Multiwire branch circuit

[JerryHonolulu]

Please explain in detail where and how a multiwire branch circuit may be used. Can the neutral conductor in a 4-wire NM cable (black, red, white, bare ground) be shared between the two energized circuit conductors?

 

[infinity]

Yes that would give you two 120 volt circuits.

 

[rlundsrud]

Depending on what code cycle you are on, it would also require a 2 pole breaker since you are sharing the neutral.

 

[augie47]

Keep in mind the NEC definition to make sure you have the correct circuit in mind:

BraBranch Circuit, Multiwire.

A branch circuit that consists of two or more ungrounded conductors that have a voltage

between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.


With changes in recent Code cycles as mentioned in the earlier posts including the requirement that the OCP devices on a MWBC open simultaneously, MWBC are not as popular as they once were since the opening of one phases ovrcurrent device necessitates the other phase(s) opening, one can cause power interruption to a lot of equipment or lighting at one time..

 

[don_resqcapt19]

Depending on what code cycle you are on, it would also require a 2 pole breaker since you are sharing the neutral.

While two pole breakers are permitted, they are not required for multiwire circuits. The rule required a common means of disconnect and that can be accomplished using two single pole breakers and a handle tie.

 

[tom baker]

The requirements for MWBC are onerous and it may be better to not use.
In addition to a two pole CB you may need to group the neutral and ungrounded conductors in the panel and all junction boxes.

See 200.4(B) and more importantly 210.4(D)

 

[don_resqcapt19]

The requirements for MWBC are onerous and it may be better to not use.
In addition to a two pole CB you may need to group the neutral and ungrounded conductors in the panel and all junction boxes.

See 200.4(B) and more importantly 210.4(D)

Not an issue based on the OP using NM cable.

 

[Gene B]

An example of a handle tie for Square D breakers:

(Note, this doesn't turn two breakers into a dual pole [common trip] breaker. It just forces the user to turn off both circuits at the same time).

 

[kwired]

Some of the changes in requirements in the past 10-15 years on MWBC's are there to protect the untrained IMO.

 

[user 100]

Some of the changes in requirements in the past 10-15 years on MWBC's are there to protect the untrained IMO.

X2 here and wouldn't be surprised at all if the good old edison circuit eventually bit the dust entirely in residential bcs in a future code passage.

 

[GoldDigger]

In detail:
A three wire plus ground (2 hot, 1 neutral, + ground) MWBC can be used anywhere that you need to run two 120V circuits to the same general area of a building using a shared neutral to both reduce voltage drop and reduce cost. It can be contained in one cable (e.g. NM) or it can be run using separate wires in the same raceway. The source of the circuit described must be a 120/240 single phase three wire panel.
(Other supply schemes and voltages, such as 208Y/120 three phase four wire, can also be used instead, but the MWBC must contain all of the phase wires plus a shared neutral.

If you have two roughly balanced 120V loads, the current in the neutral wire will be small and so the voltage drop seen by the load will be roughly half that seen with two independent neutral 120V circuits.
And you will use less copper.
And for derating the ampacity of wires in a raceway the single three wire MWBC will count as only two current carrying conductors CCCs) while two independent circuits will count as four.

 

[electrofelon]

(Other supply schemes and voltages, such as 208Y/120 three phase four wire, can also be used instead, but the MWBC must contain all of the phase wires plus a shared neutral.

Why cant you run a 2 phases+neutral MWBC? I know the neutral would count as a CCC but other than that I am not aware of a restriction.

 

[ActionDave]

In detail:
A three wire plus ground (2 hot, 1 neutral, + ground) MWBC can be used anywhere that you need to run two 120V circuits to the same general area of a building using a shared neutral to both reduce voltage drop and reduce cost. It can be contained in one cable (e.g. NM) or it can be run using separate wires in the same raceway. The source of the circuit described must be a 120/240 single phase three wire panel.
(Other supply schemes and voltages, such as 208Y/120 three phase four wire, can also be used instead, but the MWBC must contain all of the phase wires plus a shared neutral.

If you have two roughly balanced 120V loads, the current in the neutral wire will be small and so the voltage drop seen by the load will be roughly half that seen with two independent neutral 120V circuits.
And you will use less copper.
And for derating the ampacity of wires in a raceway the single three wire MWBC will count as only two current carrying conductors CCCs) while two independent circuits will count as four.

Great description of MWBC and all their advantages. Pitty they are trying to take them away from us.

 

[don_resqcapt19]

An example of a handle tie for Square D breakers:

(Note, this doesn't turn two breakers into a dual pole [common trip] breaker. It just forces the user to turn off both circuits at the same time).

And by far the worst handle tie on the market...a 6-32 screw, while not identified for the purpose, works much better on the QO breakers as a handle tie than that piece of junk. Their 3 pole one is great, but have no use for the two pole one.

 

[don_resqcapt19]

Why cant you run a 2 phases+neutral MWBC? I know the neutral would count as a CCC but other than that I am not aware of a restriction.

There is nothing in the code that says you can't use two phases and the grounded conductor from a 3 phase system as a multiwire circuit. The only issue is, as you noted, the grounded conductor is a current carrying conductor.

 

[infinity]

There is nothing in the code that says you can't use two phases and the grounded conductor from a 3 phase system as a multiwire circuit. The only issue is, as you noted, the grounded conductor is a current carrying conductor.

I agree, otherwise you wouldn't be able to hook up a 1Ø, 120/208 volt dryer or range.

 

[GoldDigger]

I agree, otherwise you wouldn't be able to hook up a 1Ø, 120/208 volt dryer or range.

Note, however, that there are limitations on putting line to line loads on an MWBC that also supplies line to neutral loads

 

[iwire]

Note, however, that there are limitations on putting line to line loads on an MWBC that also supplies line to neutral loads

Such as?

 

[kwired]

Such as?

See 210.4(C)

The exceptions however still let you do a lot of things if you have a multipole breaker.

 

[augie47]

Note, however, that there are limitations on putting line to line loads on an MWBC that also supplies line to neutral loads

Such as?

I most definitely can be mistaken, but after reading 240.15 a dozen times it appears you might not be able to use a handle tie on a 3 single pole breakers that feed line to line & line to neutral loads
(B-3 only mentions line to line loads on a 3 phase system)