Does anyone derate

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JJWalecka

JJWalecka

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New England
I can't count the amount of times I see 2", 3", or 4" E.M.T., 24 inches and longer, filled to there capacity with 12 AWG and or 10 AWG. With derating I'm sure that the ampacity of the conductors is 100 percent - 500 percent over the Code requirements. These jobs where passed by inspectors that I'm sure didn't miss it. Is derating unpracticed. Thank you for your input.

Justin W.
 
Re: Does anyone derate

Well, let?s work it out:
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  • <font size="2" face="Verdana, Helvetica, sans-serif">A 2 inch EMT can legally hold 101 conductors of type THHN, size #12 AWG.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">That many conductors would require a 35% derating factor.</font>
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  • <font size="2" face="Verdana, Helvetica, sans-serif">A #12 THHN is good for 30 amps (from 90C column).</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Derating for multiple conductors brings the ampacity to 10.5 amps.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Therefore, as long as I keep the total calculated load on each circuit below 10.5 amps, for example by having no more than 7 receptacle outlets per circuit, then this is a perfectly legal and safe installation.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">Would I design a house (or any other facility, for that matter) with that many conductors in a single conduit? No.
 
Re: Does anyone derate

A #12 THHN is good for 30 amps (from 90C column).

As long as all the terminations at the devices and OCPD are similarly rated.
 
Re: Does anyone derate

Jimwalker, Charlie B, and Thomas Zimmerman thank you for your responces. I would presume that not all the conductors were over there max, but I would imagine that alot were. It just seems to me that it would be a red flag to the Inspector to put an Amp-Probe on the conductors and possible fail the job. Some of these installations where on large Commercial projects ($250,000-$1,000,000 Electrical alone). Thank you, again, I'm only looking for opinions.

Justin W.
 
Re: Does anyone derate

Originally posted by jimwalker: Charlie,would we be required to change that breaker to a 15 ? How can i assure anyone that the load will not be higher ?
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Good point. I would say "yes." If we calculate the load at 10.5 amps, and if we use a conductor that has an ampacity of only 10.5 amps, then a 15 amp breaker is the highest we could use, and still protect the wire.
 
Re: Does anyone derate

charlie, your calculations are good but it looks like you then applied 240.4(B) and 240.4(B)(1) says you can't use this section. Since you have changed the ampacity of the wire by using the derating factors, you must use a wire that will now carry the full amperage rating of the overcurrent device. :D
 
Re: Does anyone derate

Originally posted by charlie:. . . 240.4(B)(1) says you can't use this section.
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Not true at all. I could run all 42 circuits from a 42 circuit panel, each with its own ungrounded conductor and its own grounded conductor, and still have room for 17 conductors to be used as "shared EGCs." :D
 
Re: Does anyone derate

charlie b,
Not true at all. I could run all 42 circuits from a 42 circuit panel, each with its own ungrounded conductor and its own grounded conductor, and still have room for 17 conductors to be used as "shared EGCs."
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It doesn't say "multiwire" circuits, it say "multi-outlet". Your post of 4:41P says there are 7 receptacle outlets on the circuit. As soon as there is more than one receptacle outlet on the circuit, the "round-up" provisions of 240.4(B) cannot be used.
Don
 
Re: Does anyone derate

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.
 
Re: Does anyone derate

Originally posted by don_resqcapt19: As soon as there is more than one receptacle outlet on the circuit, the "round-up" provisions of 240.4(B) cannot be used.
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My bad. :(

But that brings up an interesting twist to the story. Looking back through the math, I would now say that if you want to use 15 amp breakers, you can have no more than 20 current-carrying #12 AWG conductors in the same conduit. Similarly, if you want to use 20 amp breakers, you can have no more than 9 current-carrying #12 AWG conductors in the same conduit. The size of the conduits does not influence the math.

So back to Justin W.: Perhaps you have a better reason for concern than I had, at first, thought.
 
Re: Does anyone derate

Originally posted by charlie b:
#12 AWG conductors in the same conduit. Similarly, if you want to use 20 amp breakers, you can have no more than 9 current-carrying #12 AWG conductors in the same conduit. The size of the conduits does not influence the math.
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9 is the number to keep in mind when using 14, 12 & 10 AWG THHN with 15, 20 & 30 amp breakers.

Once the conductors are larger than 10 AWG things change.
 
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