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What is a multiwire branch circuit? (also known as "multiwire circuit", "multiwire branch circuit", "split phase shared neutral", "two circuits fed from one 12-3 or 14-3 NM-B cable", "shared neutral circuit", etc)

This question comes up from time to time on this site, and I couldn't a canonical answer about what a multiwire branch circuit is.

They seem to be relatively uncommon in residential wiring, so many homeowners may not know how to recognize or handle them, especially when taking on "simple" DIY projects like adding a GFCI or AFCI outlet or breaker.

This boils down to multiple sub-questions, but I think they are related enough to answer here:

  • What is a Multiwire Branch Circuit?
  • How do I know I have one?
  • What do I need to do differently when working with one?

A few related questions that give various pieces of the answer are:

  • I don't see an option to make this a community wiki question so others can improve it. If someone has the power to do that, feel free, otherwise, feel free to suggest edits to make it a more canonical question. – Johnny Aug 12 '15 at 0:52
  • How to know if it is one? You don't unless it's compliant with today's codes. – Mazura Aug 12 '15 at 1:39
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What is a Multi-wire Branch Circuit?

A multi-wire branch circuit is two or more circuits that is served by a a set of hot wires connected to different phases (all with the same voltage between them) and a common neutral wire (all of the hots also have the same voltage with respect to the neutral). This arrangement saves a bit on wire and can also save on conduit fill. The NEC in paragraph 210.4 (b) requires a means to disconnect all of the hot wires simultaneously. Typically this would be a single multi-pole circuit breaker.

The simultaneous disconnect requirement serves to protect against the possibility that someone working on the circuit wouldn't notice that it was a multi-wire branch circuit and would disconnect the neutral while another wire in the circuit was energized.

That said, you may run into multi-wire branch circuits that are protected by individual breakers, especially in older construction or in panels that have been modified without attention to the code (or thinking through the consequences of working on the circuit with one of the hot wires still energized).

In most residential applications multi-wire branch circuits would be 3-wire (two hots and a neutral) with 240 Volts phase-to-phase and 120 Volts phase-to-ground, but you might run into 4-wire circuits in buildings with 3-phase service (for example a high rise urban apartment building).

How do I know I have one?

You have to look carefully. There are several clues in the panel:

  • If the breakers in the panel are labeled you may get a clue by noticing that a multi-pole breaker is serving non-appliance circuits (e.g., something other that a 240 Volt appliance like a stove, oven, hot water heater, or dryer). Don't count on the labels being right…

  • Next look for any multi-pole breakers rated at 15 or 20 Amps. Most, *but by no means all, 240 Volt appliances will have 30 Amp or higher breakers. If any of those breakers serve a 120 Volt circuit – for example lighting or outlets – you've almost certainly got a multi-wire branch circuit. The exceptions that I can think of our outlets integrated with appliances, like a 120 Volt outlet on a stove.

  • Finally look for a single pole breaker with a colored (not black) wire on it. Follow that wire back to where it comes into the panel. If it is part of a cable (as opposed to a set of wires run in conduit) follow the other wire (presumably the black one) and see if it runs to a breaker (it might be unterminated as a spare). If it does, you've got a multi-wire branch circuit. It would be a good idea to replace the single-pole breakers with a multi-pole breaker and bring all of the wires in the circuit to it.

What do I need to do differently when working with one?

The big thing to know is that you've got two circuits that are interconnected because they share a neutral wire. If they are on single pole breakers and you don't notice you could get a shock working with a neutral that you thought was dead.

Another potential problem is that you can't use a single-pole GFCI on a multi-wire branch circuit.

Finally you need to ensure that the hot wires on single-pole breakers are out of phase with each other. Typically this is ensured by putting them on adjacent breakers in the panel or better yet, put them on a multi-pole breaker.

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    You can use GFCI devices on a Multi-wire branch circuit, it just takes a bit of knowledge on how GFCIs work (or a double pole GFCI breaker). Multi-wire branch circuits should have the breaker handles tied together (or be protected by a double pole breaker). Also, it's slightly wrong to use the term "out of phase" when taking about 120/240V single split-phase service. – Tester101 Aug 12 '15 at 1:11
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    @Tester101, saying the two hot wires are "out of phase" is correct from the perspective of a device receiving the power. He didn't say it was "two-phase". He said the legs were "out of phase", and they are. A electronics engineer looking at the signals in an oscilloscope would tell you that the legs are 180 degrees out of phase. What is wrong is when people say it's two-phase. A two-phase system is able define a unique direction of rotation for a revolving magnetic field; phasors separated by 180 degrees are not able to do this. – Lakey Jun 4 '17 at 21:37
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    @Lakey The two hot legs are only "out of phase", when referenced to neutral (or ground). If you connected both legs to an oscilloscope, you'd see a single 240VRMS sine wave. To see them "out of phase", you'd have to connect each one with a reference to neutral (or ground). Then you'd see two 120VRMS sine waves, 180 degrees out of phase. – Tester101 Jun 5 '17 at 10:51
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The National Electrical Code defines a Multi-wire branch circuit as follows.

Branch 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

Background

In the US, residential dwellings are often supplied using a 120/240V single split-phase system. This system consists of two ungrounded (hot) conductors, and a grounded (neutral) conductor (which is a center tap on the transformer).

Multi-wire branch circuits

A Multi-wire branch circuit takes advantage of this, by extending both ungrounded conductors and sharing a single grounded (neutral) conductor. So instead of having two circuits requiring four conductors, you can have two circuits with only three conductors.

How to recognize them

Multi-wire branch circuits will typically be supplied by two adjacent breakers in a panel, and will often be fed using 3-wire plus ground cable.

If the circuit was installed properly, the breakers handles should be tied together or a double pole breaker will be used.

What to worry about

  • If you're working on the circuit, you'll want to make sure both breakers are off.
  • You should never disconnect the grounded (neutral) conductor, while the breakers are on.
  • You'll have to take care installing GFCI devices, as it can be a bit tricky.
  • You'll have to make sure the ungrounded conductors come from separate legs of the service.
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    Just thought I'd add my 3 cents: mwbc's can also take advantage of the neutral used in 208/120Y. Granted most DIY'ers won't need to know that. :-) – Kris Aug 12 '15 at 3:39
  • @Kris I don't know--I saw another question on here today about a 3-phase espresso machine... ;-) – Craig Aug 13 '15 at 8:18

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