Conduit is a tube that protects electrical wires and gives them a path to follow. Instead of a cable with wires already inside, an electrician installs the empty tube first, then pulls the wires through it. The United States uses four main types: EMT, PVC, RMC and FMC.
What Is Conduit and Why Use It?
Conduit protects wires from damage, water and sunlight, and it keeps runs neat through walls, ceilings and underground. It also lets wires be replaced later: an old wire can be pulled out and a new one pulled in without opening the wall.
The wires inside conduit are usually single THHN/THWN-2 copper conductors, each with its own coloured insulation. Hot wires are black or red, the neutral is white or grey, and the ground is green or bare.
EMT: Electrical Metallic Tubing
EMT is the thin-wall steel tube most people picture when they think of conduit. It is covered by NEC Article 358. EMT is light, easy to cut, and easy to bend with a hand bender. It is used indoors in dry locations: surface runs in garages, basements and workshops. It is the most common conduit in indoor work because it is fast and cheap.
PVC: Rigid Polyvinyl Chloride Conduit
PVC conduit is a rigid plastic tube, covered by NEC Article 352. Its great strength is that it never rusts, so it is the standard choice outdoors and underground. The common grades are Schedule 40 for most work and Schedule 80, with a thicker wall, for places where the conduit may be hit.
PVC is joined with solvent cement and bent with a heat box that softens the plastic so it can be shaped. It expands with temperature more than metal does, so long outdoor runs need expansion fittings.
RMC: Rigid Metal Conduit
RMC is the heavyweight of the group. It is a thick-wall galvanised steel tube, covered by NEC Article 344, with threaded ends. It gives the strongest protection of any conduit, so it is used where wires face severe physical damage, in hazardous locations, and outdoors where nothing else will survive.
Diagram: Four conduit types in cross-section. EMT is thin-wall steel, PVC is thick-wall plastic, RMC is heavy thick-wall steel with threads, and FMC is a flexible spiral metal strip.
FMC: Flexible Metal Conduit
FMC, often called Greenfield, is a flexible tube made from a spiral-wound strip of steel or aluminium, covered by NEC Article 348. It bends by hand without a bender, which makes it ideal for short runs to a motor, a furnace or an air conditioner that vibrates.
Conduit Fill and Bending Rules
A conduit must never be stuffed full of wires. From Chapter 9, Table 1, one wire may fill up to 53 percent of the conduit, two wires up to 31 percent, and more than two wires up to 40 percent. In practice a half-inch EMT can hold nine 12 AWG THHN wires, while a three-quarter-inch EMT can hold sixteen.
Bending has rules too. No run may have more than 360 degrees of bends between pull points, or four 90-degree bends, or the wire cannot be pulled through. Each bend must have a minimum radius so the wire inside is not crushed.
Diagram: Conduit fill for 12 AWG THHN wires in EMT at the 40 percent limit: 9 wires in half-inch, 16 in three-quarter-inch, and 26 in one-inch conduit.
Key Takeaways
EMT, PVC, RMC and FMC are the four main conduit types in US work. EMT is thin-wall steel for dry indoor runs, PVC is rust-proof plastic for outdoors and underground, RMC is thick threaded steel for the toughest locations, and FMC is flexible metal for short connections to motors and equipment. Fill is limited to 40 percent for more than two wires, and no run may exceed 360 degrees of bends between pull points.
Objective Questions
1. Which conduit is the standard choice for dry indoor surface runs in a garage?
a) PVC Schedule 40
b) EMT
c) RMC
d) Liquidtight flexible metal conduit
Answer: (b) EMT
Explanation: EMT is the thin-wall steel tube used for dry indoor runs because it is light, cheap and easy to bend.
2. Which conduit type is best for underground and outdoor runs because it never rusts?
a) EMT
b) RMC
c) PVC
d) FMC
Answer: (c) PVC
Explanation: PVC is a rigid plastic conduit that never rusts, so it is the standard choice outdoors and underground.
3. What is FMC most often used for?
a) Long straight runs across a ceiling
b) Underground feeders
c) Short flexible connections to motors, furnaces and air conditioners
d) Service-entrance raceways
Answer: (c) Short flexible connections to motors, furnaces and air conditioners
Explanation: FMC bends by hand and is ideal for short runs to vibrating equipment, usually 3 to 6 feet long.
4. Under the NEC, what is the maximum conduit fill when there are more than two wires?
a) 31 percent
b) 40 percent
c) 53 percent
d) 60 percent
Answer: (b) 40 percent
Explanation: Chapter 9, Table 1 limits fill to 40 percent for more than two wires, 31 percent for two wires and 53 percent for one wire.
5. What is the maximum total bend allowed in one conduit run between pull points?
a) 90 degrees
b) 180 degrees
c) 270 degrees
d) 360 degrees
Answer: (d) 360 degrees
Explanation: No conduit run may have more than 360 degrees of bends, or four 90-degree bends, between pull points, or the wire cannot be pulled through.
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