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Old 03-19-2013, 09:47 PM   #1
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Hello,I am renovating a kitchen that currently does not have a dishwasher. I am thinking about using a 12/3 cable to power both the microwave and the dishwasher appliances. I was thinking of running the 12/3 to a junction box from the panel ( double pole breaker) and then running 12/2 from each device back to the junction box and tying in all the neutrals but tying in one black to the 12/3 black and one to the 12/3 red. Will that work? I have heard about the issue with the neutrals not carrying the full load back to the panel which concerns me. Any input would be great! Kevin

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Old 03-19-2013, 10:01 PM   #2
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


No problem with your plan.

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Old 03-19-2013, 11:55 PM   #3
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


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...I have heard about the issue with the neutrals not carrying the full load back to the panel which concerns me...Kevin
Hi Kevin. It's not an issue, it's a benefit. It saves copper and saves you from pulling two cables. It's like the amps from the loads on the two hot wires are "cancelling each other out". (only different) That's less work for the neutral. It carries the difference. For example, if: microwave pulls 5A and dishwasher pulls 6A then neutral carries - one amp!

Use a junction box if it's convenient. Or use a deep (larger) box for either the diswasher's or microwave's receptacle. Land your 12/3 there. Then continue to the next receptacle box with 12/2 w/ ground.

The times the 12/3 trick (Multi Wire Branch Circuit) gets ugly is when one load keeps tripping the breaker. Or if one load makes a LOT of electrical noise. Like a machine with a bunch of motors. And the other load is reeeeally finicky. Like a fancy, high end computer.

Last edited by Glennsparky; 03-20-2013 at 12:00 AM.
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Old 03-20-2013, 06:58 AM   #4
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Make sure the breakers are on opposite poles.
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Old 03-20-2013, 08:21 AM   #5
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Thanks for your input. I thought this would work but just wanted to make sure. Nice to have this site as a back up to throw ideas out and see if they make sense. Thanks again
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Old 03-20-2013, 08:43 AM   #6
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


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Make sure the breakers are on opposite poles.
What do you mean? I am going to use a double pole 20 amp breaker with each hot using one opening. Isn't that correct?
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Old 03-20-2013, 09:57 AM   #7
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Yes that's the way to do it. A DP breaker will land one leg on each phase of the service.

BTW: I think a built-in microwave has to have it's own dedicated circuit. But a countertop one does not.
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Old 03-20-2013, 10:18 AM   #8
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


I don't like MWBC circuits, personally. If your microwave trips, your dishwasher shuts off. Want to work on one? Have to flip the breaker to both. It's annoying. I figure MWBCs were a way to cut costs on installations in a way that most homeowners would not generally care about.

But if you're doing the work, why not just pull two 12/2's? 100' of 12/2 is about $50. 50' of 12/3 is also about $50. Just looked them up at Home Depot. It's a wash in price, less complicated, and more flexible to work on them later. They're not tied to each other, and can both be home runs. A 2-pole breaker also costs more than two 1-pole breakers. For all these reasons I'd just run two 12/2's.
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Old 03-20-2013, 10:56 AM   #9
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


How often do you really need to work on one appliance, especially to consider that rare chance to be an inconvenience.

BTW, the service coming into your house is a MWBC.
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Old 03-20-2013, 11:48 AM   #10
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Of course the answer there is "not very often". But it does come up. We replaced a DW. These are once in several years type of things.

We're considering putting in a transfer switch. We might want to move the Micro circuit to the transfer switch but not the DW. With it installed MWBC - not an option. So now we can't use our Microwave in a power outage cuz of limiting choices made at install time.

That's certainly another rare bit. But nonetheless, I'd say, given the cost the same, and it's simpler / cleaner to have home runs, why would you want to route your MW circuit thru your DW hookup box and deliberately tie them together adding complexity when there really isn't a benefit or savings?

I could see... if using conduit, then yes, you'd need to pull one fewer neutral, which is actual savings. Very minimal, but a smidge cheaper. If one did this for a living, repeated for many houses, it would make a difference to the bottom line for such an installer so of course it makes sense then. But with 2 12/2's vs 1 12/3 and a homeowner doing this one time, the cost is equal so I'd recommend 12/2 for 20A circuits.
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Old 03-20-2013, 04:00 PM   #11
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Wiring a microwave and dishwaher using a 12/3 multi branch circuit


Quote:
If your microwave trips, your dishwasher shuts off. Want to work on one? Have to flip the breaker to both.
I just unplug the one I need to work on.

Quote:
We might want to move the Micro circuit to the transfer switch but not the DW. With it installed MWBC - not an option.
You can still move the circuit, it just uses the one extra space. You don't have to use the DW, even if it is on the transfer switch.
Or don't transfer the MWBC and use an extension cord from a receptacle that is on the transfer switch, to power the microwave temporarily.

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