FWD: 240.4

[George Stolz]

Please give your interpretation of 240.4.B with respect to minimum wire size (75C) for the following feeder scenarios::?
(Assuming 3 PH and the neutral is a non current carrying conductor)

Parallel runs of (4) ______ MCM AL for 600A Main Breaker.

Parallel runs of (4) ______ MCM AL for 700A Main Breaker.

Parallel runs of (4) ______ MCM AL for 800A Main Breaker.

Does the code allow for choice when more than one wire size meets the "next higher" provision? For instance, would 500 MCM and 600 MCM both be proper for a 350A Breaker?

I'm a little busy to give this full attention.

 

[augie47]

There are some unknowns in your the question but to proceed:
ASSUMING, parallel in sepeate conduits and no ambient problem so no derating is required.
(4) runs of 2/0 AL at 75?would allow 540 amps and could be protected by a 600 amp breaker per 240.4 IF the calculated load is 540 amps or less,
(4) runs of 3/0 in the same manner would be rated 620 amps and would allow a 700 amp breaker.
(4) runs of 4/0 AL likewise on a 800 amp breaker with a load of 720 amps or less.

after 800 amps the story changes. On a 1000 amp breaker you would have to go with 350kcmilAL X 4

As to the second question IMO, yes, you could use either a 500 or 600 on a 350 amp breaker. Your load would determine which.

 

[david luchini]

There are some unknowns in your the question but to proceed:
ASSUMING, parallel in sepeate conduits and no ambient problem so no derating is required.
(4) runs of 2/0 AL at 75?would allow 540 amps and could be protected by a 600 amp breaker per 240.4 IF the calculated load is 540 amps or less,
(4) runs of 3/0 in the same manner would be rated 620 amps and would allow a 700 amp breaker.
(4) runs of 4/0 AL likewise on a 800 amp breaker with a load of 720 amps or less.

after 800 amps the story changes. On a 1000 amp breaker you would have to go with 350kcmilAL X 4

As to the second question IMO, yes, you could use either a 500 or 600 on a 350 amp breaker. Your load would determine which.

:thumbsup: I get the same answer, though as I reread it I think he was looking for 2 parallel sets of (4) conductors, which would be...

600A: 2 sets of 400mcm
700A: 2 sets of 500mcm
800A: 2 sets of 700mcm

 

[Robert Newland]

240.4.B

240.4.B

David: Yes, i was referring to (2) runs of conduit w/ 4 wires each. Thanks to both of you.