Bob, just for your info, my generator is marked "G" for the ground and just "F" for the field. Both of these terminals are on the generator case near the back of the unit. Both the field and the ground terminals are using what appears to be a #10 stud. The armature terminal is not marked that I can tell and is on the rear of the generator with a 1/4" stud as a connector. Also on the rear, is a small screw threaded directly into the rear cover which I assume is an auxiliary ground connection. I was told this generator was from a '62 Falcon.

 

Thanks, I worked as a mechanic at a Ford dealership in the 60's and thought "G" and "F" were right. Forgot it was marked differently on the older units until I went out and looked at my '52! Seems like '63 was about the last year for Ford to use generators, but may have been '64. - Bob

That screw on the back may have been for mounting the radio suppression capacitor - can't remember for sure if it fastened under that screw or under the head of one of the through bolts.

 

Watts law confuses things. In a conversion, you cannot work Watts law backwards to find current.

A 6V bulb with 65W has different internal resistance than a 12V bulb with 65W. Ross is correct - if you are using the original components, doubling the voltage doubles the current because the resistance stays the same.

Here's something I posted in another thread:

Ohms law is primary. Watts law is derived from Ohms law. You have to work the Ohms law problem first, then plug these into the Watts law.

Here is a simple online Watts/Volts/Amps/Ohms conversion calculator:

Watts/Volts/Amps/Ohms conversion calculator

In the 65W comparison, you have different bulbs. But let's do that anyway. To find the resistance of your 6V 65W bulb, plug in 65W and 6V into the calculator and come up with 10.8 Amps and 0.55 Ohms. If you then reset the calculator and plug in 65W and 12V, you get 5.4 Amps, but 2.2 Ohms. This is the difference between 6V bulbs and 12V bubls - for a given wattage, a 12V bulb will have higher resistance. Light is heat and resistance to voltage makes heat. This is why when you do plug that 6V 65W bulb into 12V, it grows bright as the sun for a moment before burning up because you'll generate over 260 Watts and almost 22 Amps going through it.

So yes, a 12V 65W bulb pulls half the current of a 6V 65W bulb, but it does so due to the (near) square of the resistance...because the bulb is designed for 12V usage.

Do not use Watts law backwards to determine 12V current draw when converting 6V systems. It is not applicable and will give you a wrong answer every time.


As for why 6V wires and switches are heavier if the current draw is half that of 12V, it is simple: Internal resistance is determined by wire gauge. If you look at the table here:

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

You'll see that resistance per KM of 14ga and 16ga - two common sizes in 6V and 12V systems respectively, is roughly half. This keeps the circuits operating properly. Because a little bit of resistance in a 6V system is a bigger deal than that same resistance in a 12V system. Yet another reason why good grounds are so critical in 6V systems when they are much less important (still important, but not critical) in a 12V system. So, the wires in your 6V system are heavier for resistance reduction, not because it carries double the current. Note that the table is for specific copper wires, and every type of wire is different, I used this table merely for illustration purposes - the principle applies to all conductors.

 

Good info, but I believe there's a problem with the quoted statement. - Bob

 

What do you believe is the problem? 6V wiring is typically heavier/larger gauge(smaller numerically) than 12V wiring to reduce resistance.

 

The problem is, not any instances I can think of where the draw on 6v is half that on 12v, in an OEM situation. I think you meant, " ...current draw is half that ON 12V"

 

I didn't know how to explain it, but that's it - "of" should have been "on." - Bob

 

Wow. Grammarians. I are one two!

I don't think there's anything wrong with the sentence as written. Let's break it down. First, you must quote the entire sentence, not just the prefacing statement with an added period that was not there.

I concede, I could have replaced the colon with another comma - would that have cleared it up? I don't think so, but I'm not sure I understand your objection to my word choices.

I could have phrased this as a question and answer thusly:

Would this have been clearer? Why are you stuck on the word "on"? Are you assuming a specific instance? Because the principle applies across the board.

Or perhaps you would have liked another conditioning statement such as this:

Perhaps it is because my phrasing is answering a question not specifically asked, and merely anticipated. Was that confusing? If so, it's because it was cut/pasted from another discussion.

Or am I missing your grammatical distinction somehow?

Full Disclosure: I went to Catholic school with nuns and rulers. I try to be grammatically correct, however, I fall into the trap of writing as I speak often.

 

What's confusing is: "If the current draw of 6v is half that of 12v..."
In what case is current draw of 6v half that of 12v? The only situation I can think of is when a 6v car is converted to 12v, and you're looking at a circuit that has a fixed resistance (let's say a field coil). On 12v, the current would double, but that has nothing to do with why the (original) wiring would be larger gauge.

 

To put all this discussion into perspective and make it relevant to our vehicles, can we say that 12V will draw twice the current of a 6V circuit if using the original components designed for 6V? In designing components for 12V, internal resistances are typically higher for the 12V component than for the same 6V component and usually draws about half the current for the same amount of power. This lower current is why the wiring is typically smaller than 6V systems.
Since I am completely rewiring my truck, I am using the smaller wire sizes because my components are designed to run on 12V including my generator and starter.

 

You are correct, Doug.

 

Sister Patricia would be proud...

I still fail to see the confusion. The only reason we're discussing it is a 6V to 12V conversion...

 

Thank you, thank you very much. Cougar54 has left the building!