 06:00 PM   #14
clemsonteg
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Re: Alternator wire size

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Originally Posted by gmachinz View Post
You don't need a "master control" fusible link as it were....you just need to use fusible links from the remote junctions IF you are using a spade type mini or maxi style fuse inline anywhere. I'd run a large 8ga or bigger lead from the alt charge post to any junction-then from there fuse all of your power leads whether its a fusible link or an inline fuse....then route to individual relays.
Just curious, why would a fusible link be needed if using a spade type fuse?
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Old 05-09-2015, 09:53 PM   #15
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Re: Alternator wire size

Sorrow....I meant to type IF you ARE NOT using spade type fuses, then use a fusible link-so its either one or the other but not both.
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Old 01-01-2019, 07:27 PM   #16
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Re: Alternator wire size

gmachinz,
Hello and happy new year. I'm attempting to research electrical upgrades for my 72 Jimmy. I have ordered a large box of goodies from MAD enterprises.
I can rewire an entire house, but the vehicle stuff has me moderately lost. Do you have any experience with his kits? I THINK my biggest question is my Jimmy doesn't appear to have the "bus" bar. Someone put some nice LOUD horns on it, but everything is spiced in?
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Old 01-01-2019, 08:09 PM   #17
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Re: Alternator wire size

Honestly I think the best way to re-wire the 67-72 trucks as far as the charging system/power distribution goes is to mimic the 1975-up trucks...complete with an alternator upgrade and firewall +12V common junction.
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Old 01-01-2019, 08:33 PM   #18
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Re: Alternator wire size

Thank you! I will look into it
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