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benleef 02-22-2012 10:17 AM

MWBC to Garage Questions
Hey all, long time lurker, first time poster here.

I'm in the planning stages of running a circuit to my small garage and just want to make sure all of my research has instilled in me an understanding of how to do this.

The plan:

200 Amp service in Main Panel at house. Install Double Pole 20 Amp breaker in Main from there use 12/3 UF-B through 20 feet of basement, out back of house into 18-24 inch trench underground to garage about 30 feet away. I plan to use 3/4" or 1" conduit for the whole run in case I later want to upgrade, but at this point I think a 20 Amp MWBC is more than enough for the few lights and receptacles I need. In same trench I'm going to have another 1/2" conduit to run coax cable and Ethernet to the garage.

Once the 12/3 enters the garage I will keep it in conduit to a double pole switch box for a disconnect. In the box I will join neutrals of (2) 12/2 cables and the neutral of the 12/3. I will then connect the red and black of the 12/3 to the switch, and the blacks from the 12/2s to the switch. One 12/2 will go to a tamper-resistant GFCI outlet and continue with 12/2 on the load side for additional non-GFCI outlets (but GFCI protected of course) (probably 6-8 outlets around perimeter of garage and one exterior outlet on side of garage). Also, I will make sure all neutrals are pigtailed. The second 12/2 will supply power to lights. I may install one GFCI outlet prior to the lights just in case I have the other branch of the circuit loaded up at some point. It will power approximately 2 switched lights inside and another 2-4 lights outside of garage.

I also want to have a three way switch for one or more of the outside lights so they can be turned on or off from both the garage and the kitchen in the house. Is the correct way to do this to power the lights from the garage's MWBC as described above, then from the light(s) to a three-way switch in the garage and then from that switch run 12/3 back to the house to the three-way switch in the house? Can this be run in and out of the same conduit as the MWBC?

Does all of this sound right? Also the reason I am doing this and not a sub panel is a) I don't think I will ever need that much power out in the garage and b) I want to save some money.

Thanks for any help,


Jim Port 02-22-2012 10:31 AM

It will be much easier to pull individual conductors into the conduit over the UF cable.

rditz 02-22-2012 10:54 AM

i would put a small pony panel in the garage. this way you can turn off individual circuits while keeping others turned on...

it is also a good way to organize your circuits.. it also allows you to have a mix of 120VAC and 240VAC circuits...

benleef 02-22-2012 11:02 AM

Also, I'm open to running 8/3 with 40 Amp Double Pole Breaker in Main to Garage Main Lug Sub Panel in garage. I figure this would maybe increase the cost by about $150-$200 for bigger wires, bigger and more breakers, sub panel, and ground rods. But I'm willing to spend the extra $ if it will be safer and easier.

If I do this, my ground from the sub would have to run along the length of the garage to the back where I could insert ground rods (concrete in the way otherwise on outside). Does the bare copper ground have to be run in conduit along the garage? Or just through holes in studs OK?

Also, what about 12/3 or 14/3 back to house for three-way switch? Can this be in same conduit as feeder cable?

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