Just finishing off my last couple of jobs before disappearing and thought I'd relate today's experience of one reason why I'd urge you not to go down the "whole-house" GFCI approach.

This was a big house converted from old barns -- Bathroom the size of my living room and a living room the size of my whole house; the guy paid £300,000 cash for it (about $450,000). Waaaaay out of my league! But that's irrelevant....

O.K., I was working in the yet-to-be-finished barn/garage area, having gotten several flood lights fitted high up in the 1830s beams. It was dark by the time I got to the recepts. so I turned the floods on to continue working, and shut off the 32A C/B for the recept. ring.

No problem you might think. Well, guess what happens when you take a pair of cutters to a cable and they momentarily bridge neutral and ground. Yep, <click>....<darkness>. I was expecting that.

Having descended the ladder in gloom to reset the breaker, I went back up to strip the cable ends and accidentally brushed the stripped neutral onto the grounded fixture box. D***! Back down
to the panel again. I managed to short N-to-G once more in installing that fixture. (At least the RCD/GFI is working!)

See why I don't like this arrangement?

Hey, on the positive side (no pun intended but I'll take it as a freebie anyway), you'll be svelte and fit for your adventure to find a new home after going up and down the ladder so many times!

We're required by Code to have Ground Fault Protection on any service over 1000A at 277/480!
Tripping can sometimes be achieved by almost any direct phase-to-ground fault. Even a pinched hot #18 wire to a ballast can bring down a big building.
Cool, huh?
Please write, OK?

Yep, I certainly got plenty of exercise yesterday. I could've opened the main panel again and lifted the neutrals for the ring, but I had only one outlet to move on that circuit & it was getting near quitting time -- You know how it is!

Scott,
I guess you know the problem only too well then!

Any particular reason why a 277/480 main GFI is required only above 1000 amps?

Does it have to be a single GFI or can you split to two or more panels each with its own sub-main GFI? That would at least minimize the disruption a little.

What is the typical range for the potential between a neutral and a ground? Will it always trip a GFI?

electure: We're required by Code to have Ground Fault Protection on any service over 1000A at 277/480!
Tripping can sometimes be achieved by almost any direct phase-to-ground fault. Even a pinched hot #18 wire to a ballast can bring down a big building.
Cool, huh?

And the really lousy part of it is, alot of those services are oversized by 75% by the engineers, pushing them into the 1000amp threshhold, most of the time for NO apparent reason.Engineers covering there..behinds, as usual.
By the way, whats the name of that company locally that sets the gfi's to spec's? Their name escapes me.
NS4M