Installing Conduit: Cutting, Threading, Support, and Fill

A bend is only part of a conduit job. The pipe also has to be cut square, reamed clean, joined tight, supported at the right spacing and sized so the wire will fit. Those steps decide whether the wire pulls easily, whether the run passes inspection, and whether the metal raceway can do its other job as a fault path. This guide covers the installation work on either side of the bender.

What You Will Be Able to Do

• Cut and ream EMT, IMC and RMC so conductors are not damaged.
• Thread rigid conduit and join it properly.
• Choose fittings for dry, wet and buried locations.
• Support a run according to the NEC.
• Work out conduit fill, and plan a pull with enough pull points.

Article numbers come from the National Electrical Code (NEC). Editions change and states adopt different ones, so check the edition enforced on your job.

Raceways at a Glance

Metal raceways can serve as the equipment grounding conductor (EGC), but only if every joint is tight. PVC and most flexible raceways need a separate EGC.

Cutting

EMT

Cut EMT with a band saw, reciprocating saw, hacksaw or tubing cutter. Keep the cut square, because a slanted end will not seat in a fitting. A tubing cutter leaves a raised inner edge, so ream it carefully.

IMC and RMC

Use a band saw, a portable band saw or a pipe cutter, with blades made for steel. A roll cutter leaves a heavy burr. Clamp the pipe in a vise or a stand so it cannot spin.

PVC

Use a PVC saw, a ratchet cutter or a fine-tooth blade. Wipe off the shavings, and chamfer the outside so it slides into the fitting.

Reaming

Every cut end must be reamed or finished to remove rough edges, as NEC 358.28 and 344.28 require. A burr left on the inside will cut insulation when you pull, and the damage can be hidden in the pipe until a fault appears years later. Use a reamer, a half-round file or the reaming tip on a pair of multi-tool pliers, and feel inside with a gloved finger or a pencil.

Warning: Cut pipe has razor-sharp edges, and hot metal chips fly during cutting. Wear eye protection and cut-resistant gloves, and let chips cool before you touch them.

Threading Rigid Conduit

IMC and RMC use tapered threads, cut with a die at a taper of 1 in 16, or 3/4 inch per foot, as NEC 344.28 requires. Do not use a straight-thread die.

1. Clamp the pipe firmly. A threading machine or tristand holds it for you.
2. Square the end and ream it.
3. Fit the die to the pipe and start it straight. A crooked start gives you a crooked thread.
4. Apply plenty of cutting oil to the die and the pipe as it cuts. Use a threading oil with extreme-pressure additives, not motor oil.
5. Turn the die until the end of the pipe is about flush with the far face of the die. On a 1/2 inch pipe, this gives about 3/4 inch of thread. Check the shop's chart for larger sizes.
6. Back the die off, and wipe off chips and oil.
7. Check the thread with a fitting. A good thread starts by hand and tightens with a wrench.

Tighten fittings with a wrench so that they are fully made up. The joint is part of the fault path, so hand-tight is not enough. Do not leave threads exposed on a finished run.

Where corrosion is a concern, field-cut threads need an approved, electrically conductive, corrosion-resistant compound. Do not paint the threads with anything that insulates the joint.

Threading machines can grab loose clothing and gloves. Keep sleeves tight, and use the foot switch so the machine stops the moment you step off it.

Fittings and Connectors

EMT fittings. Set-screw fittings are quick, and they are the standard in dry locations. Compression fittings use a nut and gland, and they are required where a raintight or concrete-tight seal is needed, as long as they are listed for it. Do not use set-screw fittings in wet locations unless they are listed for them.

Tight means tight. A loose locknut or set screw can leave a raceway ungrounded. Tighten locknuts with a tool. On a painted enclosure, make sure metal bites into metal, or use a bonding locknut or bushing if your job requires it.

Bushings. Where conductors of 4 AWG or larger enter a box or enclosure, an insulated bushing or a fitting with a smooth insulated throat is required, to protect the insulation from the metal edge.

Conduit bodies. LB, LL, LR and T bodies give you a corner or a pull point in tight spots. A conduit body can only hold splices if it is marked with a volume for them, and it must stay accessible.

Expansion and deflection fittings. Use these where a run crosses a building joint, or where movement is expected.

Supporting the Run

Support keeps the raceway in place, and it keeps stress off the fittings.

