Derating conductors

[freynolds]

In derating conductors/ 8 #12 THHN in pipe with 60 degree celsis Chpater 9 Table 4 gives ambient temp, do you then derate number of wires in pipe or temperature first? Can someone help me on this \. Thank you.

 

[winnie]

It doesn't matter which you do first. Multiplication is 'commutative', meaning that A * B * C = A * C * B

You just multiply the ampacity by the factor for number of conductors in a conduit and for the ambient temperature, and it doesn't matter which order you do it in.

-Jon

 

[charlie b]

In derating conductors, 8 #12 THHN in pipe, with 60 degree Celsius . . . .

Are you saying the ambient temperature is that high (140 degrees F)? That is abnormally high in any location. But you are talking about a test question, not a real situation, and I agree with Winnie’s answer.

However, you should take care to start with the ampacity value in the 90C column, when you apply derating factors, because your conductors are rated for 90C. For #12 THHN, that means you start with 30 amps.

You then apply a factor of 0.71 for an ambient temperature of 60C. You take that value from the 90C column as well. You also apply a factor of 70% for having 8 conductors in the conduit. You didn’t say otherwise, so I will assume all 8 conductors will be carrying current.

So 30 times 0.71 times 0.7 gives you 14.9. That is the ampacity of your conductors.

Please note that if the derated value was above 20 amps, the value in the 75C column for THHN, then the ampacity would be 20, and not the higher calculated value.

 

[freynolds]

Thanks Charlie, yes it is a test question, appreciated you taking the time to answer this question for me it is a great help, Thanks a lot.

F Reynolds Evansville Indiana