Where? In clean air, dirty air, a vacuum? Phase-to-phase, phase to ground? Are you building switchgear? The specification for 480V switchgear (which implies North America) is ANSI C37, I suggest you invest in a copy if that is your job function.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
This is for a heater bank in a vacuum oven that is going through a pass through. I am talking about phase to ground and I would also like to know about phase to phase. Does ANSI C37 talk about air gaps?