3

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.

2 Answers 2

7

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).

2
  • Thanks -- I'll update the question, the wires in conduit #1 are assumed to be three individual circuits: Hot 1 + Neutral 1, Hot 2 + Neutral 2, Hot 3 + Neutral 3.
    – Mike
    Dec 29, 2021 at 14:23
  • Just for clarity: People tend to use the words always and never in applying 240.4, but to be completely accurate it says (E)and (G) overrride (D), but neither of those describe most branch circuits. Dec 29, 2021 at 15:21
4

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.


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.


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)


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.

4
  • Thanks for the clarifications -- I don't actually intend on running any wiring this way, it was more of a curiosity that I wondered about. I suppose in the more general case, with the question "Can I use upsized conductors in a feeder conduit, then downsize conductors in a branch conduit", the answer "yes, assuming you follow all other rules correctly."
    – Mike
    Dec 29, 2021 at 14:26
  • I have one follow up question -- regarding 240.4(D), I know it explicitly requires 15A OCPD for a 14 AWG wire. In 210.21(B)(#), it permits us to use two or more 15A receptacles on a 20A circuit. Am I correct in interpreting that since 240.4(D) is more restrictive, I must downsize my OCPD to 15A if I am using 14 AWG wire at any point of the branch circuity? It would then follow that if I was using exclusively 12 AWG wire, I would be permitted to use the 20A OCPD and 15A receptacles.
    – Mike
    Dec 29, 2021 at 14:31
  • You can go as far as one 15A duplex receptacle on a #12 awg 20A circuit since a duplex is two receptacles on a single yoke. (See definitions in Article 100.) Dec 29, 2021 at 15:10
  • @Mike That is correct, any #14 condemns the whole circuit to 15A. (except for some really weird exceptions in industrial wiring that'll never come up in a residence). Both sockets don't have to be 15A, so a 15A "single"/simplex and a 20A simplex would suffice. Dec 29, 2021 at 20:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.