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I am planning to run two sets of four-wire cables (two hots, one neutral, and one ground) through the same conduit to feed two subpanels from a single main breaker panel. I understand that I need to derate the ampacity of the cables to 80% because I am running them in conduit without maintaining spacing, but I was wondering this table, the very last section shows the 100% rated ampacity for SE cable as a feeder, but that seems to assume 90°C terminal ratings, which the subpanels (see page 6 here) are not rated for. They have a 75°C terminal rating.

Does this mean that I need to take 80% of 75°C rating presented in the first table to get my conductor size (ex. 1GA Cu conductors for 100A capacity) or can I take 80% of the feeder wire specs (ex. 2GA Cu conductors for 100A capacity).

I understand the difference is literally one wire size, so it's probably best to oversize than undersize it, but I want to know how I should be reading those tables properly.

  • As far as ampacity derate, neutral and ground do not count. You have 4 wires in conduit. – Harper Jan 11 '17 at 21:55
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    I don't have my code book handy so I will post this as a comment. You can use the ampacity of the wire at 90 for derating but you can not exceede the 75 deg table for a final value. Also if your planned loads are continuous (+3 hour loads) that portion of the load needs to be at 125%. Unless I am setting up a sub for a dedicated non continuous load I use 125% of the feeder breaker size. But using the specs for 90 deg and derating is legal. – Ed Beal Jan 11 '17 at 21:58
  • @Harper, 4-6 counts as an 80% derating. – Hari Ganti Jan 11 '17 at 23:05
  • @EdBeal, Thanks! So it's actually a combination of the two ways I mentioned. Also, my example of 100A is 125% continuous + 100% non-continuous. – Hari Ganti Jan 11 '17 at 23:07
  • @EdBeal, If you've had a chance to look up the code, I'd be happy to properly mark your answer as accepted, if the code validates your comment. Does it warrant a proper answer? – Hari Ganti Jan 13 '17 at 7:27
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Here is the code references per 2014 NEC: 110.14.C- Conductors with temperature ratings higher than specified terminations shall be permitted to be used for ampacity adjustments corrections or both. 215.15.A- where a feeder supplies a combination of continuous loads or any combination of continuous and non continuous loads the minimum feeder size shall have an ampacity not less than the non continuous loads plus 125% of the continuous loads. Table 310.15.B.3.A- adjustment factors for more than 3 current carrying conductors. 4-6= 80% original comment:
I don't have my code book handy so I will post this as a comment. You can use the ampacity of the wire at 90 for derating but you can not exceede the 75 deg table for a final value. Also if your planned loads are continuous (+3 hour loads) that portion of the load needs to be at 125%. Unless I am setting up a sub for a dedicated non continuous load I use 125% of the feeder breaker size. But using the specs for 90 deg and derating is legal.

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