Tips for Installing and Working With PVC Conduit

What do you know about rigid polyvinyl chloride (PVC) conduit? I had almost no experience with it until I spent a brutal, windy Minnesota fall installing 4 in. PVC conduit underground at a new downtown Minneapolis bus station. It was fascinating. Up until that point, I had worked exclusively with metal conduit, so the experience was eye-opening.

I was an electrical apprentice at the time, and I learned a lot that year: hauling pipe, digging trenches, and even using a flame torch to bend plastic pipe. The job was on the site of an old bottling company, and while digging, I found an antique beer bottle from a brewery operating around 1910! Anyway, I also learned that you can use PVC at home, and I’ll give tips about this versatile, durable electrical conduit.

Types of PVC Conduit

The main designations of PVC electrical conduit are Schedule 40 and Schedule 80. (There’s also PVC plumbing pipe, but that’s an entirely different thing.) Here are the specifications for each:

• Schedule 40: A thinner-walled conduit, Schedule 40 is used when there’s no chance of physical damage. It can be used above ground and underground in both indoor and outdoor locations.
• Schedule 80: A heavier-walled, thicker conduit, Schedule 80 is used where physical damage is possible, such as emerging from grade. It can also be installed anywhere Schedule 40 is used, but it’s more expensive.

When to Use PVC Conduit

Use PVC conduit any place you need a weather-resistant, sturdy conduit that withstands corrosive environments. It’s lightweight, easy to handle, and cheaper than metal conduit. PVC is acceptable for underground use and in exposed locations. One caveat: It’s uglier than metal conduit, but if that’s not a dealbreaker, PVC is a versatile, durable wiring method.

Tips for Installing PVC Conduit

Ready to get started? Here are some key points to remember when installing PVC conduit.

Use Primer

Electricians often debate whether to use a primer before applying PVC cement. I’ve always done it, and many inspectors want to see the tell-tale purple residue on the pipe. In addition, many PVC cement brands say to use primer, so that’s what I do. (Following product labeling is an NEC requirement.) To use, liberally apply the primer to the end of the pipe, then apply glue. Push the pipe ends together and give the free pipe a quick quarter turn.

Debur PVC Conduit

Cutting PVC conduit with a hacksaw, circular saw, or PVC cutter leaves a rough edge that must be smoothed down or deburred before pulling wire through the conduit. Use a utility knife, metal file, pair of pliers or reamer to remove burs from both the inside and outside of the pipe edge. Run your fingers along the reamed edges to remove little bits of plastic.

Follow fill requirements

Conductors heat up as they carry electricity, so the NEC limits the number of wires you can install in PVC and all other conduit. This depends on the conduit size, the type (Schedule 80 or 40), the number of wires you’re pulling and their dimensions. Sometimes, it takes a bit of math to figure out, but the requirements are laid out in the NEC’s Chapter 9, Table 1, and Informative Annex C.

As a general rule, if you’re pulling a single circuit with a hot, neutral and EGC, the fill can’t be more than 40% of the conduit diameter. If the wires are all the same size, the NEC helpfully lists the maximum number of wires by wire type and conduit size.

Use plastic bushings and metal locknuts

Secure the PVC and male adapters coming into boxes with a metal locknut, then add a plastic bushing. The plastic bushing will protect the conductors from the friction of pulling them through the conduit, and metal locknuts are sturdier than plastic.

Bury PVC deep enough

The NEC has very specific depth requirements, outlined in Table 300.5, for all conduit runs underground. If you’re running a circuit out to a shed or detached garage, make sure you follow these rules to avoid damaging the circuit. The default depth for PVC is 18 inches unless the wiring meets certain qualifications. For example, if you are using PVC to run a 120-volt, GFCI-protected circuit that’s 20 amps or less, you can reduce that depth to 12 inches.

Keep bends to 360 degrees

Pulling wire through conduit gets harder with every bend, so the NEC limits the number of bends to 360 degrees before you have to put it in a junction box. This goes for all conduit runs, not just PVC. That’s four 90-degree turns before you need a box. Elbows also come in 45-degree bends and 22.5-degree bends, so add them up and keep the bends to 360 degrees or less.

Use metal elbows underground

The friction caused by pulling wire through underground PVC elbows will slice right through them, damaging the wire and leaving the pipe open to water intrusion. Instead, use metal elbows. Normally, all metal parts of a wiring system must be bonded (connected) to the grounding electrode system, but if you bury the run at least 18 inches deep, you do not need this bonding step.

Support at correct intervals

Like all conduit, PVC must be supported at various intervals to prevent sagging. The distance between supports depends on the size of the conduit, with smaller sizes requiring closer supports than larger ones. You’ll likely be using 1/2-in. or 3/4-in. PVC for home wiring and the supports for both must be no more than 3 feet apart. You can use plastic PVC straps or metal straps.

You also must support PVC within 3 feet of any conduit termination, such as an outlet box or junction box.

Use expansion fittings for long runs

You won’t believe how much PVC expands and contracts in the heat and cold, respectively. Let’s take a place like Minnesota, my old home state, where you can easily see a yearly temperature swing of 100 degrees, say from 90 degrees in the summer to minus 10 degrees in the winter (and that’s a good winter!). On a 100-foot run, the PVC will expand and contract about four inches.

If you don’t use an expansion fitting in the middle of the run, the PVC will likely crack or pull itself out of the fittings, leaving you with a potentially dangerous situation and an annoying repair job. The NEC’s table 352.44(A) outlines expected thermal expansion for various temperature swings and run lengths. You do not have to use expansion fittings underground, where temperatures are more stable.

Pull an equipment ground

Because PVC is plastic, you must pull an equipment grounding conductor (EGC) with your circuit conductors. An EGC provides a path back to the electrical panel so that any unintended current from a short circuit or ground fault can quickly trigger the breaker. Metal conduit can often serve as the EGC as long as every single connection point is bonded together. You don’t have that luxury with PVC because it’s not a conductive material.