Conduit offers superior protection and safety for wires. Even if local codes permit NM or armored cable in a basement, garage, attic, or crawlspace, consider installing conduit to protect wiring.
There are many different types and thicknesses of conduit. For most interior home installations, EMT (also called thinwall) is strong enough. Outdoors use intermediate metal conduit (IMC) or PVC conduit. Codes have detailed rules regarding conduit size, but generally 1/2-inch conduit is large enough for five or fewer wires; 3/4-inch conduit is used for more than five wires. When in doubt or if you might run more wire in the future, buy the larger size—it doesn't cost much more.
Metal conduit may serve as the path for grounding, or local codes may require you to run a green-insulated ground wire. If you use PVC pipe, you definitely need a ground wire, either green-insulated or bare copper. If there is no ground wire, make sure all the metal conduit connections are firm; a loose joint could break the ground path.
Save time by buying prebent fittings for your conduit. A coupling joins two pieces of conduit end to end. A sweep makes a slow turn, allowing wires to slide easily, while a pulling elbow makes a sharper turn. Setscrew fittings are commonly used with EMT conduit; they provide joints that are firm but not waterproof. For weathertight joints, use IMC conduit and compression fittings.
Oftentimes, it's easiest to just make a drawing of your proposed installation and have a salesperson help you assemble all the parts you need—conduit, sweeps, elbows, boxes, and clamps. And make sure to buy plenty of wire.
Anchor metal boxes to the wall with screws. For exposed wiring use handy boxes, which have rounded edges and metal covers. An offset fitting allows the conduit to run tight up against the wall.
Once the boxes are installed, measure the conduit for cutting. The surest method is to hold a piece in place and mark it, rather than using a tape measure. Remember that the conduit slides about an inch into each fitting.
Cut the conduit to fit with a hacksaw. Do not use a tubing cutter, which creates sharp edges inside the conduit that could damage wire insulation. Remove the burrs inside and out with a conduit-reaming attachment on a screwdriver.
Slide the conduit all the way into a fitting and tighten the setscrew. Test to make sure the connection is tight. (If you are not installing a ground wire, these connections are critical for grounding.)
Make sure the wires have ample room inside the conduit to slide through easily. Codes have detailed rules regarding conduit size, but generally 1/2-inch conduit is large enough for five or fewer wires; 3/4-inch conduit is used for more than five wires. When in doubt or if you might run more wire in the future, buy the larger size—it doesn't cost much more.
Anchor the conduit with a one-or two-hole strap at least every 6 feet and within 2 feet of each box. The larger the conduit, the closer the straps need to be. Check with local codes. Screws should be driven into joists or studs, not just drywall.
With every turn it gets harder for the wires to slide through a conduit. If the conduit makes more than three turns before entering a box, install a pulling elbow. Never make a splice here; just use it as an access point when pulling wires.