Does NEC allow mixing wire gauge in distinct sections of conduit when derating?

Question

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.

Answer 1

You do not need to derate the wire in conduit #2 based on wire fill, but the ampacity of #14 for receptacle circuits is still limited by 240.4(D) to 15A.

The description of wires in conduit #1 is a little confusing, but from a 240/120v single phase service 4 hots and 2 neutrals properly configured as two multiwire branch circuits would count as 4 current carrying conductors requiring 80%, or 4 hots and 4 neutrals would count as 8 current carrying requiring 70%, again subject to 240.4.

3 hots and 3 neutrals would count as 6, derated to 80%, which using 75°C or 90°C rated conductors does not reduce capacity below the maximum ampacity for most circuits of 15A for #14 awg copper in 240.4(D)(2) and 20A for #12 AWG copper in 240.4(D)(5).

Answer 2

The answer to your title question is "yes" but your example has other flaws.

You do not derate off the 90C number in the table. You derate off the maximum thermal value for that wire. For instance THW and THWN are stated in the 75C column, so they derate off their 75C number, not 90C. TW and UF are in the 60C column, so they derate off 60C.

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You are too cavalier to use the 75C number for small wires. Have you considered "why doesn't anyone else?" What you're missing is a) both terminations of that wire must be rated for 75C. Circuit breakers are; receptacles and switches aren't. And b) as NoSparksPlease discusses, 240.4(D) nixes the 75C column for #10 and smaller wires.

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In conduit #1, you only count active conductors. Delve deeper into 310.15(B)(3) and you'll see that grounds do not count (they never flow current except during fault conditions), and neutrals in between hots don't count because they only carry differential current (meaning some other conductor is carrying less).

These rules mean that if your energy source is a split-phase service (neutral between the live conductors), all circuits (a MWBC is 1 circuit) count for 2 conductors re: 310.15(B)(3)(a). Thus the derates are:

• 2-3 circuits: 80% of top ampacity (no concern to 15-40A circuits)
• 4 circuits: 70% (no concern to 15-28A circuits)
• 5-10 circuits: 50% (figure a wire size bump)
• 11-20 circuits: 40% (figure two bumps)

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So if we change your example to "eight 20A circuits in the fat distribution conduit" and "four 20A circuits on the branch conduit", then the distribution conduit would need #10 wire and the branch could use #12 wire. You can indeed splice in the junction box.