12.1 Metal (RMC, IMC, EMT) & Nonmetallic (PVC, RTRC, LFNC) Raceways

12.1 Metal (RMC, IMC, EMT) & Nonmetallic (PVC, RTRC, LFNC) Raceways

Quick Answer: Metallic raceways like RMC (Article 344), IMC (Article 342), and EMT (Article 358) provide high mechanical protection and qualify as equipment grounding conductors (EGCs) under NEC 250.118. EMT must be supported every 10 feet and within 3 feet of each box. Nonmetallic raceways like PVC (Article 352) and RTRC (Article 355) are impervious to rust and corrosive chemicals but require an internal EGC. Schedule 80 PVC is listed for areas subject to physical damage, whereas Schedule 40 PVC is prohibited where subject to physical damage. Across all raceway systems, total bends between pull points cannot exceed 360 degrees.

Selecting the proper raceway involves evaluating structural durability, chemical exposure, ambient temperature, ease of installation, and equipment grounding capability. For the Minnesota journeyman electrician, understanding the mechanical boundaries and securing rules of each conduit system is critical for passing the state licensing examination.

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Metallic Raceways: RMC, IMC, and EMT

Rigid Metal Conduit (Type RMC - NEC Article 344)

Rigid Metal Conduit is the heaviest and most robust raceway recognized in electrical construction. Manufactured from galvanized steel, stainless steel, or aluminum, RMC is threaded at both ends and provides unmatched protection against crushing, severe impact, and projectile puncture.

• Permitted Locations (NEC 344.10): Permitted under all atmospheric conditions and occupancies, including severe physical damage environments, hazardous (classified) locations, and direct burial in soil or concrete.
• Support Rules (NEC 344.30): Must be securely fastened within 3 feet (900 mm) of each outlet box, junction box, cabinet, or conduit body. Where structural members do not permit fastening within 3 feet, the distance may be extended to 5 feet (1.5 m). Standard support interval along runs is 10 feet (3.0 m).
• Straight Run Support Extension (Table 344.30(B)(2)): For straight runs of rigid metal conduit made up with threaded couplings, support intervals may be dramatically extended based on trade size:
  • Trade Size $1/2\text{ in} - 3/4\text{ in}$: Maximum support spacing 10 feet
  • Trade Size $1\text{ in}$: Maximum support spacing 12 feet
  • Trade Size $1\frac{1}{4}\text{ in} - 1\frac{1}{2}\text{ in}$: Maximum support spacing 14 feet
  • Trade Size $2\text{ in} - 2\frac{1}{2}\text{ in}$: Maximum support spacing 16 feet
  • Trade Size $3\text{ in}$ and larger: Maximum support spacing 20 feet
• Grounding (NEC 250.118): RMC is fully recognized as an equipment grounding conductor without requiring an internal copper wire, provided all threaded joints are made up wrench-tight.

Intermediate Metal Conduit (Type IMC - NEC Article 342)

Introduced as a lighter-weight alternative to RMC, Intermediate Metal Conduit features a slightly thinner wall profile while utilizing high-strength steel alloy. IMC accepts standard trade-size threaded fittings and delivers comparable mechanical impact performance to RMC at approximately $30\%$ less weight.

• Permitted Locations (NEC 342.10): Permitted in all occupancies and areas subject to physical damage.
• Support Rules (NEC 342.30): Identical to RMC: supported within 3 feet of terminations and every 10 feet along runs, with the same support spacing extensions under Table 342.30(B)(2).
• Grounding (NEC 250.118): IMC is listed as an equipment grounding conductor.

Electrical Metallic Tubing (Type EMT - NEC Article 358)

Electrical Metallic Tubing—commonly called "thinwall"—is unthreaded steel or aluminum tubing joined by setscrew or compression couplings. It is the workhorse raceway of commercial interior construction.

• Permitted Locations (NEC 358.10): Exposed and concealed in dry, damp, and wet locations (using listed wet-location fittings), and embedded in concrete with approved fittings.
• Prohibited Locations (NEC 358.12): Prohibited where subject to severe physical damage. Installing EMT in loading docks where forklifts operate or at bumper level in commercial parking garages is a direct code violation.
• Support Rules (NEC 358.30(A)): Must be securely fastened within 3 feet (900 mm) of each outlet box, junction box, cabinet, or fitting. Supports along runs must not exceed 10 feet (3.0 m). (Exception: Up to 5 feet from terminations where structural framing does not permit 3 feet).
• Grounding (NEC 250.118): EMT serves as an equipment grounding conductor when installed with listed fittings that are tightened securely.

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Nonmetallic Raceways: PVC, RTRC, and LFNC

Rigid Polyvinyl Chloride Conduit (Type PVC - NEC Article 352)

PVC conduit is nonconductive, highly resistant to moisture and corrosive chemicals, and lightweight. However, it exhibits a high coefficient of thermal expansion and becomes brittle in freezing temperatures.

• Support Spacing for PVC (Table 352.30):
  • Trade Size $1/2\text{ in} - 1\text{ in}$: 3 feet
  • Trade Size $1\frac{1}{4}\text{ in} - 2\text{ in}$: 5 feet
  • Trade Size $2\frac{1}{2}\text{ in} - 3\text{ in}$: 6 feet
  • Trade Size $3\frac{1}{2}\text{ in} - 5\text{ in}$: 7 feet
  • Trade Size $6\text{ in}$: 8 feet
  • In addition, PVC must be secured within 3 feet (900 mm) of each box, cabinet, or termination.
• Grounding (NEC 352.60): Because PVC is an electrical insulator, it cannot serve as an equipment grounding conductor. An insulated or bare equipment grounding conductor must always be pulled inside the raceway.

Reinforced Thermosetting Resin Conduit (Type RTRC - NEC Article 355)

RTRC, commonly referred to as fiberglass conduit, is manufactured by winding continuous glass filaments impregnated with epoxy resin. It provides exceptional corrosion resistance in industrial wastewater and chemical plants and has a lower coefficient of thermal expansion than PVC (Table 355.44). RTRC marked -XW is identified for areas of physical damage. Support spacing is governed by Table 355.30.

Liquidtight Flexible Nonmetallic Conduit (Type LFNC - NEC Article 356)

LFNC consists of a smooth inner nonmetallic core and an oil-resistant, water-resistant outer jacket. It is commonly used for outdoor air conditioning condensing units and motor terminations where vibration or liquid exposure is present.

• Support Rules (NEC 356.30): Must be securely fastened at intervals not exceeding 3 feet (900 mm) and within 12 inches (300 mm) on each side of every box, cabinet, or fitting, with exceptions for fishing, lengths of not more than 3 feet at terminals where flexibility is needed, and luminaire whips in accessible ceilings.

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Master Raceway Engineering Comparison Table

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The 360-Degree Bend Rule and Pull Point Architecture

Under NEC Sections 344.26 (RMC), 342.26 (IMC), 358.26 (EMT), and 352.26 (PVC), the total degrees of bends in a single raceway run between pull points must not exceed 360 degrees.

$\text{Total Bends} = \sum \theta \le 360^\circ \quad (\text{e.g., Four } 90^\circ \text{ Bends, or Two } 90^\circ + \text{Four } 45^\circ \text{ Bends})$

Every degree of bend increases the frictional pulling tension and elevates the sidewall bearing pressure (SWBP) exerted on the conductor insulation as it rides along the inside radius of the elbow. If a run requires more than 360 degrees of bends, the installer must insert an intermediate pull box, conduit body (Type C, LB, LL, LR, or T), or junction box to serve as a designated pull point.