310-16 Col 3 90 degree

[Irving Marshall]

can you use 3/0 thhn for 250amps 3 phase outside A/C unit ran in rigid conduit on top
of concrete pad outside according to 310-16 col. 3 90 degree

 

[bphgravity]

Maybe. What is the RLC or BCSC rating of the A/C unit? It may be possible to have 3/0 conductors based on sizing requirements in 440.32, however you are limited by correction factors and termination temperature ratings.

 

[drbond24]

Before you worry about using 90 C wire, make sure the terminations on both ends are rated for 90 C. Electricians around here are always wanting to use the 90 C ampacity of the wire to make the size smaller but I've never had a case where the breaker on both ends of the circuit was rated for 90 C. Your circuit is only as good as the LOWEST rating, which is usually 60/75 C.

 

[infinity]

I would forget about the 90 degree conductor rating unless you're calculating derating. The unit should be marked on the nameplate with a Minimum Circuit Ampacity (MCA). This number will be used to size the conductors. The OCPD can be larger than the typical sizes listed in 310.16, up to the Maximum Overcurrent Protection Device listed on the nameplate. You really need to provide more information to properly answer the question.

 

[mtbkl]

FYI: I was looking at using the 90C table listed conductors myself, but admit I forgot about terminations being rated for 90C. Anyhow, I called a couple conductors suppliers/manufacturers to ask about the differences between THWN (75C) and THWN-2 (90C). To sum it up, pretty much all THWN conductors #8 and above are rated as THWN-2. Seems to be an industry standard from what the suppliers/manufacturers informed me.

Also, If I recall right *not that my memory is the greatest at times, lol*, I think the THWN is listed for wet/dry conditions at 75C and only dry conditions at 90C whereas the THWN is listed for both wet/dry at 90C. Again, what the suppliers/manufacturers informed me.

 

[infinity]

Also, If I recall right *not that my memory is the greatest at times, lol*, I think the THWN is listed for wet/dry conditions at 75C and only dry conditions at 90C whereas the THWN is listed for both wet/dry at 90C. Again, what the suppliers/manufacturers informed me.

THWN is listed for 75 degrees C in wet or dry locations, THWN-2 is listed for 90 degrees C wet or dry. Look at table 310.13(A).

 

[mtbkl]

THWN is listed for 75 degrees C in wet or dry locations, THWN-2 is listed for 90 degrees C wet or dry. Look at table 310.13(A).

Opps. Forgot to put the "-2" on the second THWN. So it would be that THWN is listed for wet/dry conditions at 75C and only dry conditions at 90C whereas the THWN-2 is listed for both wet/dry at 90C. Again, what the suppliers/manufacturers informed me. That should be inline w/ 310.13(A)