jar546
CBO
Multi-wire branch circuits (MWBCs) are legal under the NEC and are often used to save copper and conduit space. They use two ungrounded conductors from opposite phases and one shared neutral. Sounds efficient—until someone makes a mistake.
Pull the neutral off the bus bar in the panel, and you’ve just wired your two 120V circuits in series across 240V. Your flat-screen TV and your desktop computer now become the load in a 240V circuit they were never designed for. Poof—damage done, often beyond repair.
This is exactly why neutral conductors should never share a single terminal lug. Even if it's code compliant for certain equipment, it's bad practice in MWBCs. A loose neutral or a shared lug that gets disconnected while energized becomes a recipe for equipment failure, especially when each leg is feeding sensitive electronics in separate parts of a home or office.
Here’s the kicker: A shared neutral on a two-pole breaker is still two circuits. The breaker ties the handles together to ensure simultaneous disconnect, which is now required in newer code cycles (NEC 210.4(B)), but it doesn’t magically turn it into one circuit. Each ungrounded conductor is still serving a separate load—you're just required to shut them off together for safety.
The takeaway? MWBCs need to be wired with care. Make sure the neutral is continuous, secured independently, and terminated properly. Don't share lugs, and always mark the neutral accordingly. Otherwise, you could be turning two harmless 120V circuits into one very expensive 240V lesson.
Pull the neutral off the bus bar in the panel, and you’ve just wired your two 120V circuits in series across 240V. Your flat-screen TV and your desktop computer now become the load in a 240V circuit they were never designed for. Poof—damage done, often beyond repair.
This is exactly why neutral conductors should never share a single terminal lug. Even if it's code compliant for certain equipment, it's bad practice in MWBCs. A loose neutral or a shared lug that gets disconnected while energized becomes a recipe for equipment failure, especially when each leg is feeding sensitive electronics in separate parts of a home or office.
Here’s the kicker: A shared neutral on a two-pole breaker is still two circuits. The breaker ties the handles together to ensure simultaneous disconnect, which is now required in newer code cycles (NEC 210.4(B)), but it doesn’t magically turn it into one circuit. Each ungrounded conductor is still serving a separate load—you're just required to shut them off together for safety.
The takeaway? MWBCs need to be wired with care. Make sure the neutral is continuous, secured independently, and terminated properly. Don't share lugs, and always mark the neutral accordingly. Otherwise, you could be turning two harmless 120V circuits into one very expensive 240V lesson.