NEC 110.26.C.2 explains doors for equipment rated 1200A or more. Do three 400A panels consititute equipment rated 1200A ?
Quite simply, no.
Well, a better look at the specific requirement might illuminate the answer a bit better. The sentence reads
"for equipment rated 1200 amperes or more AND over 1.8m (6 ft) wide that contains overcurrent devices..."
So, while a particular inspector might try to call you out on your installation because it has three 400 amp panels (and 3 times 400 is technically 1200, right), maybe you'll get lucky when you point out that your three panels, taken together, are not over six feet wide.
But all kidding aside, the intent is clearly to be applicable to large sections of switchgear or equipment with a single rating of 1200 amps or greater.
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