said by IIIBradIII:
I think he was referring to the size of house that would require 400A based on sq footage alone.
And I think that may be the case with the OP which is why I posted what I did. No where have I seen the OP mention that he needed due to demand.
Here is a post from 2010 for a 4900 sq foot home requiring 400 AMP service.
»400 amp single phase service wire size residential
1800 + 3200 = 5000 Sq ft