I have been told in many NEC classes that a #4 THWN 95 Deg. cable cannot be protected with a 100-amp circuit breaker. The reason is that the terminals on the circuit breaker are rated for 75 Deg. Therefore I should use the ampacity listed in the 75 Deg column which is 85-amps and I should use a 90-amp circuit breaker. Is this true? We have completed several audits and we see #4 wire (not necessarily sure of the temp rating) used with a 100-amp rated circuit breaker. Again is this allowed per the NEC?
Table 310.16
- Thread starter brandtew
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This is true basically. T. 310.15(B)(6) does allow an except for #4 as a service conductor for 100 amps but otherwise #4, if all termination and wire is 75C then you may use 75C column.
If the #4 is part of a nm cable or ser used as a feeder then you must use the 60C column. 334.80
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It depends on the application. In general, a #4 Thwn branch circuit or feeder would need to be protected by a 90A OCPD. But the specific applications listed in 240.4(G) may allow #4 to be protected by a higher rated circuit breaker (such as a motor application.)
Also, if this was a 120/240 single phase, 3 wire dwelling service or feeder, #4 THWN would be allowed for a 100A service (or feeder) by T310.15(B)(6).
Also, if this was a 120/240 single phase, 3 wire dwelling service or feeder, #4 THWN would be allowed for a 100A service (or feeder) by T310.15(B)(6).
The issue regarding the 75 deg ampacity is correct see 110.14(C).
Also insure that the load on that number 4 is not more than 85A. The 90 degree col is used for derating.
Regarding the 4AWG Cu protected at 100A ?? This is very common. (not always legal) lf the No 4 is used to comply with the provisions of 310.15(B) (6) then a 100 A OCPD may be OK, but this is not always the case.
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