derating conductors in conduit

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pagano21

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Hey guys need some input.Ran into a situation on a job another electrican in our company installed. He ran 12 current carrying conductors in a 1 inch conduit to supply computer equipment that only pulled 2 amps on each circut. The Inspector said according to the derating in tabel 310.15b2a you can only run 9 # 12 conductors per pipe if you plan to install on 20 amp breakers. 30 amps for # 12 times 70% per table 310-16. Now I know this rule gets broke all the time when there is min. load. What do You all think? Mike P. JW Wireman
Thanks every one
 
Re: derating conductors in conduit

I would ask the following questions:
1. Is the voltage 120/240 single phase or 120/208 3 phase?

2. Did you include the neutral in the 12 conductors?
With the load you mentioned there may be harmonics involved which can change the answer.
I do not see an exception that will allow you the skip the derating for a load of 2 amps.
 
Re: derating conductors in conduit

The Inspector is right. The purpose of the breaker is to protect the wire from an accidental or inadvertent overcurrent. The amount of current normally drawn by the load is not relevant.

If you run 9 #12 wires in a conduit, then each is good for 21 amps, and you can protect each with a 20 amp breaker. If you run 12 #12 wires in a conduit, then each is only good for 15 amps (i.e., 30 times a 50% derating factor), and you cannot use a 20 amp breaker. You can use a 15 amp breaker, or limit the run to 9 wires, or use a larger wire (a #10 would work, since 40 times 50% is 20 amps).

I'll not comment on the comment, "I know this rule gets broke all the time...."
 
Re: derating conductors in conduit

Thanks Guys. I agree with you 100% I just wanted some input. The service is 120/208 3 phase and the neutrals are counted in the calc. I understand derating but my partner does not so I wanted to take him some other Electricians and inspector comments.He did not believe the Inspector was right. Thanks again Mike P
 
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