Alternater wire Size

[78 Chevyrado]

What gauge wire size do you guys recommend?

I just ordered a 140A alternator, 3 wire  And I need to upgrade the stock charging wire.  The 78A unit I have just doesn't do it at idle anymore  but it didn't really do great when new.   The 140A was like $30 more than another 78A so i jumped on it.

My setup:

I'll be running, Headlight relay upgrade harness
I run the ac all the time and usually have the fan on high
i have an aftermarket stereo head unit that i play loud most of the time
I have a 500W amp that has a 40A Fuse  (I may possibly add another approx 500W amp in the future)
I run all this constantly and also need it to power the trailer lights and electric brakes (1 axle) on a 16' car hauler as well as the controller.

In the future I plan on an aftermarket (MSD) ignition box as well as a pair of e-fans, and fog-lights.


Where does the charging wire go?  doesn't it go to the starter?  It's been forever since I had to look at one of these.  or does it goto the junction block on the firewall?  If it gors to the junction block on the firewall will i have to upgrade any other wiring? 

Isn't there a fusible link inline with this wire, and if so what fusible link should i put in the heavier wire?

If there is no fusible link inline, do you think I should put a circuit breaker or something in the line?


Thanks fellas

[Psycho71]

I dunno what the difference would be between the charging wire on a 3 wire vs a 1 wire, but here's what I have.

I have a 100a alt. (1 wire). it runs 1-14" fan and 1-10" fan on the radiator, another 10" fan on the a/c condenser, 1-900watt amp and 1-600watt amp, aftermarket a/c system, and all the normal lights & stuff.

I have a 6 gauge wire from the alt to the batt. Never gets hot. Charges great, 13.5-14 volts with everything on and stereo wide open. Drops to around 13v with the brights on and the bass booming.

[SUX2BU99]

A heavy gauge wire from the alt. to the battery would be wise here. I run 4 ga. but the shorter the distance a wire runs, the smaller it can be for a given amount of current. That said, I'd go with 4 ga., but 6 ga. at a minimum. 4 ga. would probably be easier to find. And I'd go with a cable that has a high amount of fine-strand wire, not the stuff with a few, thicker strands like you might see in wiring for buildings. You should be able to find this wire at automotive stores, car stereo stores or even welding supply houses if you wanted to get the same kind of thing for probably much cheaper price.

You would also benefit from a 4 ga. cable running from the battery to the starter, from the battery negative terminal to the frame, and from the motor to the frame. My motor to frame ground though is a wide, flat, braided cable. All my other cables are 4 ga.

[78 Chevyrado]

I wrote back to the guy selling the alt and he recommends 4ga. also on 140A and higher alts.  I don't know what size my battery cables are, so I will definitely check them out and upgrade them too, as I don't like shortcuts when messing with wiring.  I have some of those really wide ground straps in the garage. I have one from the engine to the cab, do you think it would be worthwhile to put one from the engine to the frame?  I've never had a problem without it.  I'm going to look today to see where the alt charging wire goes to and see how its run.  I think the alt will be here mon or tues, so I need to get all my wires together.

[beastie_3]

More grounds cant hurt.

[78 Chevyrado]

More grounds cant hurt.

Most Def!


Going by my 78 shop manual, stock the alt charge wire runs from the back of the alternator to a fusible link before the junction block.  then from the junction block to the starter is another fusible link and then attatches to the same stud as the +battery cable does at the starter.  

I'm going to get fine stranded 4ga. wire.  To put it back stock like, I'll need to replace the +wire from the alternater to the junction block and then from the junction block to the starter and then from the starter to the +side of the battery.  As well as the ground wire from the -battery to the alternator mounting bracket.

The general rule is fusible links are 2 sizes smaller than the wire it protects.  So since I'm using a 4ga wire, shouldnt I use an 8ga fusible link?

The plan is (as long as I can get to the wires and looms to route them) to put in larger wire, but leave everything  in the stock configuration.

Also, why is it so hard to find sidepost battery cable connectors..   I can't find premade battery cables with the preterminated battery end in a 4ga wire and my current wires don't have their size printed on the jacket so I have to do some hunting to figure out what they even are.  Looks like I may just have to make the cable myself and get a top post battery.   Are optimas worth the money?  I usually get the high end advance auto parts store battery.

[78 Chevyrado]

I also just saw in my shop manual that the stock fusible links are 4 gauge sizes smaller then the wire they protect.  That would mean for a 4ga wire I would need a 12ga fusible link.  that seems really small.  i think I'll stick with the 8ga like I said in the last post as I don't want to have to redo a link later that was too small as the one by the starter is a pain to get to.

