Do I have to derate ampacity?

[KasseemF]

I have a 1200A service disconnect and my drawings have 4 sets of 350MCM feeding this disconnect. Which means I will have 4 cables in a pipe.

Since Table 310.15(B)(16) says Not More Than Three Current-Carrying Conductors in Raceway, Cable, or
Earth (Directly Buried) does that mean that wince I have 4 in the race I have to do a calculation to derate the ampacity and if so, what is that calculation?

 

[david luchini]

Generally, the neutral would not be counted as a current carrying conductor, so you wouldn't have to adjust the ampacity.

See 310.15(B)(5).

 

[KasseemF]

The four conductors do not include the neutral. According to the table, 350MCM is rated at 310A at 75degrees. so four in parallel would be 1240A.

If I put all four in the same conduit do i need to derate the ampacity according to NEC?

 

[david luchini]

The four conductors do not include the neutral. According to the table, 350MCM is rated at 310A at 75degrees. so four in parallel would be 1240A.

If I put all four in the same conduit do i need to derate the ampacity according to NEC?

You've lost me...are you trying to put all of the conductors of the same phase in the same conduit? If so, see 300.20.

 

[Cow]

You would need four conduits to get all your phases and neutrals installed in separate conduits, so why not put ABCN in each pipe instead and avoid potential derating issues, not to mention the pitfalls associated with an isolated phase setup?

 

[greenspark1]

You would need four conduits to get all your phases and neutrals installed in separate conduits, so why not put ABCN in each pipe instead and avoid potential derating issues, not to mention the pitfalls associated with an isolated phase setup?

Agreed. Typically each set of 350MCM (ABCN) would be run in a separate conduit, so no derating is required (unless high harmonic loads or similar special situations). If all 4 sets (16 wires) have to be combined you can use a short chase conduit (<24") and avoid the derating.