Wire Calculations

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rsalerno

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When sizing for voltage drop, temperature correction or de-rating of THHN, is it permissible to use the ampacity of the THHN in the 90 degree table to get your corrected ampacity?
 
Welcome to the forum.:)

Yes, you can start you adjustment factors using the ampacity from the 90 degree column. Keep in mind that the final adjusted ampacity can't exceed the ampacity values for the temperature rating of the terminals the conductors are connected to.

Chris
 
I thought you could but I am having trouble finding any reference in the NEC that states you can. Is there a code reference?
 
rsalerno said:
I thought you could but I am having trouble finding any reference in the NEC that states you can. Is there a code reference?
Click to expand...

Take a look at 110.14(C), specifically the last sentence....
Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Click to expand...

Chris
 
rsalerno said:
When sizing for voltage drop, temperature correction or de-rating of THHN, is it permissible to use the ampacity of the THHN in the 90 degree table to get your corrected ampacity?
Click to expand...

To be a little more specific, the final ampacity of the calculated conductor size will have to correlate with either the 75degree column or the 60degree column depending on the insulation Type. The 90degree column is used if the conductor Type is found in that column. That would be the amperage you start the calculation at.
 
Pierre,
And the 20Amp limit for #12
applies after the mathmatical derated ampacity is found.

I've had men tell me that they derated from 90 degrees C,
and can load it to 27 Amps!

I passed another inspection yesterday.
Electricians and Inspectors
are just on the other sides of the same street.
:smile:
 
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