What cable diameter?

[DRG-ler]

Hi folks, I want to replace all of my cables in my 1930 Tudor.
Can someone tell me what cable strength is installed from the alternator to the terminal box and from this to the ammeter?
The car still runs on 6V, but a modern 6V / 78A alternator is installed. I had only expected 6mm², but now that I have it in my hand it seems very thin compared to what is in the Ford. On the other hand, the old fabric cables between the terminal box and the ammeter are much thinner. Would estimate 4mm². But that would be too thin for me now, for the load that flows over it.

Normally, the same cable diameter would have to be laid from the alternator to the ammeter and from there to the starter?!?!

Greetings from Berlin / Germany
Andy

[rotorwrench]

Unfortunately for the sake of metrics, the conductors are sized in American Wire Gauge and I'm not sure how that translates to metric. Lighter duty electrics like the model A can get away with 16 AWG sizing but for higher amp conductors 14 AWG or larger can be used but it has to be able to work with the conduit and the terminal box entry points. The model A was about a 10-amp max system due to the limitations of the generators used originally. The highest draw circuits are the headlights & horn. This is why they were bused direct to the generator.

This is a good link. https://www.mafca.com/downloads/Semi...ex%20Janke.pdf

THis chart might work: https://www.google.com/search?hl=en&...4dUDCAw&uact=5

[J Franklin]

Probably nothing bigger than 2.5 mm Is needed except the battery cables We use 00 awg for those with 6 volt systems. Most wiring is 14 awg. 00 awg cable goes from the battery to the starter then you can use 12 or 14 awg to the ammeter. The table rotorwrench linked will give the comparison awg to MM

[Ak Sourdough]

If 78A means 78 amp output from your alternator, I would say forget the ammeter and put a volt meter someplace out of sight that you can take a look at occasionally. That's much more current than I would want so close to a gas tank that lives inside over my feet.

[Ak Sourdough]

A volt meter will tell you that the charging system is working even when the battery is full and is not asking for charging current.

[J Franklin]

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Is the electrical chart also applicable for DC current?

I don't use an alternator but isn't there regulation? I wouldn't want that many amps near my gas tank either but any sparking could be interesting at 20 -100 amps.

[Ak Sourdough]

The wire doesn't care about AC or DC. Amperage is a measurement of volume of flow.


An alternator will provide as much amperage as the battery and accessories ask for up to it's maximum output. If you run your battery down to nothing, then jump start and want to run lights, heater motor and electric wipers while the battery is charging the alternator may put out it's maximum capacity for a while. The problem is all of that current has to go through the ammeter and the wires that connect to it. If the ammeter has a loose nut on one post, it might arc and melt the terminal or the post. Then you have sparks and a live wire about a 1/2 inch from the gas tank. Also, a 20-0-20 ammeter may not be happy about carrying 78 amps.


The regulator controls voltage, not amperage.



I'll have my instrument panel off this weekend to replace the speedometer cable and intend to glue a piece of heavy inner tube to the tank to isolate/insulate it while I'm there.

[J Franklin]

Thank you for the education.

[DRG-ler]

Hello, and first of all thank you for your messages.

Thanks rotorwrench for the instructions. It is very interesting and revealing.

If 78A means 78 amp output from your alternator, I would say forget the ammeter and put a volt meter someplace out of sight that you can take a look at occasionally. That's much more current than I would want so close to a gas tank that lives inside over my feet.

The problem is all of that current has to go through the ammeter and the wires that connect to it. If the ammeter has a loose nut on one post, it might arc and melt the terminal or the post.

I have no problems with the 78 amps under the gastank. As long as you work properly and isolate everything correctly, there can be no problems. This is a requirement that I have of myself. I also don't want to do without the original ammeter. It must also be said that the 78 amperes do not normally flow through the ammeter. It would be technically possible, but experience has shown that the ammeter NEVER deflects fully during operation. After starting it only briefly shows no more than 15 amps and then goes to zero.
Furthermore, all consumers are powered directly by the alternator when driving. This current does NOT run through the ammeter. This only happens when the engine is stopped and the alternator is not supplying power. Then you have to pay attention to the ammeter, that it is not overloaded.

A volt meter will tell you that the charging system is working even when the battery is full and is not asking for charging current.

