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I am planning on running a multiwire branch circuit to my detached garage as described here. I have one question though on the means of disconnect for a multiwire branch circuit. Do I need a special type of disconnect because it is a multiwire branch circuit? Would something like this Eaton 60A Outdoor Air Conditioner Disconnect be adequate?

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The "means of disconnect" requirement for multi-wire branch circuits (NEC 210.4b) simply requires that there be a means of disconnect, and it disconnect all legs at once. Usually, that is simply the 2-pole (240v) circuit breaker in the panel, and that is fine; that completes the requirement.

Unrelated to the MWBC is a separate requirement that power entering an outbuilding. It must have a disconnecting means of some kind (NEC 225.31) near where the power enters, inside or out (225.32). That's where you use a disconnect switch like the above. There is no need to disconnect the neutral.

I would point out that if you're doing a sub-panel, simply use a panel that has a "main" breaker and that's your shut-off switch. It's legit to have a 30A run to a sub-panel and have a 200A main breaker in the sub-panel. The big breaker just serves as an obvious shutoff switch (and GFCI if it does that).

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As long as the breaker feeding the circuit is a double pole you should be fine because both will disconnect the ungrounded conductors simultaneously, in the case of a fault both will be opened. NEC 2014 210.4.B

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