Home Improvement → [Electrical] Conductor derating question

For individual conductors in conduit where derating must be applied for # of current carrying conductors, am I right that the overall capacity of the circuit is determined by the lower of:

-The derated ampacity of the conductor in the conduit based on the temperature rating of the conductor insulation
-The ampacity of the conductor based on the temperature rating of the terminations

In other words, is the derating factor for conduit fill applied to the ampacity of the conductor insulation or the termination temperature rating?

Logic says that it should be the lower of the two. But sometimes logic gets lost in translation.

For #12 and #14 copper wire, the temperature rating of the terminations (60 degrees C) will likely be the limiting factor. For modern wire (NM-B, THHN/THWN-2), the derate starts at the 90 degree C values (25A for #14, 30A for #12) which allows you to maintain the 15A/20A 60 degree value imposed by receptacles up to 9 current carrying conductors (and the hard and fast limits imposed by 240.4D (3),(4),(5)). More than 10 current carrying conductors in the raceway, you're going to have to bump up to the next wire size even for 60C outlets.

I think #10 wire's a little trickier since 7-9 conductors derates it to 28A.

Thanks. The situation is #6 THWN-2 (3 current carrying) and #14 THWN-2 (2 current carrying) in a single pipe. 15A OCPD on the #14 and 60A on the #6.

The termination temperature rating is the first to be considered. If the device -is- temperature rated, then that is the temperature you use for the calculation. If the device -is NOT- rated, then the 60 deg. C rating applies regardless, and conductor ampacities must be selected from the 60 degree column.

The temperature rating of the conductor is then considered. It might be advantageous to use higher temperature rated conductors even though you are using a conductor with a lower termination rating as the cross-sectional diameter is smaller (typically) and that affects the fill rules.

Maximum ambient temperature of the installation location needs to be considered, too. If the conduit is run on the roof (for example) exposed to direct sun, you'd need to further derate for those temps (usually scorching hot) and the wire gets big real fast.

So on and so forth...

Derating and fill calculations are *so* much fun...lol.

Good article that discusses the topic:

»iaeimagazine.org/magazin ··· nations/

Thanks, John, that's perfect. For future reference, here's the section that directly answers the question in my original post:

Higher-rated conductors and derating factors

One advantage for conductors with higher insulation ratings is when derating factors are applied. This advantage is noted in the last sentence of NEC 110.14(c): "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both." Derating factors may be required because of the number of conductors in a conduit, higher ambient temperatures or internal design requirements for a facility. By beginning the derating process at the ampacity of the conductor based on the higher insulation value, one may not be required to upsize the conductor to compensate for the derating. Installers/designers should bear in mind the following points (see the accompanying sidebar for an example):

%u2022 The ampacity value determined after applying the derating factors must beequal toorless thanthe ampacity of the conductor based on the temperature limitations at its terminations.

%u2022 The derated ampacity becomes the allowable ampacity of the conductor, and the conductor must be protected against overcurrent in accordance with this allowable ampacity.

Derating of the conductor is based on the temp rating of the CONDUCTOR, NOT the terminations.

The temp rating of the terminations is taken into account when figuring ampacity of the whole circuit.

In other words, it's similar to how NM cable circuit ampacity must be based on the 60 deg C column, but if you have to derate you use the 90 deg C column since that is the rating of the conductor.

It's a "three-legged stool" kind of thing...