AFAIK, there are more separate grids.
I know for a fact the Quebec and Southern Ontario grids are separate, and I heard Texas is practically on their own grid.
On the whole, I think the separte grids may be:
Quebec/eastern Canada.
South Ontario/Northeast US
South East US
Texas
Midwest US
Western Canada
Western Seaboard (including BC).
Alaska is probably a whole separate grid, and of course Hawaii will be too.
