NO, the ground is not serving any purpose other than a backup way to open a circuit breaker in case of a fault.
It does not, and should never, serve as a current carrying circuit conductor.

A 120v circuit is a line-to-neutral circuit.
A 240v circuit is a line-to-line circuit.

NO neutral is used or needed in ANY 240v load.

Yes, thanks, unfortunately that is the part I understand which is why I asked "...why exactly don't I use the neutral wire at all?"

The best answer that I've been able to find on my own is that the polarity of each 120V branch in a 240V circuit is different - one is negative, one positive. On a 120V circuits you don't have the cancellation of charges, so the neutral is used to carry the current flow back to source then to ground. I hope my summation of the more technical explanation I found didn't dillute with what i just typed.

I already understood that the neutral wasn't used on PURE 240V circuits / equipment.

K

The neutral does not carry current back to ground. It carries it back to the transformer and ultimately the power plant. The ground plays no part in power delivery. It is a safety issue or lightning protection only.

"The best answer that I've been able to find on my own is that the polarity of each 120V branch in a 240V circuit is different - one is negative, one positive."

Negative and positive do not have any real meaning when the voltage is changing 120 times a second.

The system used is a center tapped transformer winding.
Across the whole winding is 240 V.
The center of the winding is grounded and 'declared' as 0 volts.
From either end of the winding to the center is 120 V.

The net effect is that the two halves are 180 degrees out of phase with each other (NOT the same as 2-phase power though, that is 90 degrees).

When one end is at a maximum positive voltage, the other is at the maximum negative voltage. They then reverse 120 times a second to produce 60 Hertz.

Thanks guys, now that we have the least significant part of my original post answered, can someone provide an answer or clues regarding question #1 and #2... 2 is less critical as I have a feeling there is absolutely nothing wrong with using a larger gauge wire on a circuit as long as the proper breaker size is paired to the proper recepticle / equipment rating.

K

1. I doubt this is a welding circuit, sounds more like a multiwire branch circuit that was never used.
2. Go ahead and run 10/2 if you wish. Using 10/3 is a waste of money.

Guess I'll give the company that did the wiring install a call and see if they have any docs / plans that detailed that welding circuit.

The breaker is labelled "welder" circuit by the installer, because they're using company logo'd labels at the breaker and the hand written notation on the label matches the employee's signature on the installed electrical company sheet taped to the inside of the subpanel.

K

1a. A 20A circuit won't support a typical welder. Maybe some specialized type of smaller welder.

1b. A 240 V circuit really should use a 2-pole breaker, not individual breakers. In some cases it is required by code. If not always required, it is still always a good idea.

2. There is nothing wrong with oversized wire except the cost and difficulty in working with it, so long as the breaker and receptacle can accommodate it. If it doesn't fit in the termination, then you would have to splice a short piece of the "normal" gauge wire on.

3A/B I think you basically understand it. In some cases an appliance (kitchen range) may use both 120 and 240 so they do send some current back on the neutral. If it is only 240 then the neutral is not used.

I believe your 4000W heaters (16.7A) will REQUIRE a 30A circuit with 10 gauge. Electric heat is a "continuous load" and its circuit can only be loaded to 80% of capacity. Since 80% of 20A is 16A, and the load is 16.7A, you probably have to upsize even though it is only 0.7A more. This would also allow you to use the 5000W heaters (21A). You could load the 30A circuit up to 24A.