And that's exactly where the problem lies. The voltmeter does not show how much is being charged. One can differentiate whether the charging voltage is present or not. That's right. However, it is not clear whether the charge is 0.5 ampere, 8 ampere or 20 ampere. If the alternator still reaches the charging voltage but no longer supplies a proper charging current, this error is not recognized and the battery becomes empty and empty. for this reason I think the ammeter is the better solution.

I'll have my instrument panel off this weekend to replace the speedometer cable and intend to glue a piece of heavy inner tube to the tank to isolate / insulate it while I'm there.

I can understand your concerns. But I say the car has been driving like this for 90 years. The cables have worked like this for 90 years. If I renew them now, they will work again for 90 years.

Greetings from Berlin
Andy

[Badpuppy]

The wiring should be 12AWG between generator, ammeter and battery. Everything else is 16AWG. #12=2052mm, #16=1290mm, according to google search. Modern insulation is about half as thick since the pvc is now coated with polypropylene, so the wires require less space.

The ammeter should never show more than a few amps charge, 10-12A discharge since it is only a branch circuit of the generator.

Broken record here: the ammeter does not measure generator current, only battery current.

[rotorwrench]

I ran a one wire GM alternator for a while till the terminal box smoked one day due to loose terminals. It created enough resistance to melt the cheapy reproduction terminal box I had at the time. The wires all burned up too. There was no short to ground so no fire started but it got pretty smokey. I went back to a 3-brush generator and an original bakelite terminal box after that.

I see no reason to go overkill on power generation unless you want to add air conditioning and stereo equipment or some other high current draw accessory.

[Joe K]

The ampacity of wiring done to the National Electric Code is for AC wiring. Like AC voltage, AC ampacity is sinusoidal, meaning it follows the AC Voltage.

Since ampacity is sinusoidal, it may have a maximum and a zero, but the heating effect goes to the root-mean-square of the maximum value, discussed further at https://en.wikipedia.org/wiki/Root_m...mmon_waveforms. And for AC sinusoid the ampere max value divided by square root 2. Or about 0.747.

When you read amperes on an AC circuit, the value you read is this RMS value.

When you read amperes on an DC circuit, the value you read is THE VALUE

So as written - the ampacities of wire according to the National Electrical Code DO apply - but they also take into account "wire density", packing in conduit, heat development at connectors, multiple layers of insulation, and all of the minutia of residential wiring. It is a VERY conservative ampacity standard for wiring.

To show deviation from ampacity table one need look no further than the Model A generator itself (not subject to the NEC) with 18 gauge wiring used for the armature.

Of course the armature is somewhat ventilated, certainly disposable if it fails, and (usually) won't burn your car down around you when it does.

Joe K

[DRG-ler]

I see no reason to go overkill on power generation unless you want to add air conditioning and stereo equipment or some other high current draw accessory.

That was roughly my plan ...

Be retrofitted:
Turn signals all around
Hazard warning lights
Trafficators
Only the hazard warning lights together with the trafficators need 24 amps / 144 watts in emergency operation !!! I think I will retrofit LED lights here, because in emergency mode you need a long battery life.

Furthermore, these things should be installed.
Maybe Fog lights ...
Rear fog light
Reversing light
Engine compartment light
Maybe parking lights too ...?!
Water heating with an electric fan
Rear window heater
Radio with Bluetooth / MP3
Socket for telephone / navigation system

Except for the last two points, everything is built from historically old original accessories that are not younger than 1940. I don't know if you really use these things at all. But I want to have them and then they should work too. So I think I am well advised with the modern 78 amp alternator.

I ran a one wire GM alternator for a while till the terminal box smoked one day due to loose terminals. It created enough resistance to melt the cheapy reproduction terminal box I had at the time. The wires all burned up too.

In fact, this is a known problem and in the end it is just not a good maintenance. Because I am now completely rebuilding, I can check and correct such problem areas immediately. Probably the first time in 90 years.


Andy

[Ak Sourdough]

Here's a chart for automotive DC wiring. Not a lot different from the chart above, a little less conservative. Without knowing the temperature rating of the insulation I personally would stay toward the cool side of the chart to be sure it doesn't get overheated.


I didn't read the whole page, but looks like a lot of good information. Chart is at the bottom of the page..





https://www.custombatterycables.com/...ire-questions/