310.15 derating

[e57]

Situation:

Only one 1" EMT out of panel room with concrete walls through finished halls. Need 4 20A 120/208 4-wire 'round-house' (4X4=16 @#10) out to a J-box in tenent area - then switch to #12's with <9 in 3/4" EMT to other J-boxes.

I have done this before a while back with no problems - but now can not find specific wording allowing or dis-allowing it. Darned sure I can - just can't put my finger on the wording to convince someone else...

 

[infinity]

With THHN you're 4 sets of #10's are fine for 20 amps. Even at 50% derating your left with a 20 amp conductor. Same would hold true for the less than 9 #12's.

 

[e57]

With THHN you're 4 sets of #10's are fine for 20 amps. Even at 50% derating your left with a 20 amp conductor. Same would hold true for the less than 9 #12's.

Yes - but the person I'm having this debate with claims the derating applies with the "lowest conductor size on the portion of the circuit"... (Even if you switch conductors and raceways)

Where I say it is "two different conductors in two different derating situations in two different raceways". (The circuit doesn't matter) I just can find a specific wording for the situation to prove my point....

 

[charlie b]

Start with the article 100 definition of ampacity. It speaks in terms of a conductor, and the current that that conductor can carry, under its conditions of use. In your situation, you have #10 conductors in one particular set of conditions, and they will have a value of ampacity. You also have #12 conductors in a different set of conditions, and they too will have a value of ampacity. There is no reason to expect two different conductors, under two different conditions of use, to have the same ampacity, nor to have the same derating factors applied to each. The only related requirement (it comes from 240.4) is that every conductor be protected at or below its ampacity. In your case, a 20 amp breaker will protect both the #10s and the #12s.

 

[e57]

Start with the article 100 definition of ampacity. It speaks in terms of a conductor, and the current that that conductor can carry, under its conditions of use. In your situation, you have #10 conductors in one particular set of conditions, and they will have a value of ampacity. You also have #12 conductors in a different set of conditions, and they too will have a value of ampacity. There is no reason to expect two different conductors, under two different conditions of use, to have the same ampacity, nor to have the same derating factors applied to each. The only related requirement (it comes from 240.4) is that every conductor be protected at or below its ampacity. In your case, a 20 amp breaker will protect both the #10s and the #12s.

Yeah - thats what I was trying to say - No place in 310.15(B)2 does it say "Circuit" - it says "Conductor" and "Raceway"...

I think the guy is confusing it with a different situation where you might have conductors in a single raceway where it might be in say two different tempatures.... Like.... 310.15(A)2 - but in this case both the 10's and 12's are still good reguardless of length of each set of conductor.

(FWIW I have often disagreed with the image above as it implies 240.4D doesn't apply - but that is a different and well travelled topic.)

 

[iwire]

(FWIW I have often disagreed with the image above as it implies 240.4D doesn't apply - but that is a different and well travelled topic.)

Damn dude, get over it and move on. :grin:

 

[e57]

Damn dude, get over it and move on. :grin:

I am ..... :roll: This is a different topic here...