# Table 310.15(B)(2)(a) Derating wire for more than 3 current carrying conductors in a



## Greg (Aug 1, 2007)

1) You start your derating from the 90c column but once the derating is complete you can not use anything larger than a 20 breaker.

2) A MWBC doesn't have to count the Neutral when applying adjustment factors, however if it is for example a 3ph 4w panel that is supplying, say fluorescent lighting then you have to count the Neutral for adjustment factors. Because of the high harmonics caused by non-linear loads.

Single phase circuits, the *Grounded Conductor* gets counted because it is a current carrying conductor. (Neutrals do not exist in single phase)

Those of you studying to test, terminology will make or break you on a test.


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## God man (Oct 15, 2010)

Single phase circuits, the *Grounded Conductor* gets counted because it is a current carrying conductor. (Neutrals do not exist in single phase)

Do you mean 1 hot and a grounded conductor or 2 hot's and a grounded conductor?


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## HARRY304E (Sep 15, 2010)

God man said:


> Single phase circuits, the *Grounded Conductor* gets counted because it is a current carrying conductor. (Neutrals do not exist in single phase)
> 
> Do you mean 1 hot and a grounded conductor or 2 hot's and a grounded conductor?





> (Neutrals do not exist in single phase)




How is that any 120 volt circuit is single phase regardless of wether it is fed from a 3 phase service or just a single phase service.


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## God man (Oct 15, 2010)

*How is that any 120 volt circuit is single phase regardless of wether it is fed from*



Greg said:


> 1) You start your derating from the 90c column but once the derating is complete you can not use anything larger than a 20 breaker.
> 
> 2) A MWBC doesn't have to count the Neutral when applying adjustment factors, however if it is for example a 3ph 4w panel that is supplying, say fluorescent lighting then you have to count the Neutral for adjustment factors. Because of the high harmonics caused by non-linear loads.
> 
> ...





HARRY304E said:


> How is that any 120 volt circuit is single phase regardless of wether it is fed from a 3 phase service or just a single phase service.


I was asking because what he was saying did not make sense to me because a perfectly balanced single phase circuit sharing a neutral creates a neutral with azero amps on it, oh sorry a grounded conductor.


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## EMeis1114 (Mar 5, 2011)

God man said:


> I was asking because what he was saying did not make sense to me because a perfectly balanced single phase circuit sharing a neutral creates a neutral with azero amps on it, oh sorry a grounded conductor.


But how often are both legs of a a single phase mwbc balanced?


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## erics37 (May 7, 2009)

God man said:


> Do you mean 1 hot and a grounded conductor or 2 hot's and a grounded conductor?


With one hot and a grounded conductor, the grounded conductor is current-carrying for derating calculation purposes. With two hots and a grounded conductor, it depends. If it's a circuit supplied from a 3-phase Wye service then yes, the grounded conductor counts because the two phases associated with it don't vectorially cancel each other out all the way. It says it right in the code book.

If it's a 240/120 circuit from a Delta then the grounded conductor would not count if it was just carrying the unbalance between the hots.



God man said:


> I was asking because what he was saying did not make sense to me because a perfectly balanced single phase circuit sharing a neutral creates a neutral with azero amps on it, oh sorry a grounded conductor.


They're almost never perfectly balanced. The point is that the neutral will only carry the unbalanced load if the hot legs are 180 degrees out of phase. That doesn't happen with a Wye supply.


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## Dennis Alwon (May 9, 2009)

EMeis1114 said:


> But how often are both legs of a a single phase mwbc balanced?


That is not the issue. If for instance you have a single phase mwbc then there is no way that the circuit will see more than 20 amps on each hot leg. So if both legs draw 20 amps then the neutral draws zero. If one phase is 15 and the other 5 then the neutral sees 10 amps but the total is still less than 40 amps maximum on a balanced mwbc with 20 amps on each phase. 15 amps on phase A, 5 amps on phase B and 10 on the neutral. Add it up-- 30 amps. So in effect the neutral would not count because it will never be more than just the 2 phase conductors.


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## Big John (May 23, 2010)

To paraphrase Dennis: In a MWBC the total net current in the circuit doesn't change regardless of whether it's flowing on line-to-line or line-to-neutral.

No increase in current = no increase in heat = no need for additional derating.

-John


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