Multi wire branch circuit NM cable

[Jpflex]

A coworker argues with me that you can use a single neutral for two different current carrying circuits.

He wired up two receptacle 120v 20 ampere circuits each with its own 12 AWG THHN line “hot” leg connected to its individual breaker as normal but having the two circuits neutral leg spliced together sharing a common neutral with the same AWG wire size as a single line or “hot” wire

I redid his circuit to add an additional 12 AWG neutral to each individual receptacle to the neutral bus and remove the connecting splice because I figured if the two circuits were sharing a neutral and loaded up to max 80% of circuit rating then the neutrals in the circuit would be carrying 2x their max current



However, in a multi wire branch circuit with a 240 volt circuit relying on two hots and a neutral only to carry unbalance current it is clear that both hots carry close to equal current

However what I don’t understand is how code could allow a multi wire nm cable to allow both hot wires to return on one neutral for the explanation above if it’s a 120 volt receptacle circuits?

 

[don_resqcapt19]

A coworker argues with me that you can use a single neutral for two different current carrying circuits.

He wired up two receptacle 120v 20 ampere circuits each with its own 12 AWG THHN line “hot” leg connected to its individual breaker as normal but having the two circuits neutral leg spliced together sharing a common neutral with the same AWG wire size as a single line or “hot” wire

I redid his circuit to add an additional 12 AWG neutral to each individual receptacle to the neutral bus and remove the connecting splice because I figured if the two circuits were sharing a neutral and loaded up to max 80% of circuit rating then the neutrals in the circuit would be carrying 2x their max current



However, in a multi wire branch circuit with a 240 volt circuit relying on two hots and a neutral only to carry unbalance current it is clear that both hots carry close to equal current

However what I don’t understand is how code could allow a multi wire nm cable to allow both hot wires to return on one neutral for the explanation above if it’s a 120 volt receptacle circuits?

A multiwire branch circuit only works where there is 240 volts between the two hots.

If one hot has 20 amps of line to neutral load, and the other has 18 amps of line to neutral load, the current on the neutral after they are connected together will be 2 amps.

Assuming 20 amp circuits, the maximum current on the neutral will be when one of the hots has 20 amps of current and the other has zero. All other conditions of loading on the two hots will result in less than 20 amps on the neutral.

 

[infinity]

However what I don’t understand is how code could allow a multi wire nm cable to allow both hot wires to return on one neutral for the explanation above if it’s a 120 volt receptacle circuits?

Because for a 120/240 system the neutral currents in a 3-wire MWBC are not additive, meaning that 15 amps on one circuit and 15 amps on the other circuit does not mean 30 amps on the neutral conductor. The neutral current is zero.

 

[tthh]

Most importantly, it will "work" if both legs are on the same 120V side, but the neutral will be overloaded by up to 2x and will be dangerous. I find these mis-wired in the panel every so often. DIY, handyman or a careless/clueless professional.

 

[Little Bill]

You were once allowed to have a MWBC without having handle ties or a 2-pole breaker. The breakers could be anywhere as long as they were on opposite legs, meaning 240V between the conductors. Now, either a handle tie or 2-pole breaker is required. Perhaps the co-worker is thinking of how it used to be and is unaware of the new requirements.

 

[Sberry]

I foundone the master did wrong in a congested panel,,, not much a deal as it was for 2 fixed light circuits, no recepts. Even less a deal since he had used 12/3 to lead out to a Jbox where it went to 2 cables, was on 15 breaker.

 

[Jpflex]

Because for a 120/240 system the neutral currents in a 3-wire MWBC are not additive, meaning that 15 amps on one circuit and 15 amps on the other circuit does not mean 30 amps on the neutral conductor. The neutral current is zero.

The neutral is not zero amperes in 2 hot wire 240 volt circuit with a shared neutral but runs the unbalanced current as I explained.

However a line to neutral 120 volt circuit running a multi wire circuit with two hots using one neutral was my concern with double amperes on neutral perhaps lines have to be on separate phases

But if on true same phase current travels in same direction and neutral current doubles

 

[infinity]

The neutral is not zero amperes in 2 hot wire 240 volt circuit with a shared neutral but runs the unbalanced current as I explained.

We're discussing a MWBC and I said when the two loads are each 15 amps then the neutral current is 0 amps. Theoretically you could disconnect the neutral and the two loads would operate normally in series.

 

[Jpflex]

Most importantly, it will "work" if both legs are on the same 120V side, but the neutral will be overloaded by up to 2x and will be dangerous. I find these mis-wired in the panel every so often. DIY, handyman or a careless/clueless professional.

So lines per circuit would have to be on different phases such as A and B for current not to be doubled on neutral?

 

[roger]

The neutral is not zero amperes in 2 hot wire 240 volt circuit with a shared neutral but runs the unbalanced current as I explained.

Yes it is if both loads are equal as Rob describes.

 

[roger]

So lines per circuit would have to be on different phases such as A and B for current not to be doubled on neutral?

Correct.

 

[don_resqcapt19]

The neutral is not zero amperes in 2 hot wire 240 volt circuit with a shared neutral but runs the unbalanced current as I explained.

However a line to neutral 120 volt circuit running a multi wire circuit with two hots using one neutral was my concern with double amperes on neutral perhaps lines have to be on separate phases

But if on true same phase current travels in same direction and neutral current doubles

There is no neutral or unbalanced current on a two wire 240 volt circuit, so the neutral is zero amps. You can't have neutral current without line to neutral loads.

The definition of mulitwire branch circuit tells you that the ungrounded conductors of that type of circuit cannot be on the same leg or phase.

Branch Circuit, Multiwire. (Multiwire Branch Circuit)
A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a neutral conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral conductor of the system. (CMP-2)

 

[roger]

Here is a single phase MWBC

 

[Jpflex]

We're discussing a MWBC and I said when the two loads are each 15 amps then the neutral current is 0 amps. Theoretically you could disconnect the neutral and the two loads would operate normally in series.

Yes if loads are equal like you say and only if lines are on opposite phases otherwise no current could travel if you disconnect neutral

 

[winnie]

However a line to neutral 120 volt circuit running a multi wire circuit with two hots using one neutral was my concern with double amperes on neutral perhaps lines have to be on separate phases

But if on true same phase current travels in same direction and neutral current doubles

This is exactly the point that @infinity was making.

A properly made up multi wire branch circuit, with the hots coming from different legs of the supply, will not overload the neutral.

An improperly made up MWBC can easily overload the neutral.

The essential requirement is that the two hots be fed from different supply legs.

Jon

 

[roger]

Yes if loads are equal like you say and only if lines are on opposite phases otherwise no current could travel if you disconnect neutral

If lines are not on opposite phases you don't have a MWBC period.

 

[don_resqcapt19]

Yes if loads are equal like you say and only if lines are on opposite phases otherwise no current could travel if you disconnect neutral

The current still flows when you open the neutral and the loads are not equal, however, that often results in letting the magic smoke out of the most expensive load.

 

[infinity]

Yes if loads are equal like you say and only if lines are on opposite phases otherwise no current could travel if you disconnect neutral

Yes that's correct and since the title of this thread is "multi wire branch circuit" that is what I was commenting on.

​

 

[Sberry]

Got to be careful on some older gensets and welding machines. They have 120 outlets and they have 240 and that is not the same as a 120/240. They are not made to hook up to premise power.

 

[CaneCorso]

Multi wire circuits need to be on the same breaker. You would need this installed on a 2 pole breaker.