Don’t worry that this doesn’t make sense from a “real world” point of view. The narrative is just a highly simplified device to set up the real question that I’ll get to in a moment.

I’m doing some automotive electrical work. Specifically, I’m replacing a 30/35 amp generator with an 80 amp alternator.

The old setup had two wires that ran between the generator and the voltage regulator – a 10 gauge wire carried generated current from the generator to the voltage regulator and an 18 gauge wire going from the voltage regulator back to the generator. The new alternator has a built-in voltage regulator and only has one terminal so I can bypass the voltage regulator and go directly to the battery. The 10 gauge wire should be beefy enough to carry all of the current from the alternator to the battery.

But that means that I’ll have the 18 gauge wire left hanging there. Of course I could either just let it hang there or tape off the ends (I don’t want to cut it), but why not connect it between the alternator and battery, too?

So my question is, if I were to connect both wires between the alternator and battery, would this behave as a single, larger diameter wire and carry the 80 amp current more easily? Or would they act as if they were parallel (almost 0 ohm) resistors each carrying its share of the current – in this case up to 40 amps each – which would probably overheat the 18 gauge wire?

 

Yes, you can connect the 18gauge in parallel with the 10-gauge wire and the total copper cross section of the two wires becomes the conductor.

 

10 gauge wire is not big enough for 80 amps you need to have something like a 6 gauge

 

something like a 6 gauge

Yeah. I'd go with (3) ten gauge wires of matched length and make sure they have high temperature insulation rated to survive oil.
Can't remember right now if that's MTW or THWN, or THHN.

 

Your missing something:

There is usually three wires on an alternator and ground.

The large one to the battery supplies current to charge the battery. It has to handle 80 Amps. there's a wire calculator at www. powerstream.com.

One of the wires typically is connected to the IGN circuit. If you don't, the internal regulator will discharge the battery through leakage paths. This also serves to measure the 12 V system voltage essentially at the fuse block. It may seem to be connected to the same place, but it isn't.

The third wire provides the "lamp" function.

 

There is usually three wires on an alternator and ground.

The new alternator has a built-in voltage regulator and only has one terminal

I think you two are working on different alternators.

 

The 10 gauge wire is for charging the battery and for carrying the electrical load of the car. The 18 gauge wire is not intended to charge anything but rather to control the alternator - sort of the "On / Off" switch for the Alt. If you're set up for experimenting then with the alternator spinning, check for voltage (13.6 to 14.4 volts - possibly higher). Connect the 18 gauge wire for one test and disconnect it for a second test. I'm betting when the 18 is disconnected you get no voltage output from the alternator.

30 amp alternator? Most cars have at least 60 amp that I'm aware of - but I don't possess a worldly knowledge of alternators, so maybe I'm wrong. 30 amp alt. may be (in my mind) part of a John Deere Tractor or something small - not automotive.

Like KISS said - in automotive alternators there's usually a wire for either the idiot light or for a meter of some kind (volts or amps), and from experience I know this to be even smaller than the 18 gauge wire. Your diagram showed nothing like that.

Perhaps if you get to the root of what it is you are trying to accomplish we can better help you. Drawings help a lot. Often it's easier to read a diagram than to decipher what someone said in paragraph after paragraph (me being the KING of paragraph's). A drawing can say a lot with minimal effort. Even drawn on a piece of paper and a photograph of it will help.

 

Maybe I mis-spoke about the testing part of my comment - you should either get current output from the alternator with the 18 connected to the battery and none with the 18 disconnected. You'll see the battery voltage sit around 12.6 volts if the alternator is not putting out current and 13.6 to 14.4 volts on the battery when the alternator IS putting out current.

 

Thanks for all of the great information. This is exactly what I need to know.

10 gauge wire is not big enough for 80 amps you need to have something like a 6 gauge

I wondered about that, too. I had previously received advice on a different site that the 10 gauge would be good enough, but I think that they were going on the assumption that the alternator would never actually put out all 80 amps. I think that I'm going to have to try to figure out how to add heavier gauge wiring without turning the truck into spaghetti.

 

Replace your alternator output with a piece of welding cable.
The welding cable is also good for the starter supply leg.

 

Several people questioned the wiring set up of the alternator. I intentionally left it out because I didn’t want to clutter up the original question with unnecessary information.

This is for a 1953 Chevrolet pickup truck that has been converted from 6V to 12V. Currently the truck has a generator (hence the low 30/35 amp output – and also the reason I’m changing to an alternator). The old generator as two terminals – “arm” (the 10 gauge wire) which carries the current output from the generator to the voltage regulator and then on to the rest of the vehicle and “fld” (the 18 gauge wire) which the voltage regulator uses to control the generator.

From the voltage regulator it goes to the ammeter on the dashboard (no light), the ignition switch, battery and rest of the electrical system. Yes, current for the entire vehicle is going through the truck cab through cloth insulated wiring (which has since been upgraded to modern automotive wiring).

I’ve attached a schematic of the truck’s electrical system. It’s a little low-res so I’ll try to attach a better scan later on.

The new alternator is a replacement for the old generator. Since it has a built-in voltage regulator, it only has one output terminal. That should really go to the ammeter, not the battery, if the rest of the electrical system can handle it. I don’t want to hijack my own thread so I’m planning to post a separate question about that this evening.

 

Your new alternator at 80 amps requires a minimum cable size of 8 gauge
Welding cable +1
If this is a single terminal alternator, it is self-excited and does not require an excitation lead from a lamp or ignition. It is done by residual magnetism
You can run this single wire back through your ammeter to use as an indicator
This unit if built correctly will not cause a parasitic drain.
Cap off the 18 gauge wire, remove it or use it as a second. It does not matter as long as it follows the same path as the large cable.
If it does not start up around 1500 rpm, you will need to "flash" the alternator to restore the residual magnetism.

 

But getting back to my original question, could someone please explain why the two separate wires act as a single conductor?

I mean, let’s say that I had a multi-strand wire in a circuit and let’s say that the wire had 20 separate strands. If I were to split that wire open and separate the strands into two separate bundles of 10 strands each, I would expect each bundle to carry half of the current of the original wire.

But let’s say I separated the strands into a bundle of 19 strands and one single strand. I guess that means that the 19 strand bundle is carrying 19 times the current of the single strand, but why?

Maybe I’m asking a low level physics question, so if I am; please try to give an answer that a guy that got a “C” in High School Chemistry (like me) can understand. Thanks!

 

Can't remember right now if that's MTW or THWN, or THHN.

Those are "machine" wires. They will work in an automotive situation, but, they are not an ideal automotive use wire. Wire for cars have a higher number of smaller individual strands, that makes them more flexible and better suited for the vibration in a automotive environment.

 

One of the assumptions with automotive electrical is that there is resistance in a conductor and people tend to think of the wire as a resistor and I this may be your source of confusion. Although any conductor has some resistance, the length of wire you are using coupled with the very low amount of resistance in the wire make it a very clean pathway for electrons to travel. Those of us in the automotive trade who have spent any time working with DC electrical, pay very little attention to this resistance. So if we take this out of the equation and look at it as a clean pathway, basically all you are doing is providing more surface area for the electrons to travel along. It would be like taking a 1 " plumbing pipe which can carry water safely, and adding a 1/4" pipe beside it. Some water will travel through it but most will travel through the large pipe. Basically you are looking at Ohm's Law on a very small scale.

 

A loose larger wire could divert all of the current to the smaller one. Don't forget to add a fuse or fusible link in the wiring unless the alternator does not require it. A single wire is the best way to go.

 

I had completely forgotten about polarizing a voltage regulator for a generator.
Good call.