derating calc after engineer calcs

[220wire]

Could I use the va values that an engineer states on their plans as part of the derating calculation?

My example is I have an office building with about 900va calcd for the load
on each office circuit. I'd figure that the load would be mostly linear and that the grounded conductor will count as a current carrying conductor.

I want to put as many 120v circuits with dedicated grounded conductors as I am allowed.
If this were true maybe 9 #12's in a 1/2" emt?

 

[charlie b]

You can use 9 #12 conductors in the same conduit, even if all count as current-carrying conductors, presuming that you are using a 90C rated wire, and the amount of load on any circuit has nothing to do with the derating calculation. A #12 is good for 30 amps at 90C. Multiply that by a 70% derating factor for 9 CCCs, and you get 21 amps. You can still use a 20 amp breaker to protect that set of conductors. Note that I did not use the value of 900 VA in any way.

 

[220wire]

So if my breaker's terminals are only rated 75*, does that all go out the window?

 

[charlie b]

Not at all. The 75C rating of a #12 wire with 90C insulation is 25 amps. We have to protect it with nothing higher than 20 amp breaker for reasons that do not have to do with ampacity. So when you derate from 30 amps to 21 amps, you are still higher than 20, and that is less than the 75C rating of the wire, so the resultant ampacity value is still 20 amps.

 

[220wire]

thanks charlieb. I gotta quit second guessing myself

 

[chris kennedy]

110.14(C) allows you to start derating above the terminal rating.

110.14(C) Temperature Limitations.
The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.