# multiwire branch circuit



## jjdh (Sep 8, 2009)

In a conduit with 3-MWBC [multiwire branches circuit] (balanced 120 or 277-volt loads on 3-phase circuits serving lighting). Do I have a derating problem.
Some people say I'm wrong. Am I thinking right?

H = Hots N = Neutrals CCC = Current Carrying Conductors MWBC = # of Multi-wire Branch Circuits 

3-H + 0-N = 3-CCC x 3-MWBC = 9-CCC 70% DERATING
2-H + 1-N = 3-CCC x 3-MWBC = 9-CCC 70% DERATING
1-H + 1-N = 2-CCC x 3-MWBC = 6-CCC 80% DERATING


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## Speedy Petey (Jan 10, 2007)

Well, if that is all you have in the pipe then you are fine, assuming #12 wire. 

Three 3-phase MWBCs is nine CCCs.


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## william1978 (Sep 21, 2008)

We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.


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## 480sparky (Sep 20, 2007)

william1978 said:


> We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.


 
Some are, some aren't.

Check out 310.15(B)(4)(a) - (c).


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## gilbequick (Oct 6, 2007)

william1978 said:


> We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.


If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?


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## BadSplice (Sep 5, 2009)

gilbequick said:


> If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?


 Unswitched lighting?


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## 480sparky (Sep 20, 2007)

BadSplice said:


> Unswitched lighting?


Until one light goes out. :whistling2:


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## BadSplice (Sep 5, 2009)

480sparky said:


> Until one light goes out. :whistling2:


That won't hold up. That's like an inspector failing you because your ground rod impedance might go up if it doesn't rain as much next year.


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## 480sparky (Sep 20, 2007)

BadSplice said:


> That won't hold up. That's like an inspector failing you because your ground rod impedance might go up if it doesn't rain as much next year.


 
So one light goes out right before the inspector shows up.



FAIL.


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## BadSplice (Sep 5, 2009)

480sparky said:


> So one light goes out right before the inspector shows up.
> 
> 
> 
> FAIL.


Never happen! :thumbsup:


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## 480sparky (Sep 20, 2007)

BadSplice said:


> Never happen! :thumbsup:


 
I'm glad you live in a perfect world where you never get a defective lamp or luminaire.:blink:


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## Bob Badger (Apr 19, 2009)

gilbequick said:


> If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?


The balancing has nothing to do with the NEC not counting the neutral as a current carrying conductor.

The neutral can be carrying current all the time and still not need to be counted as a current carrying conductor. As Ken pointed out, check out 310.15(B)(4)(a) - (c).


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## BadSplice (Sep 5, 2009)

480sparky said:


> I'm glad you live in a perfect world where you never get a defective lamp or luminaire.:blink:


Jealous?


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## Magnettica (Jan 23, 2007)

I predict as soon as the electrician understands derating factors and what is and what is not a current carry conductor, that electrician moves up to another level. You need to fully understand derating and CCC's before moving on to motors and motor calculations. Buy some Mike Holt books, you'll get it.


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## RePhase277 (Feb 5, 2008)

The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.

If all three are loaded differently, then the neutral will carry minimal current, but then so will the least loaded circuit(s). In other words, whether the loads are balanced or not, the heating effect is similar, so the neutral can be discounted as non current carrying.

Inductive loads are a different story according to the Code, so the neutrals are counted.


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## Ray Cyr (Nov 21, 2007)

InPhase277 said:


> The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.


Nice explanation, I hadn't formed a mental picture of that before, Thank you:thumbsup:


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## Bob Badger (Apr 19, 2009)

InPhase277 said:


> Inductive loads are a different story according to the Code, so the neutrals are counted.



Not 'inductive loads' it's linear vs non-linear loads.

A motor would be a linear load.

A computers power supply would be a non-linear load.


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## McClary’s Electrical (Feb 21, 2009)

InPhase277 said:


> The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.
> 
> If all three are loaded differently, then the neutral will carry minimal current, but then so will the least loaded circuit(s). In other words, whether the loads are balanced or not, the heating effect is similar, so the neutral can be discounted as non current carrying.
> 
> Inductive loads are a different story according to the Code, so the neutrals are counted.


No, it carries the difference of the two circuits.


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## RePhase277 (Feb 5, 2008)

mcclary's electrical said:


> No, it carries the difference of the two circuits.


That would be true for a single phase MWBC. We are talking about 3-phase.


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## RePhase277 (Feb 5, 2008)

Bob Badger said:


> Not 'inductive loads' it's linear vs non-linear loads.
> 
> A motor would be a linear load.
> 
> A computers power supply would be a non-linear load.


You're right. I could have been a little more descriptive. Not all inductive loads are non-linear.


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