Working With PVC Conduit
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 great tips from commercial electrician Jason Bouchard to help you make wiring runs with conduit.
Schedule 40 vs. 80
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.
Buy THHN wire
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.
Deburr with a utility knife
If you do end up with a rough cut, don’t forget to deburr the inside of the conduit. Burrs can damage the insulation on the wires. There are a lot of fancy deburring tools available, but Jason just spins a utility knife on the inside of the conduit to smooth it out.
Larger conduit and bigger boxes
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 doesn’t need primer
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. Measure as carefully as you can so you can avoid dry-fitting your connections. Unlike PVC plumbing pipes, PVC conduit and fittings can be difficult to pull apart once you shove them together. And always wear gloves, unless you want to spend half the evening picking glue off your hands.
Mount one box, then the conduit, then the next box
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. This process makes it easier to fasten the connectors, nuts and bushings to the box first and then glue the conduit to the connector.
Metal hangers work best
Use metal hangers even with PVC conduit; they hold up better than plastic. Choose the single-hole type. One screw is more than 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.
Cut it with a circular saw
There are lots of ways to cut PVC conduit, but a circular saw fitted with a metal blade gives you a smooth, fast, burr-free cut. If you don’t have a metal blade, a regular construction blade will do the job, but you may have to deburr the end of the conduit after you cut.
Install metal locknuts
It’s OK to use plastic conduit locknuts, but just as with conduit hangers, Jason prefers the metal ones. He says plastic locknuts can strip out and break.
Keep elbow totals no more than 360 degrees
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. Jason rarely goes beyond 180 degrees because it’s easier to install an additional box and pull the wire a shorter distance.
Use weatherproof boxes outdoors
Install weatherproof boxes (sometimes called bell boxes) outside. Unlike regular boxes, weatherproof boxes usually have threaded knockout holes to create a water-resistant connection. Many come with caps to plug the hole you don’t use. Make sure the box you buy has holes where you need them.
Drill a hole to let water out
There’s still a strong possibility that water will get inside your weatherproof box. Jason drills a 1/4-in. hole in the bottom of the box, so if water gets in, it can get out. You can drill the hole before or after you install the box.
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 our expert ties them on: First he strips 4 in. of sheathing off two wires. Then he cuts half the strands off the two exposed wires (less bulk to pull through). Next he loops the remaining exposed wires through the eyelet of the fish tape. Finally, he wraps all three wires in electrical tape all the way up to the eyelet of the fish tape.
Bushings protect wires
Even if you deburr the end of your pipe, you can still damage wires when pulling them past conduit edges. A bushing provides a nice rounded, smooth surface for the wires to slide by. It’s cheap insurance, and your electrical inspector will be impressed.
Meet the Expert
Jason Bouchard has been a journeyman commercial electrician for more than 22 years. He’s worked on everything from traffic lights to waste incinerators. He even wired And LIT the huge torch featured in the 1996 Summer Olympics in Atlanta.