Double derating - heard the term, what scenario would imply

[bjp_ne_elec]

(Just a note on the Subject line - I wanted to say - "what scenario would imply it? - but the field is a few characters shy)

Have heard the term "double derating" and references to 215.2(A)(1) - - but what does this really imply? It's been inferred, that this section is written so this double derating does not occur, but I'm having difficulty pulling that interpretation out of the verbiage.

If you have a mixed load of say 80 amps continuous and 100 non-continuous - is there a different result if one scenario was fed by three conductors in a raceway and the other one happened to be fed with four conductors? In the four conductor case, you have a situation where 310.15(B)(2)(a) applies.

Thanks

Brett

 

[don_resqcapt19]

Brett,
The rule in 215 calls for a minimum ampacity for the conductors in the circuit. The adjustment factors in 310.15(B)(2) and the temperature correction factors at the bottom of the ampacity tables actually change the ampacity of the conductors. All three factors apply.
Don

 

[haskindm]

You may have to "double derate" if you have over three current carrying conductors in a raceway and a high ambient temperature. You will need to derate for both conditions.