8.3 Metallic and Nonmetallic Raceways
Key Takeaways
- Rigid Metal Conduit (RMC, Article 344) and Intermediate Metal Conduit (IMC, Article 342) offer the highest level of mechanical protection, are permitted in all atmospheric conditions, and allow extended support intervals up to 16 or 20 feet for straight horizontal runs with threaded couplings per Table 344.30(B)(2).
- NEC 344.26, 352.26, and 358.26 mandate that the total degrees of bends between pull points (boxes, conduit bodies, or cabinets) shall not exceed 360 degrees (four 90-degree bends) to prevent insulation damage during wire pulling.
- Electrical Metallic Tubing (EMT, Article 358) must be supported within 3 feet of every box or fitting and every 10 feet thereafter; set-screw fittings are restricted to dry indoor locations, while listed raintight compression fittings are required in wet and outdoor environments.
- Rigid Polyvinyl Chloride Conduit (PVC, Article 352) requires Schedule 80 wherever subject to physical damage per 352.10(F), and outdoor installations must include expansion fittings calculated from Table 352.44 (0.406 inches per 100 feet per 10°F temperature change) whenever thermal movement reaches 0.25 inches.
- Flexible Metal Conduit (FMC, Article 348) and Liquidtight Flexible Metal Conduit (LFMC, Article 350) require an internal or external wire-type equipment grounding conductor whenever the run exceeds 6 feet or the circuit overcurrent device exceeds 20 amperes under NEC 250.118.
8.3 Metallic and Nonmetallic Raceways
Exam Fast Fact: Raceway systems form the backbone of commercial, industrial, and institutional electrical construction. On the Colorado PSI Journeyman examination, candidates must know: (1) the 360-degree maximum bend rule across all conduit articles, (2) the support spacing tables for RMC (Table 344.30(B)(2)), EMT (358.30), and PVC (Table 352.30), (3) PVC thermal expansion calculations using NEC Table 352.44, and (4) the strict 6-foot / 20-ampere equipment grounding boundary for flexible conduits (FMC and LFMC) under NEC 250.118.
Conduit raceways provide mechanical protection, allow conductors to be pulled, upgraded, or replaced throughout a building's lifespan, and frequently serve as the equipment grounding conductor (EGC) for the enclosed circuits.
Heavy-Wall Metallic Raceways: RMC & IMC (Articles 344 & 342)
Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC) are threaded, heavy-wall circular raceways designed for extreme mechanical durability.
Material Differences
- Rigid Metal Conduit (Article 344): Heaviest metallic wall. Manufactured from galvanized steel, stainless steel, aluminum, or red brass. Listed for all atmospheric conditions, corrosive environments, hazardous (classified) locations (Class I, Division 1), and direct burial in soil or concrete.
- Intermediate Metal Conduit (Article 342): Intermediate wall thickness. Manufactured from high-strength alloy steel. Slightly lighter than RMC (roughly 30% lighter), providing similar mechanical strength and utilizing identical threaded fittings.
Threading & Field Reaming (NEC 344.28 & 342.28)
Field threads must be cut using standard pipe dies with a 3/4-inch taper per foot (NPT - National Pipe Taper). After cutting, all conduit ends must be reamed to remove sharp interior burrs that would abrade wire insulation during pulling.
THE 360-DEGREE TOTAL BEND RULE
Pull Point A Pull Point B
┌────────────┐ ┌────────────┐
│ Junction ├───────┐ │ Pull Box │
│ Box │ │ 90° Bend │ │
└────────────┘ ▼ └──────▲─────┘
│ │
│ 90° Bend │ 90° Bend
└───────────────►───────────────┐ │
│ │
▼ │
└────┘
90° Bend
Total Bends: 90° + 90° + 90° + 90° = 360° Maximum Permitted!
(Exceeding 360° requires an intermediate pull box or conduit body)
Maximum Bends Between Pull Points (NEC 344.26 & 342.26)
The total degrees of bends between pull points—such as junction boxes, pull boxes, cabinets, or conduit bodies (e.g., Type LB, T, or C fittings)—shall not exceed 360 degrees (the equivalent of four 90-degree bends). Every bend introduces mechanical friction during a wire pull; exceeding 360 degrees causes pulling ropes to burn through insulation or snap conductors.
