Should you derate twice for two separate conditions?

[Sora Komi]

Hi - I have come across a situation where there seem to be to derating conditions that could apply to a wiring
project. It is for a test lab, and the products in the test lab will be run continuously (continuous use) and also at
ambient temperatures as high as 140 degrees F. So, in calculating the wire size, do I derate for both the ambient
temperature and continuous duty, or just one condition? And how is the derate calculation performed if both
conditions need to be accounted for? Thanks for the help, I have never run across this before, and haven't found
an answer that I trust online. (oh, and the minimum circuit ampacity of the unit is 500 amps, if that makes any difference.)

 

[wwhitney]

First, as to the temperature effect on the equipment terminations, that is to be handled by the equipment. Its specs or manual should state that 140F =60C ambient is acceptable, and if any conductor upsizing is required at the terminations, should specify that. The 125% factor for continuous loads is part of this termination sizing.

Then, as to the derating due to temperature, it applies to the run of the wire between the equipment and the source. The 125% factor for continuous loads does not apply here; the ampacity of a wire is already a continuous rating.

So as an example, if your equipment requires 40A continuous, the termination wire size would be 50A, from either the 60C or 75C column depending on the termination rating (assuming the equipment doesn't specify larger wires). For the run of the circuit at 60C ambient, assuming 90C conductor insulation, the ampacity needs to be 40A, but the column value gets derating by a factor of sqrt((90-60)/(90-30)) = 0.707, so you'd need a conductor whose 90C column entry is at least 40A / 0.707 = 57A.

Cheers, Wayne