The tables in Articles 344 and 352 list other sizes. Rigid metal can sometimes extend farther on straight threaded runs, because the pipe is stiff. For example, 1-inch RMC can go up to 12 feet between supports under those conditions.

Securing methods:

• One-hole and two-hole straps on a wall or strut.
• Strut channel, such as a 1-5/8 inch channel, with pipe clamps for racks of conduit.
• Beam clamps on steel, with the right rod or clamp for the load.

Use a strap sized to the pipe. Do not hang conduit from ceiling grid wires or from other trades' pipe. Support must come from the building structure. Keep raceways from being used to support other things, unless the code allows it.

Fill

Too much wire in a conduit makes the pull hard, damages insulation and traps heat. NEC Chapter 9, Table 1 sets the fill limits:

Two conductors have the lowest limit because they can jam against each other as they pull around a bend. A short nipple of 24 inches or less can be filled to 60 percent.

To calculate fill:

1. Find the area of each conductor from Chapter 9, Table 5, using the insulation type.
2. Add the areas together, including EGCs.
3. Compare the total with the allowed area of the raceway from Chapter 9, Table 4.

If all conductors are the same size, Annex C lets you skip the math. Its tables list how many fit.

Worked Example: Mixed Conductors

You want to pull three 8 AWG THHN, one 10 AWG THHN and one 12 AWG THHN in 3/4 inch EMT.

1. Three 8 AWG: 3 x 0.0366 = 0.1098.
2. One 10 AWG: 0.0211.
3. One 12 AWG: 0.0133.
4. Total: 0.1442 sq in.
5. Allowed: 0.213 sq in for 3/4 inch EMT.

It fits. A code limit is still a maximum, so an upsized pipe makes a long pull easier, and leaves room for future circuits. Note that more wires in a raceway can also force you to reduce conductor ampacity, which is a separate calculation.

Planning the Pull

Total bends between pull points cannot exceed 360 degrees, or four 90s, under NEC 358.26 and 344.26. That is the maximum. A long run with three 90s is often hard to pull, so add a pull box or conduit body when you can.

Before you pull:

1. Finish and support the raceway first. Wire goes in after the raceway is complete.
2. Run a fish tape or blow in a pull string.
3. Use pulling lubricant listed for the insulation.
4. Have someone feed at the start, keeping the wires straight and in the right order.
5. Pull steadily and try not to stop, because restarting takes more force.

For heavy pulls, use a power puller and the right rope, and keep clear of the rope's line. A rope under load can snap and whip with enough force to injure.

Leave at least 6 inches of free conductor at each box.

Going Outdoors and Underground

• Conduit going from a warm space to a cold one can form condensation. Seal the raceway at the transition with a duct seal or other approved material.
• Minimum cover depends on the wiring method. The usual values are 6 inches for RMC and IMC, 18 inches for PVC and 24 inches for direct-buried cable. Driveways and other locations change this, so read Table 300.5 or your edition's equivalent.
• Service conductors that are buried and not encased in concrete need a warning ribbon in the trench, at least 12 inches above them.
• PVC expands in the heat. It grows about 4 inches per 100 feet for a 100-degree temperature change, so long exposed runs need expansion fittings.

Common Mistakes

Check Your Understanding

1. A 1-inch EMT run goes 26 feet along a wall from one box to another. How many supports do you need at minimum?

Three, such as at 3, 13 and 23 feet. That keeps one within 3 feet of each box, with no more than 10 feet between supports.

2. How many 12 AWG THHN conductors can you put in 1/2 inch EMT?

0.122 / 0.0133 = 9.17, so 9 conductors.

3. You need to pull wire into a run with two 90s, a 30-degree offset, and another 90. Is the run legal?

90 + 90 + 60 + 90 = 330 degrees, so it is legal. It will still be a stiff pull, so use lubricant, and consider a pull point.

4. Why does a threaded joint need to be wrench-tight?

The joint carries fault current. A loose joint can stop a breaker from tripping quickly and leave metal parts live.

Key Takeaways

• Ream every cut end, or the pipe will damage the wire.
• Thread with a 3/4-inch-per-foot taper, using plenty of cutting oil.
• Metal raceway joints are part of the fault path, so they must be tight.
• Support EMT and rigid within 3 feet of a box and every 10 feet.
• Fill limits are 53, 31 and 40 percent for one, two and more than two conductors.
• Keep a run to 360 degrees of bends between pull points, and finish the raceway before you pull.