Neutral on MWBC

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OHHV

Member
I have a question that I'd like to have clarified if possible. 120/240v 1 phase panel supplying 2 dedicated receptacles with a 12/3 cable on a 2 pole breaker. 120v per receptacle with a shared neutral. My question is if each receptacle/hot leg has a load that draws 18A and they're both running at the same time, will the neutral overload?

roger

Moderator
Staff member
No, the neutral would have zero amps on it in this scenario.

Roger

Rewire

Senior Member
OHHV said:
I have a question that I'd like to have clarified if possible. 120/240v 1 phase panel supplying 2 dedicated receptacles with a 12/3 cable on a 2 pole breaker. 120v per receptacle with a shared neutral. My question is if each receptacle/hot leg has a load that draws 18A and they're both running at the same time, will the neutral overload?
The neutral conductor carries the unbalanced load,the difference between A and B phase amps will be the neutral amps.
Aphase18A - Bphase18A=0A on neutral.

gndrod

Senior Member
If it is a green contract, I'd use separate neutrals just in case.

roger

Moderator
Staff member
gndrod said:
If it is a green contract, I'd use separate neutrals just in case.

Why?

BTW, In the OP's example, there wouldn't be any neutrals if two grounded conductors were used. :wink:

Roger

iaov

Senior Member
Nope. The neutral only carries the ubalanced load. If one conductor was carrying 16 amps and the other 18, the neutral current would only be 2 amps.

220/221

Senior Member
If the circuits were on two single pole breakers and on the same phase/leg, you would have an overloaded neutral with 36 amps.

Dennis Alwon

Moderator
Staff member
OHHV said:
I have a question that I'd like to have clarified if possible. 120/240v 1 phase panel supplying 2 dedicated receptacles with a 12/3 cable on a 2 pole breaker. 120v per receptacle with a shared neutral. My question is if each receptacle/hot leg has a load that draws 18A and they're both running at the same time, will the neutral overload?

First, a multiwire branch circuit much use two phase in order for the grounded conductors to carry the unbalanced load. If, not in your example, you hooked both ungrounded conductors to the same phase then you would overload the grounded conductor.

roger

Moderator
Staff member
220/221 said:
If the circuits were on two single pole breakers and on the same phase/leg, you would have an overloaded neutral with 36 amps.

There wouldn't be a neutral in this case and if you were using a #8 conductor it would be fine.

Roger

walkerj

Senior Member
This might be silly,but would there really be a reason to wire with a "super neutral"?

480sparky

Senior Member
walkerj said:
This might be silly,but would there really be a reason to wire with a "super neutral"?

Circuits with known harmonics, such as lighting on dimmer circuits, or circuits feeding large electronics.

stickboy1375

Senior Member
walkerj said:
This might be silly,but would there really be a reason to wire with a "super neutral"?

I believe it was designed to minimize the effects of harmonic currents on the neutral conductors generated by non-linear loads ?

walkerj

Senior Member
But why a MWBC sharing the same phase and a Super neutral

I'm feeling brickwallish today

roger

Moderator
Staff member
walkerj said:
This might be silly,but would there really be a reason to wire with a "super neutral"?

It is not silly but, it would only be necessary in very few instances where non linear loads are the majority but, this would not be the same thing as using a grounded conductor large enough to carry the combined load of ungrounded conductors on the same leg or phase.

Roger

walkerj

Senior Member
roger said:
It is not silly but, it would only be necessary in very few instances where non linear loads are the majority but, this would not be the same thing as using a grounded conductor large enough to carry the combined load of ungrounded conductors on the same leg or phase.

Roger

The former I understand, sortakinda.
The latter I dont see any reason to:-?

480sparky

Senior Member
walkerj said:
But why a MWBC sharing the same phase and a Super neutral

I'm feeling brickwallish today

If the two circuits are on the same phase, then you don't have a MWBC. You have two circuits that will overload the neutral. The definition of a neutral is one that carries the unbalanced load, not the total load.

OHHV

Member
Thanks. I was thinking of it as both being on the same phase and that's where the confusion came from and not 180 degrees apart. The original conversation started because the electrician was running one homerun of 12/3 to two bathrooms, with a dedicated gfci in each. Can you guys also clear up the confusion that if these 2 gfci receptacles are installed using the 12/3, will sharing the neutral affect the way they operate?

Dennis Alwon

Moderator
Staff member
walkerj said:
This might be silly,but would there really be a reason to wire with a "super neutral"?

Doesn't seem possible to wire with the super neutral with the 2008 code requiring a dp or a listed breaker tie on MWBC.

gndrod

Senior Member
Y

Y

roger said:
Why?

BTW, In the OP's example, there wouldn't be any neutrals if two grounded conductors were used. :wink:

Roger

Limits stand-alone AC-DC inversion. rbj

Smart \$

Esteemed Member
walkerj said:
This might be silly,but would there really be a reason to wire with a "super neutral"?
No, provided we remain in the realm of a 120/240V system...

The first thing everyone brings up is the harmonic current of non-linear loads. However, it is only the triplen harmonic currents of 3? systems that add up to more current on the neutral conductor than the ampacity of the line conductors. This does not happen on 120/240 systems because like harmonic currents have a 180? phase shift respective of its counterpart.

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