I must take an examination on Friday and am struggling a bit with problems pertaining to ampacities and to the proper procedure for derating ampacities. I show below a sample problem and my proposed solution. I would like to know whether or not the proposed solution is correct, and, if so, whether the underlying logic is correct. If you can help, I would be much obliged!

**Problem**

A 240V/120V single-phase, three-wire feeder circuit supplies a continuous load of 180 A and a non-continuous load of 100 A. Given that:

(1) the terminations for the feeder are rated for 75 degrees C,

(2) the ambient temperature is 40 degrees C,

calculate the size of THHN conductors that are required for this application. What size overcurrent protection device is required?

**Proposed solution**

Step #1: Calculate the true load: 180 A + 100 A = 280 A

Step #2: Adjust true load for presence of continuous load: adjusted load = 1.25 X 180 A + 100 A = 325 A

Step #3: Use result of step #2 to establish minimal size of THHN conductors:

Using 90 degree C column of Table 310.16, see that 350 kcmil THHN conductors are the smallest that have an uncorrected ampacity, 350 A, that is equal to or greater than 325 A.

Step #4: Adjust uncorrected ampacity for 40 degree C temperature

Using the 90 degree C column of Table 310.16, I see that the correct factor is 0.91.

Therefore, adjusted ampacity = 350 A X 0.91 = 318.5 A

Step #5: Compare adjusted ampacity with 75 degree ampacity of 350 kcmil conductors:

318.5 A is greater than 310 A, which is the 75 degree C ampacity of 350 kcmil conductors

Therefore use 310 A rather than 318.5 A as being the allowable ampacity of the conductors

Allowable ampacity = 310 A

Step #6: Compare allowable ampacity with true load:

310 A is greater than 280 A, so conductors will carry the load

Step #7: The conductors must be protected according to their allowable ampacity, which is 310 A. In addition the results of step #2 establishes a minimal rating for the overcurrent protection device, namely 325 A, that actually exceeds 310 A. Unfortunately 325 A does not correspond to a standard size, so we must select the next largest standard size, which is 350 A.

CONCLUSIONS: 350 KCMIL THHN CONDUCTORS SATISFY ALL REQUIREMENTS OF THE NEC PROVIDED THAT THE OVERCURRENT PROTECTION DEVICE IS RATED AT 350 A.

I have a few specific questions:

(1) Is step #3 correct? Approximately 50% of my classmates insist that we must use the 75 degree column even at this early stage of the calculation because the termination devices are rated at 75 degrees C. I do not see how to justify this point of view for circuits in excess of 100 A.

(2) Is the use of the correction factor for 90 degree conductors in step #4 correct?

(3) Is the comparison with the true load in step #6 correct? I note that if the proper comparison should be with the adjusted load, then 350 kcmil conductors would not be acceptable.

If there should be anyone out there in cyberspace with the time and inclination to be of assistance I would greatly appreciate hearing from you.