I understand derating based on bundling and temperature, but how does conduit size come into play. NEC states 16 #12AWG in 3/4 emt based on 40% fill. Bundling adjustments decrease the ampacity of #12 THHN to 15A based on 10 - 20 conductors in a raceway and 21A based on 7 - 9 conductors in a raceway. Does this mean that if I run a 1 1/2" emt that I can only pull 7 - 9 #12 current carrying conductors through it! I hope someone can clear this up since its been a pretty hot topic for arguments on the job.
Derating and conduit size
- Thread starter sparky519
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- Chapel Hill, NC
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- Retired Electrical Contractor
Conduit size has nothing to do with derating. You can have a 2" emt with 7 #12 thhn conductors and you still need to derate.
- Location
- Massachusetts
It doesn't at all.
Ten - 14 AWGs in a 4" EMT would get the same derating as ten -14 AWGs in 3/4" EMT.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
#12 thhn is rated 30 amps. At 70%- 7-9 conductors- you can still use a 20 amp cb on it. Once you go over that and even with a 4" conduit you must derate 50%-- thus you would have to upsize your conductors to #8 to be able to use a 20 amp OCPD.
conduit locations
conduit locations
I agree that bundling is different for adjustment Factors of conductor carrying current quantity. Maybe looking into using Correction Factors (2008 Table 310.16) for temperature environments of conduit (I.e. size on roofs) would be a better answer to what you are asking. The Copper Development Association (CDA) has a great slide calculator and pamphlet for outdoor design temperatures that lists 500 cities in the US for conduit 0n-roof inside temperatures derived from the 30 year ASHRAE Handbook. Google up www.copper.org and search for the free Derating pamphlet online. Hope this helps.
conduit locations
I agree that bundling is different for adjustment Factors of conductor carrying current quantity. Maybe looking into using Correction Factors (2008 Table 310.16) for temperature environments of conduit (I.e. size on roofs) would be a better answer to what you are asking. The Copper Development Association (CDA) has a great slide calculator and pamphlet for outdoor design temperatures that lists 500 cities in the US for conduit 0n-roof inside temperatures derived from the 30 year ASHRAE Handbook. Google up www.copper.org and search for the free Derating pamphlet online. Hope this helps.
Back in the 90's, there used to be a blurb in the code in 310 probably be 310.8 that would allow you to not have to derate for heat - if, and only if- your pipe was 10% or less of the total length of th conductor. It was a nice "gothcha" moment on testing...
Thinking this is what you are referring to... It's no longer in the NEC
Thinking this is what you are referring to... It's no longer in the NEC
- Location
- Massachusetts
Moved, maybe changed?
If your question is, "Does empty internal space in the conduit affect derating?", IMO it should, but maybe not enough to show up in tables or else the code just rounds off on the side of safety.
With an almost empty conduit the conductor insulation mostly sees the temperature of the air inside the conduit.
With an almost full conduit the conductor insulation mostly sees the inside surface temp of the conduit, and with metal being a better thermal conductor than air the conductor insulation should see a temp. nearly the same as the ambient air outside the conduit, so this second case should have a lower conductor temp. rise above ambient, given equal currents in both cases.
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
If the conduit is horizontal all the conductors would be bundled on top of each other and heating would occur. Perhaps there should be an exception for vertical runs and percentage fill. :grin:
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