What do you electrical gurus think?  8ga, 12ga or even 10ga?   All your Thoughts are appreciated!

[SUX2BU99]

I don't have a fusible link in the wire from my alt. to my battery. I don't tend to like fusible links very much and would rather have a real fuse. You can get ANL (wafer) fuses in 100A or 150A sizes if you like. I have one on my stereo system in a slick looking fuse holder.

I have a yellow top Optima in my truck. It's okay but they can be senstive to total discharging. The color matched well to my truck though and they are popular with high-end stereos. That sounds like a silly reason to buy one but that's okay   I had a red top when they first came out in the early 90's and it served me really well.

[78 Chevyrado]

I don't have a fusible link in the wire from my alt. to my battery. I don't tend to like fusible links very much and would rather have a real fuse. You can get ANL (wafer) fuses in 100A or 150A sizes if you like. I have one on my stereo system in a slick looking fuse holder.

I have a yellow top Optima in my truck. It's okay but they can be senstive to total discharging. The color matched well to my truck though and they are popular with high-end stereos. That sounds like a silly reason to buy one but that's okay  I had a red top when they first came out in the early 90's and it served me really well.

I forgot about those ANL fuses, I have one of those on my amp too.   I think I'll go with them, a pair of 150A fuses with watertight holders.  4ga wire.  I think I have terminals laying around, we'll see.    Thanks for the help sux2b!


Does anybody know what size the big stud on the starter is;  the one the battery cable attatches to?  I need to buy 4ga terminals and don't know what size ring to get.  the other ones I can find out easy. 

Thanks!

[SUX2BU99]

No problem! The stud is large but how large I don't know. I do know that a 9/16" wrench is what fits onto the nut that goes onto the stud. If you get a regular 4 ga. wire terminal, the hole in it will be large enough to fit onto that stud I'm sure. It's not as big though as a battery post. It's about as big though as a bolt that would thread into a side-post terminal.

[78 Chevyrado]

I got the alternator put in today.  I got the wire i needed from best buys scrap wire collection for $5  The Fuse holder and fuse was $20 on ebay.  I just ran the wire from the alt straight to the battery.  I didnt even half feel like trying to run it in the stock location.  This way doesnt look bad at all so It stays.  I'll decide what to do with it when the engines out some time.  All the new wires including grounds are 4 gauge.

The pics make the wires look like they're rubbing on sharp edges everywhere....    I assure yall they're safe and out of harms way.




The Grounds

This one is on a preexisting unused stud at the bottom of the brake booster bracket.


I'm getting 14.5v with only engine running and I'm getting 14.2V  with everything I can turn on turned on and at idle.

All my current ground wires showed no connection problems on the old ohm meter so I just added the 2 extras.  The fuel pump one goes ground to frame,  and the booster is for body to frame.  I'm debating replacing the stock body to engine strap...  it has a good connection, real good.  when everything is running I only show 2 amps running through it anyway.

Some amp reading i took at various places while running at idle (700 RPM) and voltmeter reading 14.2 volts and everything running except backup lights, interior lights & cigarette lighter.

AMP Readings  @ 14.2 Volts

+ Battery Cable  -  18.3A
- Battery Cable  -  15A

NEW Alternator Charge Cable  -  26.3A
OLD Alternator Charge Cable  -  31.5A

Ground at Fuel Pump  -  42A
Ground at Brake Booster  -  31A
Ground strap from firewall to RH cylinder Head  -  2A

Junction Block to AC Fan Power Cable  -  22A   -  Fan set to HIGH
AC Compressor Clutch Power  -  5.3A

Headlight/Parklight Harness  -  Low Beams-12A   HighBeams-13.7A

500W Amplifier  -  10A to 20A depending on the song playing


I'm debating on at least replacing the Stock alternator wire to the junction block...   Its pulling higher amps than most of the other wires so i might run an 8ga wire back there just to do it.

Edit:  i did a test run for redoing the original charging line and it made no differenct whatever so I'm going to leave it alone.   I'm all done!

[SUX2BU99]

Looks awesome    The body to frame ground is a good idea. 0.3V drop with everything on is nothing, especially if it's all staying above 14V!

[Grim 82]

Have you installed the headlight relay harness yet? It's probably the most useful mod that I have done to my truck.

[78 Chevyrado]

Not yet.   The only thing I really added so far is the Amplifier and keyless entry.  I'll have it one day though, and I may put those jeep headlights in to go with it.