derating because of conduit fill?

[myronelectrician]

I have 7 #12 conductors an 5 #10 conductors all thhn in 1 inch emt.
The run is only 20 feet long he says I have to derate the conductors due to conduit overfill, I don't think so according to the conduit fill chart. Is the
Inspector wrong as I suspect? Thanks Guys.

 

[pierre]

depending on how the neutral conductor configuration is, you will have some adjustments to make. How are the neutrals configured? Give us some more info.

 

[jbfan74]

The inspector is right. Anytime you have more than 3 current carrying conductors in pipe you must derate.
Look at table 310.15(B)(2)(a) to see the percentages.

 

[bphgravity]

Raceway fill and ampacity adjustments/corrections are two separate issues.

See 310.15 of the NEC.

 

[charlie b]

Re: derating because of conduit fill?

. . . he says I have to derate the conductors due to conduit overfill. . . .

If you really mean "overfill," which is to say that you put too many conductors in the conduit and exceeded the 40% limit on cross-sectional area, then that is a violation in itself.

However, if you mean that you have more than three current-carrying conductors in the same conduit, then that triggers a requirement for a derating factor.