Conductor ampacity derating

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bhernandez

Member
Location
Florida
I recently failed a rough inspection becuase the inspector rejected the use of 9 #12 THWN Cu conductors in 3/4" ENMT that will be be on 20A breakers. He is basing his decision on using 20 amps as the starting point of the 70% derating. I explained that for derating purposes you can use the 30 amp listed in 310-16 for 90C #12 THHN CU (20 amp being the maximum overcurrent protection allowed). 70% of 30a is 21 amps and ok for use with 20a breakers. Am I right or wrong?
 
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david

Senior Member
Location
Pennsylvania
Re: Conductor ampacity derating

Are there no other current carrying conductors in the ? conduit. Is this a three phase or single phase system.
I agree you would start with the 30 amps for de-rating in the example you have given.
 
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rb

Member
Location
Tennessee
Re: Conductor ampacity derating

Your post indicates that THWN conductors are used in the ENT. Table 310.13 identifies THWN as a 75 degree conductor. This would require the derating to be applied from the 25 amp rating in the 75 degree column of Table 310.16. If these conductors are part of a multioutlet branch circuit supplying plug connected loads, 240.3(b) will prohibit the rounding up to the next standard size breaker and a 15 amp circuit breaker would be the maximum overcurrent device permitted to protect these conductors.

More likely than not, these conductors are actually THWN-2 conductors and the inspector is overlooking Footnote No. 4 to Table 310.13. In which case you are right and the inspector is wrong
 
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