Another derating question

[theshockdoc]

Here is the scenerio. A raceway contains three 3 phase, 3 wire circuits which supply three seperate 45 amp continous loads. The conductors are THHN and the terminations are 75 degrees.

The way I've been lookig at it. I multiple my loads of 45 amps by 1.25 continous factor which gives me 56.25 amps. The OCPD I select is 60 amps. I go to 310.16 and select a #8 conductor rated for 65 amps at 75 degree terminals but I take the value of 75 amps from the 90 degree value and derate at 70% (9 current carrying conductors) 75 x .7 = 52.5 amps. Here's where I have a problem. I can either 1) use the #8 because I selected it at 65 amps before applying any derating factor and once derated to 52.5 I can still protect the conductors at the next standard OCPD of 60 amps or 2) now that I've derated the conductors to 52.5 amps the conductors are below the continous load calculation of 56.25 and I must select a higher conductor value.

Thanks

 

[iwire]

You need to use 6 AWG or separate one of the circuits.

As you showed yourself a 52 amp rated conductor is not large enough for a 56 amp load.


Edit, based on Bob's comment you will need 4 AWG

 

[Bob NH]

Here is the scenerio. A raceway contains three 3 phase, 3 wire circuits which supply three seperate 45 amp continous loads. The conductors are THHN and the terminations are 75 degrees.

The way I've been lookig at it. I multiple my loads of 45 amps by 1.25 continous factor which gives me 56.25 amps. The OCPD I select is 60 amps. I go to 310.16 and select a #8 conductor rated for 65 amps at 75 degree terminals but I take the value of 75 amps from the 90 degree value and derate at 70% (9 current carrying conductors) 75 x .7 = 52.5 amps. Here's where I have a problem. I can either 1) use the #8 because I selected it at 65 amps before applying any derating factor and once derated to 52.5 I can still protect the conductors at the next standard OCPD of 60 amps or 2) now that I've derated the conductors to 52.5 amps the conductors are below the continous load calculation of 56.25 and I must select a higher conductor value.

Thanks

You must have a different 310.16 that I do. Mine says that #8 is rated for 50 Amps at 75 C. I think you picked your Ampacity values from the #6 row.

 

[theshockdoc]

I was so concerned with getting the wording of my question right I overlooked the obvious. The original conductor should have been a #6 not a #8. Thanks