Here's the hypothetical scenario. I have two segments of conduit:
- Conduit 1 (Distributor), direct from electrical panel to a distribution junction box 1
- Conduit 2 (Branch), from distribution junction box 1 towards branch junction box 2 with a single outlet
Per NEC table 310.15(B)(3)(a), I must derate ampacity relative to tables 310.15(B)(16) through 310.15(B)(19):
- For 4 to 6 conductors, derate to 80% of its table maximum value
- For 7 to 9 conductors, derate to 70% of its table maximum value
The question is: Would I be allowed to run the following wires in each conduit, or do I need to upsize the wires in Conduit 2?
- Conduit 1: 12 AWG THHN (Ground), 6 x 12 AWG THHN (Circuits 1, 2, 3, 120V Line + Neutral)
- Conduit 2: 14 AWG Ground, 2 x 14 AWG THHN (Circuit 1)
Assuming all terminations are 75C rated, my reasoning is:
- 12 AWG feeders in Conduit 1 have ampacity of at least 21A after derating, assuming standard ambient temperatures.
- The Conduit 2 raceway only contains 2 current carrying conductors, thus I do not need to derate them. On for 14 AWG THHN, can I use the 75C ampacity of 20A?
Conduit 2 would only ever feed a 15A or 20A outlet, which in turn is allowed to be protected by a 20A breaker.