Derating and OCPD


Old 04-21-05, 11:57 AM
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Join Date: Oct 2002
Posts: 161
Derating and OCPD


I have a load (120v) that requires derating the conductors that run in a raceway - 310.15B2a. Before derating, the OCPD was 15a. My conductors are now larger in their ampicity.

Since my conductors are now larger, do I also have to increase the size of the OCPD?

Please let me know what you all think.

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Old 04-21-05, 12:22 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
There are a whole bunch of numbers missing from your post, so it's not possible to be specific, but you never have to increase the size of the OCPD just because the wires are bigger.
Old 04-21-05, 12:36 PM
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Join Date: Oct 2002
Posts: 161
Hi John,

Voltage= 120

Continuous load (non-motor) - 10x1.25=12.5a
OCPD - Next size up=15a
Conductors - 14AWG

14 condcutors will be running through the raceway. Derate at 50%.

Using the 90 degree column I find that #12 can be derated by 50% and provide 15a of ampacity. (However, on the plan one of the keynotes states that they want to run #10 in these particular circuits (?)).

So, if I 'm using #10, which has a higher ampacity than 15a (my OCPD), should I resize the breaker to accomodate the ampacity of the #10 or is the load just the load, and I've already sized it correctly.

Old 04-22-05, 01:54 AM
Join Date: Nov 2002
Location: SC
Posts: 171
Sorry to butt in, couldn't stop myself. Ques? Are all conductors current carrying? If so and you have a continuous load of 12.5A (such as lighting) you need a circuit at least 125% of the load, which would put it at 15.625A, so you need a 20A circuit. #10 90C derated 50% will give you that 20A. Note that when you derate the conductors, that is their new full load carrying capacity. In this instance you could never put a OCPD over 20A on the #10s.(Unless used for motor starting)

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