Why Conduit Type Selection Matters
Conduit protects conductors from physical damage, moisture, and corrosive environments. Choosing the wrong conduit type can result in conductor insulation failure, shock hazards, costly rework, and failed inspections. Each conduit type is governed by a specific NEC article under Chapter 3 — Wiring Methods and Materials, and each has defined locations where it is permitted and prohibited.
EMT — Electrical Metallic Tubing (NEC Article 358)
EMT is the most widely used conduit in commercial construction. It is a thin-wall steel tube (thinner than RMC and IMC) that is light enough to bend with a hand bender or hydraulic bender, and is connected with set-screw or compression fittings — no threading required.
- Material: Galvanized steel or aluminum
- Weight: Light — the thinnest-wall metal conduit
- Permitted uses: Exposed and concealed work, through walls and floors, in concrete (if listed for that use), wet locations (with listed fittings)
- Prohibited uses: Where subject to severe physical damage, in cinder fill, where protection from corrosion is required but not provided
- Connections: Set-screw or compression couplings; rain-tight fittings for wet locations (NEC 358.42)
- Trade sizes: 1/2 inch through 4 inches
EMT cannot be threaded. It provides a continuous metallic raceway that can serve as the equipment grounding conductor when fittings are properly installed (NEC 358.60).
RMC — Rigid Metal Conduit (NEC Article 344)
RMC (also called rigid steel conduit or RSC) is the heaviest and most durable metal conduit. It can be threaded, and it provides the highest level of mechanical protection available in a conduit system.
- Material: Galvanized steel, stainless steel, or aluminum
- Weight: Heavy — approximately 3x the weight of EMT in the same trade size
- Permitted uses: All locations, including exposed outdoor, underground (direct buried, concrete encased), areas with physical damage exposure
- Prohibited uses: Very few; aluminum RMC is prohibited in contact with concrete containing chlorides
- Connections: Threaded couplings, elbows, and conduit bodies
- Trade sizes: 1/2 inch through 6 inches
RMC is the go-to choice for service entrance conduit, exposed outdoor runs at grade level, and any location where the conduit will be exposed to vehicle or forklift traffic. It also serves as an equipment grounding conductor (NEC 344.60).
IMC — Intermediate Metal Conduit (NEC Article 342)
IMC occupies the middle ground between EMT and RMC. It has thicker walls than EMT but thinner walls than RMC, can be threaded, and weighs approximately 30% less than RMC in the same trade size.
- Material: Galvanized steel
- Weight: Medium — approximately 70% of RMC weight
- Permitted uses: Same as RMC — all locations including outdoor and underground
- Connections: Threaded couplings (same thread pitch as RMC — NPSM taper), or listed fittings designed for IMC
- Trade sizes: 1/2 inch through 4 inches
IMC is common in outdoor feeders and service entrance work where RMC would be adequate but IMC saves weight and cost. Its permitted uses mirror RMC under NEC 342.10.
PVC Conduit (NEC Article 352)
PVC (polyvinyl chloride) conduit is the standard choice for underground direct burial and concrete-encased installations. It is non-conductive, highly corrosion-resistant, and significantly less expensive than metallic conduit.
- Schedule 40: Standard wall thickness; suitable for most direct burial and concrete encasement applications
- Schedule 80: Heavier wall thickness; required where exposed to physical damage (NEC 352.10(G)); has a smaller interior diameter than Schedule 40 in the same trade size
- Permitted uses: Underground direct burial, concrete encased, wet and dry locations, exposed (Schedule 80 where subject to physical damage)
- Prohibited uses: Hazardous locations (unless specifically listed), support of fixtures, where ambient temperature exceeds the conduit rating
Thermal expansion is a critical design consideration for PVC. PVC expands approximately 3.38 inches per 100 feet per 10 degrees F temperature change — far more than steel conduit. NEC 352.44 requires expansion fittings for long straight runs. A 200-foot outdoor PVC run may require an expansion fitting at each end.
