400 Amp & 800 Amp Feeders

[kunk]

Can you use 500 MCM Cu for a 400 Amp feeder and 2 sets of 500 MCM Cu for an 800 amp feeder?

 

[Dennis Alwon]

Can you use 500 MCM Cu for a 400 Amp feeder and 2 sets of 500 MCM Cu for an 800 amp feeder?

Assuming 75C you run 500 MCM as long as the calculated load is less than 380 amps . Article 240.4(B) allows it.

Same is true for parallel-- 500mcm is good for 380 X 2= 760 amps. If the load is not higher than that then you can use an 800 amp breaker

 

[ron]

But your logic takes a vacation when you go to 1200A breakers. You may not use (3) sets of 500's. 240.4(C)

 

[iwire]

But your logic takes a vacation when you go to 1200A breakers. You may not use (3) sets of 500's. 240.4(C)

An important fact to make clear for sure.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions
are met:

(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cordand-
plug-connected portable loads.

(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).

(3) The next higher standard rating selected does not exceed
800 amperes.