Thanks for all the info guys. Sorry to grill you all on this one, but when I was in the States I only studied residential 120/240V service in detail, so I wasn't familiar with common 3-ph. practice over there.
All of our LV 3-phase in the U.K. is 4-wire 240/415V Y without any oddball systems like corner-grounded delta or 4-wire delta (no offense intended!).
I hadn't realized that it's usual to use different colors for 277/480 than for 120/208, but if both systems are often present in the same building I can certainly see how it makes sense.
I knew the NEC allows gray as an alternative to white for neutrals, but again I didn't realize that gray would be chosen for the higher voltage system where white is already in use for a lower voltage. (I figured it was just a little-used option reserved for neutrals because white could soon look like gray in some environments!)
Some more questions posed by your answer:
What exactly does SDS stand for? Supply Distribution System is all I can think of that seems appropriate to the context.
On the 4-wire delta systems, why is the high-line defined as phase C in service equipment and phase B elsewhere? Come to that, what exactly do you define as "service" equipment?
(Maybe Oscar Wilde wasn't too far wrong when he described America & Britain as two countries divided by a common language!)
I've also seen references to an industrial 347/600V system. Anyone care to comment?
As for electrons only working through the right color wires, try convincing someone that holding a cord up high will slow down the electrons because they have to go uphill.....!!
