Conduit Types, Properties, and NEC Installation Requirements

Conduit Types, Properties, and NEC Installation Requirements

Conduit is the protective raceway that houses electrical conductors in a building's wiring system. Selecting the correct conduit type for a given environment is not optional—it is a code requirement and a safety obligation. The National Electrical Code (NFPA 70, commonly called the NEC) dedicates separate articles to each conduit type. This lesson covers the five types you will encounter most often: EMT, IMC, RMC, PVC, and FMC.

The Five Common Conduit Types

Electrical Metallic Tubing (EMT) — Article 358. EMT is a thin-wall, unthreaded steel (or aluminum) raceway. Because its wall is thinner than IMC or RMC, it is the lightest and least expensive metal conduit. It is joined with set-screw or compression fittings, never threaded. EMT is permitted in exposed and concealed work in most dry and damp locations, and in concrete if it is listed for that use. It is not permitted where subject to severe physical damage or in hazardous (classified) locations unless specifically listed.

Intermediate Metal Conduit (IMC) — Article 342. IMC has a thicker wall than EMT but a thinner wall than RMC. It is threaded and offers good mechanical protection. IMC is suitable for all the same locations as RMC, including outdoor, wet, and concrete-encased applications, and it is lighter than RMC, making it a popular choice on commercial projects.

Rigid Metal Conduit (RMC) — Article 344. RMC (also called rigid steel or galvanized rigid conduit, GRC) has the thickest wall of all metal conduits. It is threaded and provides the highest level of mechanical protection. RMC is permitted in all occupancies and all environments, including wet locations, direct burial, concrete encasement, and hazardous locations. When in doubt about the harshest environment on a job, RMC is typically the safe choice.

Rigid PVC Conduit — Article 352. PVC conduit is a nonmetallic plastic raceway. It resists moisture and many chemicals, making it ideal for underground, direct-burial, and wet locations. PVC does not corrode. However, it requires an equipment-grounding conductor run inside because the conduit itself does not provide a metal grounding path. PVC expands and contracts significantly with temperature changes; expansion joints are required on long runs. PVC is not suitable for exposed locations where it may be subject to physical damage unless Schedule 80 is used, and its use in some hazardous locations is restricted.

Flexible Metal Conduit (FMC) — Article 348. FMC is a spiral-wound, interlocked metal raceway that is flexible along its length. It is used for final connections to motors, equipment with vibration, or where rigid conduit cannot be routed efficiently. The NEC limits the length of FMC used as an equipment-grounding conductor path; consult the code for specific length limitations and grounding rules. FMC is generally not permitted in wet locations unless it is specifically listed for that use (liquid-tight FMC covers that application).

Trade Size, OD, and ID

Conduit is specified by trade size (½", ¾", 1", and so on), which is a nominal designation—not an actual measurement. The actual outside diameter (OD) and inside diameter (ID) vary between conduit types of the same trade size because wall thickness differs. IMC and RMC have thicker walls than EMT, so for the same trade size, their ID is slightly smaller than EMT. This matters when calculating conduit fill. Always use the conduit-specific tables in NEC Annex C or Chapter 9.

Conduit Fill Rules — NEC Chapter 9 and Annex C

The NEC limits how much of a conduit's cross-sectional area may be occupied by conductors:

• One conductor: 53% fill
• Two conductors: 31% fill
• Three or more conductors: 40% fill

These percentages apply to the conduit's usable interior area. NEC Annex C provides pre-calculated tables showing the maximum number of conductors of each wire type and AWG size that fit in each conduit type and trade size. Use Annex C for efficiency on the job. For mixed wire sizes or types, use the Chapter 9 tables and calculate the total conductor area against the allowable fill area.

Bend Count and Support Spacing

The NEC limits cumulative bends between pull points (junction boxes or conduit bodies) to 360 degrees total. A 90° sweep equals one quarter of that budget. Exceeding 360° makes pulling conductors extremely difficult and can damage insulation.

Support spacing requirements are specified by conduit type in the relevant NEC article. As a general rule for metal conduits, supports are required within a specified distance of every box, cabinet, or fitting, and at intervals along the run. RMC and IMC are permitted longer intervals between supports than EMT in many cases. Always verify against the specific NEC article for the conduit you are installing.

Matching Conduit to Occupancy and Environment

| Environment | Typical Choice | Reason |

|---|---|---|

| Dry indoor walls | EMT | Economical, adequate protection |

| Outdoor exposed | RMC or IMC | Mechanical protection, weather resistance |

| Wet or underground | PVC or RMC | Corrosion resistance |

| Concrete encased | RMC, IMC, or PVC | Listed for that use |

| Motor final connections | FMC | Flexibility, vibration isolation |

| Hazardous locations | RMC (verify listing) | Maximum protection; check specific article |

Safety Notes

Before working on any existing conduit system, follow lockout/tagout (LOTO) procedures—verify the circuit is de-energized with a tested meter before opening any fitting. Wear safety glasses when cutting conduit; metal shavings are a serious eye hazard. Ream all cut conduit ends to remove burrs that can damage conductor insulation. When handling PVC solvent cement, work in ventilated areas and wear chemical-resistant gloves. On elevated work, follow fall-protection requirements and secure all tools and conduit sections.