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Motor Disconnects, Part 3

Motor Disconnects, Part 3Can you wire a motor disconnect so one pole disconnects a permanently grounded conductor? Yes, but only if the disconnect simultaneously opens all circuit conductors [430.105]. It’s important to clarify what the NEC means by “grounded conductor.”

This isn’t the “grounding conductor,” which there’s no need to open. A grounding conductor connects non-current carrying metal parts to the system grounded conductor and/or the grounding electrode conductor [100]. Think “green wire” (though the actual color might be some variation such as green with yellow stripes, see Article 250 marking requirements).

The grounded conductor is a circuit conductor that’s intentionally grounded [100]. This is typically, but not always, the neutral. Think “white wire” (though the actual color or marking could differ).

While there’s no reason to disconnect the grounding conductor, you may have reasons for the disconnect to open all of the circuit conductors including the grounded conductor.

For example, the facility contains 277V lighting and your grounded conductor is the neutral. While the motor supply is presumably on a different circuit, its grounded conductor could still pick up current from the grounded conductor of the lighting circuit. That disconnect is what’s going to be locked out/tagged out to isolate the motor from its supply. So in this case, you want to open that grounded conductor of the motor circuit when the disconnect is opened for maintenance.

« Part 2 | Part 4 » | Source: Mark Lamendola | Mindconnection