Multiconductor derating - which temperature rating?

[lielec11]

I have to de-rate conductors because they want to run more than 3 in a raceway. My question is which temperature column do I use in chart 310.15(B)(16)? I remember readsing somewhere you could use the 90degC column but can't find it.

Also, an example in the 2014 code uses the 90degC column (page 263 of the handbook) so that leads me to believe this as well.

Thanks!

 

[david luchini]

I remember reading somewhere you could use the 90degC column but can't find it.

What you remember is found in 310.15(B).

 

[infinity]

Use whatever the ampacity is for that insulation type, for example THHN is a 90° C conductor so you would use the 90° C ampacity.

 

[lielec11]

310.15(B) transfers me to 110.14(C) which stipulates "... Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both". Guess that's it... so thanks!

 

[Julius Right]

See also NEC Art.310.15 (B):
The temperature correction and adjustment factors shall
be permitted to be applied to the ampacity for the temperature
rating of the conductor, if the corrected and adjusted
ampacity does not exceed the ampacity for the temperature
rating of the termination in accordance with the provisions
of 110.14(C).

 

[Carultch]

I have to de-rate conductors because they want to run more than 3 in a raceway. My question is which temperature column do I use in chart 310.15(B)(16)? I remember readsing somewhere you could use the 90degC column but can't find it.

Also, an example in the 2014 code uses the 90degC column (page 263 of the handbook) so that leads me to believe this as well.

Thanks!

Most single conductor wire made today has a 90C insulation rating. The ampacity is usually going to be limited to either the 60C or (usually) 75C column of 310.15(B)(16) for terminations, but the value in 90C wire is that it gives you flexibility to make your derate factor calculations. The two derate factors compound together (i.e. multiply), if you have both more than 3 CCC's in a raceway and more than 30C ambient.

You don't apply a bundling derate factor to the terminations, because wire isn't bundled at the terminations. Less intuitive is that you also don't need to apply an ambient temperature derate (or rooftop elevated temperature derate) to the termination rating either.