phase a, phase a , and neutral ?

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ibew441dc

ibew441dc

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I have a reasonable understanding of electrical theory and am comfortable with the NEC........

But I can not pin down a rule that specifically restricts running circuits in this manner.(A,A,& Neutral or B,B,B & neutral, ect.)


Originally I thought I would find the answer around one of these sections and came up short 310.4 ,Article 100 def. multi-wire branch circuit,300.3(B).....

Anybody got an answer for me?

Thanks
 
Now, that definition does not say that you can't do what you are asking.

But there are some other articles, sections, that will apply. Give us a few minutes....
 
I can't think of a code section, but the electrical gods would be unhappy with you, using the same phase for both or all three hots can seriously overload the neutral, If you have 15 amps on one circuit on "A", added another "A" circuit with say, 15 amps to make it simple, would produce 30 amps on the neutral. But if you changed one of the circuits to "B" or "C" the neutral load would be zero. If the "B" or "C" phase was 10 amps then the load on the neutral would be 5 amps.
 
I'm with ya all the way on the definition , but I still feel somethings missing.

The definition branch circuit , multi wire is a clear definition, but I still can not find anywhere that does not permit/restrict running 2 phase A's and a common neutral.
 
Avoid jamming the internet read a past post.

http://forums.mikeholt.com/showthread.php?t=101674&page=6

I can't think of a code section, but the electrical gods would be unhappy with you, using the same phase for both or all three hots can seriously overload the neutral, If you have 15 amps on one circuit on "A", added another "A" circuit with say, 15 amps to make it simple, would produce 30 amps on the neutral. But if you changed one of the circuits to "B" or "C" the neutral load would be zero. If the "B" or "C" phase was 10 amps then the load on the neutral would be 5 amps.
Click to expand...

Well you are on track but there is a bit more math to it.

neutralcurrent_edited-1.jpg
 
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There is no restriction in the NEC to install either of the the scenarios you have presented in your question.
As long as the grounded conductor is the proper size, you can install as many phase conductors with 1 grounded conductor as you please.

Multiwire Branch Circuit is a definition, not a code requirement.
 
hillbilly1 said:
I can't think of a code section, but the electrical gods would be unhappy with you, using the same phase for both or all three hots can seriously overload the neutral, If you have 15 amps on one circuit on "A", added another "A" circuit with say, 15 amps to make it simple, would produce 30 amps on the neutral. But if you changed one of the circuits to "B" or "C" the neutral load would be zero. If the "B" or "C" phase was 10 amps then the load on the neutral would be 5 amps.
Click to expand...

Sure wouldn't want to make them unhappy (lol). But In the scenario you just spelled out overloading the neutral would not be a problem if it was over sized.
 
Daniel, at the moment, I am agreeing with you. The A-A-Neutral doesn't qualify to be a MWBC... but so what? A-A-Neutral isn't a MWBC. But neither is A-Neutral, but A-neutral is okay. Where does it say that A-A-Neutral is not legal?

Now, I am certain everyone understands the dnager here. But we are talking code and what it literally says. I certainly don't think that A-A-Neutral should be legal, but is it specifically illegal?

One thought is 210.19 sizing of branch circuit conductors. The neutral may need to be upsized???

215.4 actually has some circumstances where you could have multiple circuits of the same phase sharing a neutral. This is for feeders.

This is a great question!:smile:
 
Pierre C Belarge said:
There is no restriction in the NEC to install either of the the scenarios you have presented in your question.
As long as the grounded conductor is the proper size, you can install as many phase conductors with 1 grounded conductor as you please.
.
Click to expand...

Thats what I originally thought, but I had a gut feeling it was incorrect and am still not totally convinced it is legal.
 
What really got me thinking was the new requirements for muti-wire branch circuits (Having to simultaneously disconnect all ungrounded conductors, regardless of whether or not they terminate to the same yoke. 210.4)

If it is true that I could run 10 A phase and 1 neutral (properly sized), similar hazards could be made present as in a multi-wire branch circuit.
 
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The only positive thing I can see is that if you had 10 A phase & 1 properly sized neutral and the neutral opened somehow , you wouldn't deliver an overvoltage to your equipment.
 
You would have to know 20 amps on a and 20 amps on a would put 40 amps on the neutral which would overload the ampacity of the neutral as in the ampacity tables specifically prohibiting it.
 
ibew441dc said:
The only positive thing I can see is that if you had 10 A phase & 1 properly sized neutral and the neutral opened somehow , you wouldn't deliver an over voltage to your equipment.
Click to expand...

Just might deliver an over voltage to the guy trying to figure out what the Hell you did. This may be "technically legal" but it sure sounds like a BAD idea.
 
Pierre C Belarge said:
There is no restriction in the NEC to install either of the the scenarios you have presented in your question.
As long as the grounded conductor is the proper size, you can install as many phase conductors with 1 grounded conductor as you please.
Click to expand...


I agree entirly with Pierre. :smile:
 
Here is an example from the the NECH

commonneutral.JPG


Those are all 20 amp circuits and the neutral is sized for the maximum possible current.


In the real world I can't think of a time this could be done efficiently.
 
hillbilly1 said:
I was only figuring for single phase, which is correct, as you noticed I did not give an example for a three phase set up. those figures I quoted do not work for three phase, thanks.
Click to expand...

Actually you did: ;)

hillbilly1 said:
But if you changed one of the circuits to "B" or "C" the neutral load would be zero
Click to expand...

There would only be a C in a 3 phase system.
 
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