Amp Guage



Converted my '53 Studebaker to 12 volts. The truck was originally equipped with a 30 A 6V generator. The Amp guage just reads +/-. I’m installing a 100 amp single wire alternator which goes in one side of my Amp guage and out the other to the battery. Will my amp guage handle 100 amps or will I fry it?


An amp gauge that was designed to read only 30 amps max probably won’t handle 100 amps. Amps=heat.



I was running, for a while a 50A alternator with no problems to the guage. The Alternator went out, and the shop talked me into upgrading to a 100A. Also, there is this to consider: Ohms law. V=IR, If volts increase from 6 to 12, then (assuming load is the same) I should be able to pass through twice as many amps right?


I’m not going to try to explain it. Read this.

And why on earth would you need a 100 amp alternator on a 53 Studebaker pickup?



Unfortunately, the correct answer is maybe.

It’s not clear whether you’ve been running the car at 6 volts or 12 volts previous to installing the 100A alternator. If you were running it at 6 volts, you are now going to put twice as much current (I = E/R) through the “ammeter”, generate four times as much internal heat (P = EI), and maybe fry the ammeter. If you’ve been running it at 12 volts, then the “ammeter” can handle the higher voltage and current.

Except that you’ve presumably changed the many of the loads (e.g. light bulbs) to 12 volt versions which will probably draw less current and therefore you haven’t really doubled the current flow through the “ammeter”. You may even have reduced it.

In any case, the theoretical answer is that it is the components of the car that use electricity and the output voltage of the generator/alternator that determine how much current is drawn, not the rated capacity of the generator/alternator. However, in practice, a higher capacity alternator is less likely to bog down (has a lower “source impedance” if you like technical terms) under load, so your “ammeter” might see a bit more current with the 100Amp alternator than it did with the 50Amp 12 volt(?) alternator that you were previously using(?).


Wrong. Bump the voltage to double, and the amp load cuts in half. P=V*I, where P = power required. However, the truck will never pull 100 amps from this alternator unless you get a short.

The 100 amps is total capacity available from the alternator. The electrical system will only draw as many amps as needed. In this truck, that will never be close to 100 amps. If it cost more, you just threw some money away for nothing.

The gauge should be fine. Especially if it worked with the the 12V 50amp alternator.


It will probably handle the current just fine… Until you leave your lights on or run your battery partially dead somehow. Then when you start the car, the output of the alternator will ramp up, and you will likely damage or completely burn up the gauge and wiring if it hasn’t been upgraded, and possibly have a nice dash fire.

If you insist on doing this, I’d consider a 50A inline fuse to keep things from going up in smoke.


The ammeter does not go between the alternator and the battery. It goes between the battery and everything else except the starter.
An ammeter between the alternator and battery will never show minus amps. You want the ammeter to show how much the battery is being charged or discharged, not the total amps put out by the alternator.


Good link. If the OP would add another resistor of the same resistance and wattage as the shunt resistor in parallel with the shunt resistor, he should be o.k. The gauge itself certainly doesn’t handle the full current output of the alternator. Of course, the amperage markings on the ammeter won’t mean anything.


You’re correct. The current load of the vehicle should drop in half by doubling the voltage if the load is the same. The ammeter doesn’t care what the voltage is, it is measuring current so it will still handle 30 amps in a 6 volt or 12 volt system. I also agree with B.L.E.'s statement of the location of the meter. It is measuring current to or from the battery.

Looking at Testers web link took me back to school days soooo many years ago. I was having flashbacks.