Basic wiring strategy for garages
Whether you're rewiring or adding more wiring to an older garage, or running wires in a new garage, you have a decision to make. Do you plan to leave the walls open or close them up with drywall? If you intend to leave the walls and ceiling open, you have to follow the special rules that we show here. The same goes for sheds, workshops or other structures with unfinished walls outside the living space of the house.
In a house or a finished garage, electrical cable is protected from damage by permanent wallcoverings like drywall, plaster or even wood, but that’s not the case in an unfinished garage. The key to safe, code-compliant exposed wiring is to use the framing members to protect and support the wires. That means not spanning stud or joist spaces with wires. Keep all the cables closely hugging the studs, plates and ceiling joists so they're not subject to abuse (see “Dangerous Wiring Mistakes” at the end of this article).
In this story, we'll show you a common, cost effective and code approved way to run exposed electrical cable. But there's one downside to this method: If you ever do decide to finish the area, you'll have to completely rewire because the surface-mounted cables will make drywalling impossible.
In an unfinished garage, the trick is to follow the framing. Instead of connecting switch, light and outlet boxes by spanning framing spaces, route cable along studs, top plates and ceiling joists (Photo 3), along whichever framing member leads to the next box (see lead photo). That means you'll use a lot more cable, because the paths are rarely direct. But cable's cheap and quick to run. Keep all cables exposed and easy to see and never run cables on the tops of walls; keep them on vertical surfaces. That way the wires won't ever have anything resting on them like hooks or garden tools (Photo 6).
Set the boxes and run the wires
Begin by planning your wiring scheme and nailing up all the electrical boxes. To run a cable from one box to another, pull cable off the coil, strip at least 8 in. of sheathing off the wires and thread the wires and about 1/2 in. of sheathing through the wire opening in the box. Untwist the cable and staple it at least every 4 ft. to the framing, all the way to the next box without cutting it to length. When you reach the next box, stretch out the cable and mark the entrance point on the cable, cut the cable about a foot long, strip the sheathing and feed the wires into the box. Then nail in the staples near the box.
The code requires that you hold all wires at least 1-1/4 in. (Photo 1) from the back and front surface of studs and joists. With 2x4 framing, that means you have to hold two cables running side by side tight to each other. If you're running three or more cables, you'll need special “stacker” staples (Photo 4) so you can piggyback cables on top of one another. Stacker staples aren't always sold at home centers, but you can find them at electrical supply stores. Sometimes these staples are the only option because of the number of wires in a stud space.
Cable should be stapled at least every 48 in. between boxes, but as rule, the more staples the better. Avoid sagging cables and keep turns tight by using as many staples as necessary. Electrical boxes may have built-in metal or plastic cable clamps. Many duplex boxes only have knockouts. Cable should be stapled within 8 in. of boxes without clamps and within 12 in. of boxes with clamps (Photo 4).
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How to run surface wiring on solid walls
Many garages contain one or more walls made of concrete or already drywalled. Cable installed on these exposed surfaces has to be protected by conduit. We show EMT (electrical metallic tubing; Photo 7), but you can also use rigid PVC conduit.
Use 1/2-in. tubing for one cable (14-2 or 12-2) and 3/4-in. for two cables. Whichever conduit style you use, choose boxes, straps and connectors that are specifically made for that type of conduit. Screw boxes to walls through the back into solid backing and use straps at least every 4 ft. to fasten the conduit to the wall. Route conduit until you reach the open stud spaces and then continue wiring as we’ve described. Never exit wires through cut ends of conduit because the sharp metal edges can cut into the sheathing. Always exit cable through connectors.
DON'T run wires across open joists.
DON'T run wires across open stud spaces.
Dangerous Wiring Mistakes
Avoid running cables like we show in the photos below. Cables that span stud spaces or ceiling joists are in constant jeopardy of nicks or cuts from sharp shovels, opening car doors or nearly anything else you’ll find in a garage. As if that’s not bad enough, it's too tempting to use them for tucking, hanging or trapping all kinds of paraphernalia to get it off the floor and out of the way.