November 19, 2021—Learn with me
I’m having an electrical sub panel installed in my garage. I’ve always wanted more lights and outlets than the current circuits can handle. And plus I’m beginning to need 240V for power tools.
As I plan ahead I’m having to learn about all the different outlets, wire gauges, breakers, and circuits related to both 120V and 240V.
One thing I’m learning about is something called a multi-wire branch circuit (MWBC).
A MWBC gives you two circuits that are out of phase and share a neutral.
Here in the United States the electrical service that comes into your house is essentially a MWBC. Generally half of the 120V appliances will be on one leg and the other half will be on the other leg. Bigger appliances that require 240V service, like an electric stove or HVAC system, will be wired across both legs. Like this:
Transformer | Electrical panel | Appliances
+120V ------ ------------------ ---+-------------+
\ \
| Fridge |
/ |
Neutral ---- ------------------ ---+ | Stove
\ |
| Microwave |
/ /
-120V ------ ------------------ ---+-------------+
Notice how the 120V fridge and microwave share a common neutral?
It’s important that the two 120V legs are exactly out of phase. When they are out of phase then the neutral line only has to carry the difference in current between the two legs. But if the two legs are in phase then it has to carry the sum of the currents.
In a MWBC (where the 120V legs are out of phase) you can use the same gauge wire for the neutral as for the two legs. In other words you can use a single 12/3 copper wire to feed two 20A 120V circuits. The highest load the neutral will see is when one leg is using a lot of current and the other is using none. Paradoxically, the neutral will see zero current when both legs are using exactly the same amount of current.
But if the legs were in phase then the neutral would have to be sized to carry twice the capacity of the legs. The highest load the neutral would see in that case is when both legs are using a lot of current. And the neutral would only ever see zero current when neither leg is using any current.
Here’s why: in a MWBC if the two legs are using the same amount of current then the same amount of current will be flowing into one leg as out the other, because they are exactly out of phase. But if they were in phase then the same amount of current would be flowing into one leg as into the other, and those two currents have nowhere to go but through the neutral.
Pick an amperage, like 20A. Get a 20A double pole breaker and some 12/3 (which is rated for 20A) and a NEMA 5-20 (which is rated for 120V 20A) duplex outlet. The 12/3 will have four conductors in it:
Because you’re using a double pole breaker then the black and red lines will be out of phase.
Break the hot tab on your duplex outlet so that the hot side of the two receptacles are no longer electrically connected.
Then wire it like this:
Tada! You now have an outlet that can pump 20A out of both receptacles at once!
In addition to all the rules for normal wiring, a MWBC additionally has these requirements:
When you flip a breaker it has to turn off both legs. This is to prevent any nasty surprises when someone flips the breaker to do work inside the outlet’s box—you want both legs to be dead, not just one of them. A double pole breaker solves this.
You also must take care that you don’t lose the neutral. If you want a MWBC to feed multiple outlets (not just multiple receptacles, but multiple outlets in a room) then you have to wire them together in a way that keeps the neutral connected for all the other outlets if you remove an outlet, or if the neutral breaks or falls off of an outlet. If you lose the neutral on our example MWBC above then there will still be 240V between the two hot sides of the receptacles exactly as there was before. Which in theory is fine without the neutral if both receptacles are pulling the exact same amount of current. But as soon as there is a current imbalance then there will be a voltage imbalance—one device might see 200V while the other only sees 40V—a good way to start a fire. You also get odd behavior such as lights only working while the toaster is on. Wiring the outlets in with pigtails helps to keep the integrity of the neutral line independent of the integrity of the outlet.
A MWBC has to be wired properly to work. But it can be a useful way to get lots of power out of a single duplex outlet. Or a useful way to put a NEMA 6-20 outlet in the same box as a duplex NEMA 5-20 outlet with only a single 12/3 wire.