For outdoor wiring—hooking up a hot tub, adding outlets to a deck, powering a shed—installing conduit makes a lot of sense, and many times it’s required. Conduit even has its uses inside, wherever wires would otherwise be exposed and could be damaged. And if you have a circuit you may want to extend someday, conduit will enable you to pull additional wires.
We’re going to show you how to install plastic (PVC) rigid conduit rather than metal conduit. Plastic conduit is less expensive, lighter and much easier to work with. Here are some great pro tips from commercial electricians to help you make wiring runs with conduit.
Schedule 40 conduit is cheaper and has a larger inside diameter, so it’s easier to pull wires through it. The plastic on Schedule 80 is thicker, but the conduit has the same outside diameter as 40, so the inside diameter is smaller. Always install Schedule 80 conduit in high-traffic areas or any other areas where it could get damaged, like behind your woodpile. By the way, the fittings (such as adapters and turns) are the same for each type.
THHN (thermoplastic high heat resistant nylon-coated) is the best wire for pulling through conduit. Other types of wire have a sticky rubber sheathing that makes them almost impossible to pull. Stranded THHN is used on most commercial jobs—it’s more flexible than solid wire, which makes it easier to pull, and it doesn’t spring back when you push it into the box.
Install 3/4-in. conduit instead of 1/2-in. if (1) you need to pull more than three wires through one section of conduit; (2) there’s any chance you’ll add wires in the future; or (3) if you have a long and winding run. The 3/4-in. conduit doesn’t cost that much more, and it’s a heck of a lot easier to pull wire through. Whatever size conduit you use, don’t fill it more than 40 percent with wires.
Single-gang electrical boxes will work, but if you have two or more conduit sections connecting to one box, buy double-gang. The male connectors on the ends of the conduit take up quite a bit of room inside the box, leaving little room for devices. GFCI receptacles and other large devices, like dimmers, fit better in deeper boxes (2-1/8 in.).
Conduit in a trench
If you’re running rigid PVC conduit, most trenches need to be 18 in. deep, but ask your electrical inspector how deep to dig the trench for your specific project. You can run Schedule 40 in a trench, but use Schedule 80 wherever the conduit comes up out of the ground (see “Schedule 40 vs. 80” above). Assemble all the conduit first and plop the whole thing into the trench when you’re done. Much easier and cleaner than working in a trench!
Some PVC pipes require primer, but you don’t need to use primer when gluing conduit and fittings. Home centers usually sell the appropriate cement near the the conduit and fittings.
It’s tempting to start by attaching all the boxes to the walls and ceiling first and then run the conduit, but don’t do it. It’s easier to secure one box and then run the conduit from that box to the next one. Fasten the second box to the wall or ceiling after you fasten it to the conduit. Then you won’t have to fight the conduit trying to bend it into position. This is especially important if you have two boxes in close proximity because it’s difficult to bend short sections of conduit.
Use metal hangers even with PVC conduit; they hold up better than plastic. Choose the single-hole type. One screw is enough support, and compared with the two-hole strap, installation will go twice as fast.
Your job will look better if you install the kind of hanger that offsets the conduit the same distance from the wall as the knockout on your boxes. For 1/2-in. through 1-in. conduit, the maximum spacing between supports is 3 ft.
If you have a long run with a whole bunch of twists and turns, consider splitting up the span with junction boxes. Every elbow you install makes pulling wire more difficult. And installing turns totaling more than 360 degrees (four 90-degree elbows) is not allowed on one run. Pros rarely go beyond 180 degrees because it’s easier to install an additional box and pull the wire a shorter distance.
Hook on Old Wires to Pull New Ones
If you’re adding wires to existing conduit and have to pull them a long distance, hook the new wires to an existing one—including a replacement wire for the one you’re using—and pull them through that way. You’ll have to buy extra wire, but you’ll save a lot of time and frustration.
It’s easy to push wires short distances, but if it’s necessary to pull them a long distance with fish tape, here’s how pros tie them on: First strip 4 in. of sheathing off two wires. Then cut half the strands off the two exposed wires (less bulk to pull through). Next loop the remaining exposed wires through the eyelet of the fish tape. Finally, wrap all three wires in electrical tape all the way up to the eyelet of the fish tape.