This is a question that was brought up at work and never really gave it thought before. I did do some research here and in the NEC and wanted to post my findings for thoughts and commits. Basically we have a 1 & 1/4" EMT w/ 22-#12's and #12 gnd. This will be feeding various computer work stations that will not be active even 50% of the time and even then not all circuits will be in use. The contractor brought up the derating issue. Never really thought about it before when dealing w/ 120v, 20a circuits that are not at full load. Usually only worried about it w/ big distribution and services. Anyhow the way I understand table 310.15(b)(2)(a) applies to FULL load conductors and can't really apply to conductors with varying loads. Annex B, Table B.310.11 applies to load diversity and thus can apply to my situation. So using this table (70% derating) and using insulation rating of 90 degree C I think this is legal as per the NEC. Any thoughts, commits, suggestions would be appreciated. Thank you.
#12 conductor derating....
- Thread starter blmoll
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fc
Senior Member
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- New Jersey
Re: #12 conductor derating....
Do a search on this web site and you will find a lot has been said about load diversity. You should find your answer.
Do a search on this web site and you will find a lot has been said about load diversity. You should find your answer.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: #12 conductor derating....
It does not matter that any given station will be running less than half the time. Many people turn their computers off before they go home at night. It might help if not all the stations will be in use at the same time, but how can you be sure that will always be the case? What stops the owner from bringing in extra people to work on a project, and having all the computers running at the same time? If you wish to take advantage of the ?load diversity? discussed in Table B.310.11, then you need to be able to justify the 70% value to the satisfaction of the AHJ. That may require a calculation sealed by a PE. And speaking as a PE, I don?t think I would go along with your proposed configuration.
Why could you not run another conduit or two?
It does not matter that any given station will be running less than half the time. Many people turn their computers off before they go home at night. It might help if not all the stations will be in use at the same time, but how can you be sure that will always be the case? What stops the owner from bringing in extra people to work on a project, and having all the computers running at the same time? If you wish to take advantage of the ?load diversity? discussed in Table B.310.11, then you need to be able to justify the 70% value to the satisfaction of the AHJ. That may require a calculation sealed by a PE. And speaking as a PE, I don?t think I would go along with your proposed configuration.
Why could you not run another conduit or two?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: #12 conductor derating....
This scenario reminds me of work that was typically done in NYC before they adopted the NEC in 2001. It was very common to run a few 1.25" EMT's out of the panel and pull in 24 or so #12 THHN conductors. Prior to 2001 the maximum required derating according to the NYC Electrical Code was 70%. So after derating, #12 THHN was still good for 21 amps. This allowed you to pull in as many as the code allowed according to the maximum conduit fill and still use 20 amp circuit breakers.
Although this system would seemed highly flawed according to NEC standard for derating, I don't recall hearing about any building fires that used this method.
[ March 28, 2005, 06:47 PM: Message edited by: infinity ]
This scenario reminds me of work that was typically done in NYC before they adopted the NEC in 2001. It was very common to run a few 1.25" EMT's out of the panel and pull in 24 or so #12 THHN conductors. Prior to 2001 the maximum required derating according to the NYC Electrical Code was 70%. So after derating, #12 THHN was still good for 21 amps. This allowed you to pull in as many as the code allowed according to the maximum conduit fill and still use 20 amp circuit breakers.
Although this system would seemed highly flawed according to NEC standard for derating, I don't recall hearing about any building fires that used this method.
[ March 28, 2005, 06:47 PM: Message edited by: infinity ]
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