# Conduit fill and de-rating



## BruceB (Sep 21, 2009)

Scenario: Panel located in basement of commercial building, feeding about 40 20A circuits on main floor. Thinking about running a few 2" EMT conduits straight up into the ceiling of the main floor (about 20 ft), terminating in a J-box, then spreading out with conduit throughout the floor. Conduit fill table says I can put 101 #12's (all same type insulation), but then the de-rating comes into play. Can this be done, and if not, then can't the conduit fill table be thrown in the trash?


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## MDShunk (Jan 7, 2007)

I think you're going to find it more compliant to run about (8) 3/4" pipes.


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## Celtic (Nov 19, 2007)

BruceB said:


> Conduit fill table says I can put 101 #12's (all same type insulation), but then the de-rating comes into play. Can this be done, and if not, then can't the conduit fill table be thrown in the trash?


Sure you can do it...but those #12's are now derated to 35%.

Upsize the conductors, down size the conduits and use more conduits.


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## RePhase277 (Feb 5, 2008)

BruceB said:


> Can this be done, and if not, then can't the conduit fill table be thrown in the trash?


Sounds like you are going to do it no matter what, anyway.


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## Magnettica (Jan 23, 2007)

The problem with doing the way you propose is that the conductors need to be derated. Now if you don't know how to derate conductors than you need to ask how to derate conductors. #12 THHN is good for 25 amps for derating purposes. So, 35% of 25 amps is 8.75 amps. Now that new "20 amp circuit" is good for only 8 amps. The reason for derating is to contain heat build up inside the conduit.


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## 480sparky (Sep 20, 2007)

Magnettica said:


> The problem with doing the way you propose is that the conductors need to be derated. Now if you don't know how to derate conductors than you need to ask how to derate conductors. #12 THHN is good for 25 amps for derating purposes. So, 35% of 25 amps is 8.75 amps. Now that new "20 amp circuit" is good for only 8 amps. The reason for derating is to contain heat build up inside the conduit.


You must have a different code book than me. Mine says 12 THHN is good for 30 amps.


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## Selectric (Aug 18, 2009)

480sparky said:


> You must have a different code book than me. Mine says 12 THHN is good for 30 amps.


Yeah, so does mine.


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## seo (Oct 28, 2008)

Mine reads the same a 480sparkys.


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## McClary’s Electrical (Feb 21, 2009)

ditto


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## lectro88 (Jul 13, 2009)

Magnettica said:


> The problem with doing the way you propose is that the conductors need to be derated. Now if you don't know how to derate conductors than you need to ask how to derate conductors. #12 THHN is good for 25 amps for derating purposes. So, 35% of 25 amps is 8.75 amps. Now that new "20 amp circuit" is good for only 8 amps. The reason for derating is to contain heat build up inside the conduit.


I'm not attacking anyboby or trying to show that I know more, like the others have stated, for derating purposes ONLY you can use 90 degree table that permits 30A X .35=10.5 A. max amp OCP. per conductor. Agreed Men? This is a backwards way to attempt a install and should be attempted only by a professional.


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## 480sparky (Sep 20, 2007)

lectro88 said:


> I'm not attacking anyboby or trying to show that I know more, like the others have stated, for derating purposes ONLY you can use 90 degree table that permits 30A X .35=10.5 A. max amp OCP. per conductor. Agreed Men? This is a backwards way to attempt a install and should be attempted only by a professional.


To expound on this, ampacity of a conductor starts in 310.15. 



> *310.15 Ampacities for Conductors Rated 0–2000 Volts.*


Usually, the first step is 310.15(B), 



> (B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.16 through Table 310.19, and Ampacity Table 310.20 and Table 310.21 as modified by (B)(1) through (B)(6).


which is then modified in this case by 310.15(B)(2)(a).



> (a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.


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## lectro88 (Jul 13, 2009)

Sorry. Forgot the Ref. 110.14C Temp. Limitations for derating.


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