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This weeks post comes from a couple of questions asked these past couple weeks about sizing a service.  I was asked why you have to add the breakers up to get the size of the main!  I told them to look at section 230.90(A) Ex.. #3in the NEC and in the Florida Building Code Residential E3603.3.1 exception.  This is not a new so I don’t understand how someone could come to the conclusion that the main “MUST” be based on adding up all the breakers in the panel or all the mains at a service.  The exception to this section allows us to size the main based on a calculation, and not by adding the breakers together, because if that was the correct way to size the main I would need 600 amps for the main in my home and my 2/0 wire would be way to small.  In the slides below there is a 400 ampere service with four 200 ampere mains, which to the layman would look like an 800 ampere service but in reality it is only a 400 ampere service, because the calculated load was only 340 amperes and we used 500 Kcmil Cu. wire which has an ampacity of 380 amperes at 75c which is good for a 400 amp service see 240.4(B).

Hope this helps.

Jake