NCEES Power problem 104

[BamaBino]

I agree that the answer to this question is the AWG 4 conductor.

But since the circuit is at less than 100amps and according to NEC Section 110.14C1, shouldn't the left-most column for 60 deg C be used?

In this particular problem with the current 66.7A, the 60 and 75 deg C columns both say to use AWG 4.

But let's says the total current was was 71A, then doesn't 110.14C1 say that AWG 3 is the minimum size?

 

[dray]

The 60 deg should be used for circuits rated at 100A or less so yes to your first question and yes to your second question.

 

[ElecPwrPEOct11]

I wasn't previously aware of this Code requirement. Since this question asks only about the minimum conductor size, if the total current was 71A I would still use the 75o column. This basically assumes that all of the equipment (terminal blocks, fittings, etc) is rated for this temperature (per 110.14©(1)(a)(3)). You might have to use the 60 degree column if your equipment isn't rated for 75, but best case (minimum size) scenario is still #6 for a 71A load.

 

[EEVA PE]

The way I understand this is if the circuit is less than 100 A and all equipment is satisfied under 75 deg C, you still must check that same size conductor meets the conditions for 60 deg C. Then you can use the 75 deg C conductor. There are also other conditions that are listed in 110.14 that will allow use of 75 deg C conductor without satisfying the 60 deg C requirement.

 

[ElecPwrPEOct11]

This issue came up again in the CI test 2 problem 72. They specify 90 degree conductor but the total load is 62.5A. The answer is #6 conductor, which I agree with. I think that you can use the 90 degree ampacity column of Table 310.15 since you can buy 90 degree rated equipment & terminations to match.

110.14©(1)(a)(3) states: "Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors".

I'd like to get a consensus on this question since it is likely we'll see a motor conductor sizing question on the test. Any permanent forum residents want to weigh in? Thanks!