# De-rating



## Deep Cover (Dec 8, 2012)

If you did it their way you would be derating the circuit twice.


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## smiley mcrib (Sep 25, 2011)

You upside the wire which increases the amps to compensate for derating.

OR you downside the breaker size.

Example - number twelve good for twenty five amps, derate it because you have nine CCC, which say gives you 18 amps. so you have to upsize the wire to ten, which are all on a twenty amp breaker.

Or you downside the breaker to a 15 amp and be fine, since after derating the 12 awg is still good for a maximum of 18 amps.

All numbers are just thrown out there, for those of you who will do the actual math and correct me


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## wildleg (Apr 12, 2009)

keepdry said:


> Have I been doing this wrong??
> In situations where I have multiple multi-wire branch circuits in EMT:
> Where the prints call for 20 Amp breakers:
> And I have more than 9 current carrying conductors in said EMT:
> ...


you must have fallen asleep in class, because you are missing the point entirely (although you are correct).

the point is this: the ampacity of the conductors need to be adjusted for bundling, ambient temperature, and wire fill. This adjustment to the ampacity means that if you go up to a suitable wire size and derate or adjust it you will end up with an ampacity that is suitable for the current you want to place on the conductor, or alternatively you could lower the current possible by providing a lower fuse or breaker, but in either case, you are correcting things so that the *allowable ampacity of the conductor is not exceeded *for the installation


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## Barjack (Mar 28, 2010)

Ampacity is a conductor's ability to carry current. The higher the ampacity, the more current it can carry. It has NOTHING to do with the size of the over current device. When you derate a conductor, you are reducing its ampacity due to certain factors, like amount of CCC's in a raceway, ambient temp, rooftop temp, etc. 

Check out Art. 310

Nowhere in the NEC does it say to apply a smaller OCPD instead of derating the conductors.

This practice could potentially create an intentionally overloaded circuit.


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## keepdry (Jul 24, 2012)

Thanks for the help. 
Barack, you explained it exactly as I understand it. 
Thanks for the confirmation.


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## bradly26 (Apr 19, 2013)

If your insulation type falls in the 90 c column use 30 amps to de-rate.
12 ccc correction factor is 50% which makes #12 cu good for 15 amps. That is the reason you have to use the smaller breaker. But that is for #12cu. #10 in the 90 c column is good for 40 amps so after the 50% correction factor it leaves you with 20 amps. There would be no need to use a smaller breaker.


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