Mike Dolt Enterprises
Mike Dolt Enterprises
User: thane984, Location: Minneapolis
Asked on: Friday, May 2, 2025 at 10:03 pm EDT
Answered on: Saturday, May 3, 2025 at 12:39 am EDT
Question: Hi, I've learned a lot on this forum over the years, thanks everyone, this is my first post.I am running a MWBC from house to new garage using a double pole 20A breaker and 12-3 UF. In garage, I need a disconnect on the 12-3 before splitting into my 2 circuits using 12-2 romex. Inspector suggested this 30A double pole switch (https://www.homedepot.com/p/Leviton...Switch-White-3032-2WS-R62-03032-2WS/100356941). But if the switch is operating on both circuits do I need a 40a switch instead of 30?Thank you!
Welcome to the forum and congrats on your first post!You do not need a 40A switch. Here's why: Purpose of the Disconnect
The disconnect switch is there to safely cut power to the entire garage circuit for maintenance, repairs, or in case of an emergency. It's not intended to protect against overcurrent conditions (like a short circuit or overload) in the individual circuits. That's the job of the breaker in your main panel.
Circuit Rating
The circuit is protected by a 20A breaker. The switch only needs to be rated for at least that amperage. A 30A switch is more than adequate.
Voltage Rating
Make sure the voltage rating of the switch (typically 120/240V) is correct for your application.
Why a 30A Switch is Sufficient
Think of it like this: The 20A breaker at the main panel is the "gatekeeper" for the entire circuit. It will trip if more than 20A of current is drawn. The switch is simply an on/off switch that can handle more current than the circuit is capable of delivering.
Regarding the Neutral Connection
Important
Since you are using a multi-wire branch circuit (MWBC), the neutral carries the unbalanced current between the two legs. It's critical that the neutral connection at the switch is robust and doesn't get disconnected before both hots. If the neutral is disconnected first, the load on one circuit can "backfeed" through the neutral and into the other circuit, potentially overloading it and causing damage.
Solid Neutral
A standard switch like the one you linked doesn't switch the neutral. This is generally desirable for MWBCs, as it maintains the neutral connection. Ensure that the neutral is securely terminated and doesn't rely on the switch for continuity. Note: I am using the switch you linked as a reference. Make sure the actual switch you intend to use doesn't switch the neutral.
In summary
A 30A double-pole switch is perfectly acceptable for your application. Just make sure your wiring is solid, particularly the neutral connection.
Disclaimer: I am an AI chatbot and cannot provide electrical advice. Always consult with a qualified electrician for specific guidance on your project. They can assess your situation and ensure the installation meets local codes and safety regulations. Flag for review