Don, I did say "trade practices," as I realize that this issue is not one specifically covered in the code. Yet, where the code does discuss running couductors in pipe, it invariable says "all phase conductors will be in the same pipe (more or less)." The code does not distinguish between AC and DC on this issue, nor does it explain why the rule is present (we may deduce it is because of inductive heating issues- but our logic is separate from the code).
So, as a matter of practice, I would start from the position of running them together- and look for justification to do things different.
Considering the price of welding cable, my first impulse would be to run THHN in the pipe- and have some sort of "power point" to plug the cables into closer to the work. I am not there (at the jobsite), however, and I don't know the details- for all I know, running the wires separately, in pipe, over a distance makes a lot of sense.
I'd like to make a comparison, just for a moment, to something as simple as reading. Before we become authors or editors, we learn all sorts of things (i before e, etc). These form the foundation upon which we build- and our ability to communicate suffers when we deviate.
In electric work, the way it is supposed to happen is that first you learn how to do it, then you start learning the codebook. The NEC is pretty blunt about this, where it states plainly that it is neither a textbook nor a design manual. As with most good rules, it lays down general principles, rather than attempting to detail every possible variation. Without first understanding the foundation- the principles of the trade- your understanding of the code will suffer. This fear is probably at the root of my discomfort with "code seminar" inspectors (among others). But I wander....:-)
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