Multiwire branch circuit

A few years ago a friend of mine who owns an old house was telling me a story about some electrical work he was doing.  He was working on a circuit with some old wiring, and when he disconnected one of the wires all of the lights on that circuit doubled in brightness.  He was wondering what had happened.

So I explained to him that he had simply disconnected the neutral wire in a multiwire branch circuit.  Pretty simple really.  But he wasn’t at all familiar with that concept.  Most people aren’t.  But it’s an important concept in electrical wiring, and here is how it works.

First off, go outside and look at your service drop – the electrical wires coming to your house from the utility pole.  (Your service might be underground, in which case you have a service lateral.  You can skip this step.)  There are three wires coming into your house from the utility.  There are two hot wires and one neutral wire.  Often times the neutral wire in the service drop isn’t even insulated.

Have you ever wondered how two hot wires can share a single neutral wire?  Of course we use alternating current, and the two hot wires are on different poles of the electrical system.  So when one of those hot wires is trying to push electricity into the house with 120 volts the other wire is trying to push electricity out of the house with 120 volts, and those direction change 60 times each second.  If we don’t use a neutral wire at all we have a 240-volt circuit, used for high-powered equipment like electric ovens, electric water heaters, electric dryers, and central air conditioners.

The two hot wires share the single neutral wire, and since they’re pushing electricity in opposite directions the neutral wire is only seeing the difference in the electrical current between the two hot wires.

And we can do this with circuits inside the house too.  Two circuits, with wires coming off of two different circuit breakers, can share a single neutral wire.  This works as long as those two circuit breakers are on different poles of the electrical system.  This is a multiwire branch circuit, and here’s how it works.

Here’s a typical electrical panel.  There are two hot wires (red arrows) and one neutral wire (white arrow).  The hot wires provide power to the bus stabs, the places where the circuit breakers attach.  The neutral wire connects to the two neutral bus bars (these two bars are connected together by a copper rod that’s kind of hard to see – yellow arrow).

And this is the way that the bus stabs are powered.  The wire that powers the breakers alternates down each column, so that any two breakers that are right next to each other come from different poles of the electrical service.

To form a multiwire circuit we take two adjacent circuit breakers and attach hot wires, and we attach a wire to the neutral bus bar.  Now we run our circuits, kind of like this.

Remember that the red and blue wires have opposite voltages, so that when the red wire is trying to push electricity into the bulb the blue wire is trying to push electricity back to the electrical panel. And so the shared neutral wire is stuck in the middle – the two circuits are trying to push electricity in opposite directions.  So the neutral wire only sees the difference between the currents flowing in the two separate circuits.  If for example the red circuit is carrying 9 amps and the blue circuit is carrying 6 amps, then there will only be 3 amps on the neutral wire.

There are two major benefits of a multiwire circuit.  You use less wire, so that’s a cost saving.  You also have less voltage drop since there’s less current flowing through the neutral wire.  Less voltage drop means less wasted energy, so that’s a good thing.

One big drawback is that using a GFCI device can be problematic, since the two sides of a circuit are seeing different amounts of electrical current.  That would trip off a GFCI device.  And for arc fault protection (AFCI) you’d need to use a special double pole device, as opposed to just using two separate circuit breakers.

One important issue to keep in mind is that when using a multiwire branch circuit the two circuit breakers must have their handles tied together.  Look again at the circuit diagram with the bulbs.  If we shut off the red circuit breaker then you might think that circuit is safe to work on.  But it isn’t safe, because the neutral wire still has current flowing over it from the blue circuit.  So in order to work on either of these circuits both of the circuit breakers need to be shut off.  Installing a handle tie on the two breaker handles ensures that both would be shut off.

As an aside, the point of connecting the handles together is not to force both circuit breakers to trip off in the event of an overload condition.  That won’t happen, and I’ve personally seen the situation where one breaker tripped off but the handle wasn’t enough to trip off the other breaker.  The point of the handle tie is to force a person who’s shutting off one of the breakers to shut off both at the same time.

So, going back to my story at the start of this blog, what happened to my friend is he disconnected the neutral wire in his multiwire branch circuit.  Look again at the circuit diagram above.  Each bulb is seeing 120 volts, so they’re illuminated normally.  If you disconnect the neutral wire as in the diagram below then suddenly both bulbs are connected through a 240-volt circuit.  The voltage they’re seeing just doubled, and so they’ll become twice as bright.  Some people consider this another drawback to a multiwire branch circuit, that if there’s a problem with the neutral wire everything will see 240 volts.  That can cause a big problem with some electronic equipment.

Also keep in mind that a neutral wire can share only two hot wires, and only if those two hot wires are on different poles of the electrical system.  If a neutral wire is connected to three or more hot wires then at least two of those wires will be on the same pole.  In that case, the current that the neutral wire sees is the sum of the two currents.  This could easily cause the current flowing through the neutral wire to be too high, and that can cause overheating and it’s a fire hazard.  A multiwire branch circuit can consist only of two hot wires on different poles of the service and one neutral wire.