Electrical Forum - Derating Ampacity

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Subject - Derating Ampacity
Scott Vickrey	Here's a simple question everyone should know. What is the maximum number of current carrying #12 THHN copper conductors that can be pulled in 1-1/4" EMT or larger and still be hooked up to 20 amp breakers?
JimmyDee	Wow! Look at that. I got 4 stars under my name. I guess thats from 500 posts. I wonder if Scott sends me money or something now
JimmyDee	OK I see the error of my ways. I should never think I have one up on an inspector. Actually have an amp to spare. Jim
Scott Vickrey	Wow, that got some responses! The way I see it is 20/30 = .66 or 20 amps are 66% of 30 amps. This means we can have up to a 66% derating of #12 THHN and still use 20 amp breakers. When we refer to Table 310.15(B)(2)(a) we see we can't have 10 because that would require a 50% derating. We can pull 9 because the required derating for 7 to 9 CCCs is only 70%.
Ryan_J	:)
aussiesparky	OK lets see how it works upsidedown #12 = 4.0mm2 I'll assume THHN is 7/0.85 single insulated V90 building wire and 1-1/4" is 32mm steel conduit. you can install 13 conductors in 32mm conduit, so we'll asume 4 single phase circuits. 4 circuits of 4.0mm2 installed touching in a 32mm conduit has a derating factor of 0.65. 4.0mm2 installed in a wiring enclosure has a current carrying capacity of 37amps. 37*.65=24.05. so the upsidedown answer is 12
tony thrower	I agree with Ryan provided you meet the conditions of Art. 240.4 (B)
JimmyDee	#12 thhn copper conductors are good for 25 amps. Derate for 6 in a conduit and that gives you 20 amp wire. Derating at 9 conductors would be 70% and .70 x 25 = 17.5 amps. Code says you can put on next larger size breaker if one is not sized for the wire. Is that your justification and are the inspectors allowing that for small conductors? Jim
Ryan_J	Without looking it up in table 310.15(B)(2)(a), I will say 9. After that I know it takes a 50% ampacity adjustment. How did I do for not looking it up?
JimmyDee	6 conductors. 4-6 conductors are derated at 80% and .80 x 25 = 20. Jim
Ryan_J	quote: Originally posted by JimmyDee #12 thhn copper conductors are good for 25 amps. Jim Jimmy, you might want to look at 310.16 one more time. Remember that we can use the 90 degree column for derating.