PVC conduit does not provide a ground path — a separate EGC must be pulled inside per NEC 352.60.
FMC — Flexible Metal Conduit (NEC Article 348)
FMC (also called "Greenfield" or "flex") is a spiral-interlocked steel conduit that provides flexibility for connections to equipment that vibrates, moves, or is difficult to reach with rigid conduit. It is not liquid-tight.
- Permitted uses: Where flexibility is required, motor terminations, equipment subject to vibration, lengths to luminaires
- Prohibited uses: Wet or damp locations (use LFMC instead), in concrete, underground, where subject to physical damage, where exposed to corrosive vapors
- Maximum length for equipment: 6 feet for motor connections and similar equipment terminations (NEC 348.20(A)(1)). Longer runs require approval or a different wiring method.
- Grounding: FMC can serve as the EGC only in trade sizes 3/4 inch or smaller and lengths of 6 feet or less, with listed fittings (NEC 348.60)
LFMC — Liquidtight Flexible Metal Conduit (NEC Article 350)
LFMC is FMC with a plastic outer jacket that makes it resistant to liquids, oils, and moisture. It is the correct choice for motor connections in wet, damp, or oily locations — including outdoor AC units, food processing equipment, and machine tools.
- Permitted uses: Wet and damp locations, motor connections where flexibility is needed, outdoors (with listed fittings)
- Maximum length: 6 feet for motor terminations, same as FMC (NEC 350.20(A))
- Grounding: Same rules as FMC — 3/4 inch or smaller, 6 feet or less for use as EGC (NEC 350.60)
ENT — Electrical Nonmetallic Tubing (NEC Article 362)
ENT is a corrugated plastic tubing (commonly blue in color) that is pliable by hand and is used in residential construction concealed inside walls, floors, and ceilings. It is not rigid PVC conduit.
- Permitted uses: Concealed in walls, floors, and ceilings; in concrete; in one- and two-family dwellings and multi-family dwellings up to 6 stories
- Prohibited uses: Exposed work (except in accessible areas per NEC 362.10(3)), hazardous locations, in plenums, above suspended ceilings used as air plenums, where ambient temperature exceeds 50 degrees C, in commercial buildings unless specific conditions are met
Conduit Type Comparison Table
| Type | NEC Article | Material | Wall | Can Thread | Ground Path | Best Use |
|---|---|---|---|---|---|---|
| EMT | 358 | Steel/Aluminum | Thin | No | Yes (with fittings) | Commercial interior |
| RMC | 344 | Steel/Aluminum/SS | Heavy | Yes | Yes | Outdoor, service entrance, physical damage areas |
| IMC | 342 | Steel | Medium | Yes | Yes | Outdoor feeders, moderate duty |
| PVC Sch 40 | 352 | PVC plastic | Standard | No | No | Underground, concrete encased |
| PVC Sch 80 | 352 | PVC plastic | Heavy | No | No | Exposed, physical damage areas |
| FMC | 348 | Steel (spiral) | Flexible | No | Limited | Motor connections, dry locations, max 6 ft |
| LFMC | 350 | Steel + PVC jacket | Flexible | No | Limited | Motor connections, wet/oily, max 6 ft |
| ENT | 362 | PVC (corrugated) | Flexible | No | No | Residential concealed only |
Conduit Fill Calculations (NEC Chapter 9)
NEC Chapter 9, Table 1 limits how many conductors can occupy a conduit to prevent overheating and insulation damage during conductor pulling:
- 1 conductor: 53% fill (maximum percentage of conduit cross-sectional area)
- 2 conductors: 31% fill
- 3 or more conductors: 40% fill
To calculate, sum the cross-sectional areas of all conductors (from NEC Chapter 9, Table 5 for insulated conductors) and compare to the available area in the selected conduit (from NEC Chapter 9, Table 4). Select the smallest conduit that keeps you at or below the applicable fill percentage. Annex C in the NEC provides pre-calculated tables showing how many conductors of a given size fit in each conduit type and size.