What is the importance of neutrals in multiwire branch circuits?

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In a multiwire branch circuit, two hot conductors share a single neutral. The current in the neutral isn’t always zero—the neutral carries the difference between the currents in the two hot legs. When the two loads aren’t equal, current flows on the neutral, so it must be treated as a current-carrying conductor that’s properly terminated at the panel and at devices.

The reason for tying the two breakers together is safety. If one hot circuit is turned off while the other remains energized, the neutral could still be carrying current from the live circuit, creating a shock hazard or leaving parts of the circuit energized even though you think it’s off. A common disconnect (a tied breaker or a handle-tie that trips both breakers together) ensures that all conductors for the MWBC are de-energized simultaneously, protecting anyone who might work on the circuit and preventing hazardous conditions on the neutral.

It’s also important to recognize that the neutral is not the same as the equipment grounding conductor. The neutral can carry current during normal operation, whereas the grounding conductor is for fault protection and is not used to carry normal circuit current.