What are my options for installing 240v receptacles in an attached garage?

Question

My workshop (attached garage) is currently wired with 120V receptacles fed from the main panel in the house by #10/3 with ground (1 black, 1 red, 1 white, 1 ground) attached to a double-pole 30A breaker.

I'd like to power a 13A, 240V table saw, and need to install a 240V receptacle. It would also be convenient to install a couple of "extension" receptacles around the perimeter so that I have of the option of changing my shop layout. I see two options, and would welcome advice.

Option 1:

Extend the existing wiring around the shop, using double-ganged boxes for a 120V receptacle on one side, and a 240V receptacle on the other side. The 240V receptacle would use both hots and the ground, and for the 120V side, I would draw from either hot leg to create a split receptacle.

Option 2:

Install a subpanel in the workshop, fed from the currently-installed 30A breaker in the main panel, and create separate circuits for 120V and 240V. Each circuit would receive its own breaker.

Questions:

1. Option 1 seems most straightforward. But is it correct or safe?
2. If choosing Option 2, what size breakers should I use for the 120V and 240V circuits?

Answer 1

A subpanel is probably the safest bet. It will allow easier access to cut power in an emergency, and it will offer better protection of the wiring.

If you go with option 1, you'll have to step down to #12 or #14 wire at some point to feed 120V receptacles. Which means you'll potentially be able to draw 30 amps on a #12/#14 wire, before the breaker trips.

If it were me, I'd install a subpanel with a 30A main breaker. Then use a 20A single pole breaker to feed the 120V receptacles, and a 20A double pole breaker to feed the 220V receptacle. You could use a 15A breaker for the 120V receptacles, but since it's a shop (where you'll likely use high draw devices) I'd go with 20A. If you do use 20A for the 120V receptacles, make sure you use receptacles rated for that amperage and #12 wire.

Answer 2

You shoul install a 60 amp sub panel and use 20 amp breaker single pole for your plugs with a wire # 12 gauge 1 hot, 1 neutral and 1 ground, you can share 1 neutral for 2 circuits and for your 220 v use 20 amp doble pole wire# 12, 2 hots and 1 ground.

Answer 3

Option 1 is called a "multi-wire branch circuit" or MWBC. These let you use 3 wires instead of 4, but are prone to dangerous mistakes. So there are extra rules to fend off these problems.

• as of 2011 code, you MUST use a 2-pole breaker that trips both sides of the MWBC at once. If the room goes dark when a breaker trips, that’s a code violation.

• MWBC makes it impossible or expensive to use GFCI or AFCI breakers. Use GFCI outlets instead.

• You must pigtail neutrals (you can’t daisychain using the 2 terminals on an outlet). That is, only one neutral can go to a receptacle, and that goes to a wirenut to be joined to the upstream and downstream wires.

• sizing breakers is very simple: Breakers protect wires. What is the smallest wire in the entire MWBC, including grounds? 14gauge=15A. 12ga=20A. 10ga=30A. That’s your max breaker size.

• The outlets must match the breaker size exactly. (with a key exception: common 15A household outlets are permitted on 20A circuits: NEC 210.21B3). Most likely you’ll be stuck at a 20A breaker.

• inspectors dislike MWBCs, because they so often have flaws.

Option 2, sub-panel, makes all these issues moot. In which case: use a 30A main breaker (10ga wire) and size the panel breakers as described above. If metal conduit works for you, it’ll be safer and you don’t need to pull a ground.