Multiwire circuits are electrical circuits where two different circuits use the same neutral. The reason for doing this is purely economical. It saves one wire, the other neutral that would have been used.

These are very dangerous circuits, due to the fact that they can be connected incorrectly and work apparently normal.

This type of circuit is used on single phase circuits that require one hot wire and one neutral, such as 120 volt lighting or, 277 volt lighting for that matter.

The idea is to connect two circuits to a double pole breaker and use one neutral for both. The current will balance leaving only the unbalanced load on the neutral.

The problem is that usually there are more than one of these circuits and if the circuits are mixed up or only single pole breakers are used, then only one side of the circuit can be turned off, leaving the other on.

When only one side of this circuit is turned off at the breaker, the lights or whatever goes off, but THE CIRCUIT IS STILL HOT ALL THE WAY BACK TO THAT BREAKER, due to the feedback from the other breaker on that circuit.

If you must use multiwire circuits, then use double pole breakers and make sure you match the circuits when making up the panel and any j-boxes.

If you are doing electrical trouble shooting or repairs, and you suspect multiwire circuits have been used, then check the voltage at the load side of the breaker to ground after turning it off.

Any switches in the circuits will have the same effect as the breaker, the switch can be off and both wires will be hot.