Derating

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joeyww12000

Senior Member
Location
Chatsworth GA
Ok guys Im doing some derating practice this morning and I remember having a conversation with another electrician at work about this. From reading the posts on here I have concluded you start derating from the 90 degree C column of table 310.16 for most applications depending on insulation of wire being used. The question I have is how do you calculate in the lugs at termination points since they are usually not rated for 90 degree C? Would you not derate to allow for the lowest temperature rated material in the circuit?
 
Derate the conductors, according to the insulation rating. (Often, but not always 90?) You don't need to take the rating of the terminations into account - yet.

(4) #8 THHN @ 55 amps
55 x 80% = 44 amps.

However, say, the terminations are only rated 60?

#8 @ 60? = 40 amps. So the #8 would still be allowed 40 amps.

You are still restricted to the lowest rated "link in the chain" for your circuit ampacity, but you can take advantage of the higher rated insulation of the conductor for derating.
 
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