My local church runs a camp that recently had a gym built on site. The father in law of one of the members did the wiring of the gym lights. He ran a 3/4 emt conduit, with seven circuits running to the lights. Each circuit fed no more than 4 400w, 120v lights. He used #12 THHN to feed the lights. Figuring the de rating on those wires in the conduit, we start at 30a, derate 70% to bring us to 21a, he placed each circuit on a 20a breaker. Now figuring the draw on the lights, each light draws 3.4a, with no more than 4 lights on a circuit, we are less than 14a total load. The inspector failed it, saying that he should have began his de rating calculation a 20a instead of 30a. When my friend approached me with this, I can't find any fault with the installation, is the inspector correct and I'm missing something? Any input is appreciated. I brought this problem to another electrician friend and he opened a whole other can of worms, saying he thinks the de rating should be down to 50% because you count the neutrals, I don't do a ton of de rating calcs, but I've never counted the neuts and never had a problem...Thanks in advance for any replies.
Derating
- Thread starter simplyme
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Re: Derating
The inspector is wrong and your friend may be right. You haven't given quite enough information.
Is this a 120Y/208v or a 277Y/480v system, or is it a single phase system and how many neutrals are in the conduit?
See 310.15(B)(2)(a)
Roger
[ May 27, 2005, 09:36 PM: Message edited by: roger ]
The inspector is wrong and your friend may be right. You haven't given quite enough information.
Is this a 120Y/208v or a 277Y/480v system, or is it a single phase system and how many neutrals are in the conduit?
See 310.15(B)(2)(a)
Roger
[ May 27, 2005, 09:36 PM: Message edited by: roger ]
Re: Derating
Roger, thanks so much for the reply, my de rating has always been done on single phase. And the gym is a single phase service. But the neutrals would be counted on 277/480 as the neuts carry an imbalance, am I right? BTW, beautiful state, your North Carolina. We visit the Charlotte area often, as we have friends there. We have thought about moving there ourselves from time to time.
Roger, thanks so much for the reply, my de rating has always been done on single phase. And the gym is a single phase service. But the neutrals would be counted on 277/480 as the neuts carry an imbalance, am I right? BTW, beautiful state, your North Carolina. We visit the Charlotte area often, as we have friends there. We have thought about moving there ourselves from time to time.
Re: Derating
Simplyme,
Where a problem occures is when only two phases of a three phase system is used with a neutral the neutral will carry pretty much the same current as the other conductors.
Being that you have a single phase system I am going to assume (That can be dangerous
) you have L1-N-L2, L1-N-L2, L1-N-L2, and L-N, this will give you 8 current carrying conductors which will equate to a 70% derating of the conductors so your 21 amp number is correct.
Roger
Simplyme,
you are kind of right. Actually this is true in the pure definition of "Neutral Conductor" in single or three phase systems.
Where a problem occures is when only two phases of a three phase system is used with a neutral the neutral will carry pretty much the same current as the other conductors.
Being that you have a single phase system I am going to assume (That can be dangerous
) you have L1-N-L2, L1-N-L2, L1-N-L2, and L-N, this will give you 8 current carrying conductors which will equate to a 70% derating of the conductors so your 21 amp number is correct.Thank you, it is a nice place to live.
Roger
john m. caloggero
Senior Member
Re: Derating
Assuming that the lighting is incandescent and that the installer has balanced the load, and used a common neutral for every two circuits; there will be 8 currren-carrying conductors which puts you in the 70 percent adjustment factor.The derating factor is applied to the ampacity of the conductor. No. 12awg has an ampacity of 30A, therefore, .70 x 30A = 21A. The size of the overcurrent device cannot be any greater than 20A. The inspector is incorrect.
Assuming that the lighting is incandescent and that the installer has balanced the load, and used a common neutral for every two circuits; there will be 8 currren-carrying conductors which puts you in the 70 percent adjustment factor.The derating factor is applied to the ampacity of the conductor. No. 12awg has an ampacity of 30A, therefore, .70 x 30A = 21A. The size of the overcurrent device cannot be any greater than 20A. The inspector is incorrect.
stew
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Re: Derating
and the reason he is incorrect is because you start with the ampacity of the conductor in the 90 deg column not the 60 deg column for derating puposes.
and the reason he is incorrect is because you start with the ampacity of the conductor in the 90 deg column not the 60 deg column for derating puposes.
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