Support Spacing for RMC & IMC (NEC 344.30 & Table 344.30(B)(2))
- General Rule (344.30(A)): Supported within 3 feet (900 mm) of each box, cabinet, conduit body, or termination fitting, and at intervals not exceeding 10 feet (3.0 m).
- Structural Exception: Where structural framing does not permit fastening within 3 feet, the distance may be increased to 5 feet (1.5 m).
- Straight Horizontal Runs with Threaded Couplings (Table 344.30(B)(2)): For industrial and commercial straight runs utilizing threaded couplings, support intervals may be extended based on conduit trade size:
| Conduit Trade Size (Inches) | Maximum Support Distance Between Rigid Metal Conduit Supports (Table 344.30(B)(2)) |
|---|---|
| 1/2" and 3/4" | 10 feet |
| 1" | 12 feet |
| 1-1/4" and 1-1/2" | 14 feet |
| 2" and 2-1/2" | 16 feet |
| 3" and larger | 20 feet |
Electrical Metallic Tubing: EMT (NEC Article 358)
Commonly referred to as "thinwall," Electrical Metallic Tubing (EMT) is the most widely installed raceway in commercial electrical construction due to its lightweight handling and rapid installation.
Characteristics & Prohibitions
- Unthreaded Wall: EMT has a thin wall that cannot be threaded. Any attempt to cut threads into EMT violates NEC 358.28(B).
- Uses Permitted (358.10): Exposed and concealed installations in dry, damp, and wet locations (with proper fittings); embedded in concrete; installed in direct contact with earth where provided with corrosion protection.
- Uses Not Permitted (358.12): Where subject to severe physical damage; in direct burial without concrete encasement (unless specially listed); or in hazardous (classified) locations except where specifically permitted by Articles 501 through 505.
Fittings: Set-Screw vs. Compression (NEC 358.42 & 314.15)
- Set-Screw Fittings: Utilize one or more clamping screws that seat against the EMT wall. Approved only for dry indoor locations. In concrete, set-screw fittings must be taped to achieve concrete-tight integrity.
- Compression Fittings: Utilize a split compression ring and threaded gland nut. Standard compression fittings are concrete-tight without taping.
- Raintight Compression Fittings: Outdoor and wet location installations require compression fittings equipped with internal elastomeric sealing rings/gaskets and marked "Raintight." Standard set-screw and indoor compression fittings are illegal in wet locations.
Securing and Supporting (NEC 358.30)
- Fastened within 3 feet (900 mm) of every outlet box, junction box, device box, cabinet, or fitting.
- Supported at intervals not exceeding 10 feet (3.0 m).
- Unbroken Length Exception: Where unbroken lengths of EMT pass through framing members, supports may be spaced up to 5 feet (1.5 m) from terminations.
- Maximum bends between pull points: 360 degrees (NEC 358.26).
Rigid Polyvinyl Chloride Conduit: PVC (NEC Article 352)
Rigid PVC conduit is a nonmetallic, nonconductive, flame-retardant raceway impervious to rust, chemical attack, and soil corrosion.
Schedule 40 vs. Schedule 80 PVC
| Technical Feature | Schedule 40 PVC (NEC 352.10) | Schedule 80 PVC (NEC 352.10(F)) |
|---|---|---|
| Wall Thickness | Standard heavy wall | Extra-heavy wall (roughly 40% thicker) |
| Physical Damage Rating | General underground and aboveground; NOT permitted where subject to physical damage | MANDATORY where subject to physical damage (exterior risers, vehicular areas, loading docks) |
| Internal Diameter | Standard internal area | Smaller internal area due to thicker wall |
| Conduit Fill Impact | Standard Chapter 9 Table 4 fill | Reduced wire capacity! Must use specific Schedule 80 Table 4 values |
Thermal Expansion Calculations (NEC 352.44 & Table 352.44)
PVC expands and contracts with temperature changes at roughly 5 to 6 times the rate of steel.
NEC 352.44 Rule: "Expansion fittings for PVC conduit shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44, is expected to be 0.25 in. (6 mm) or greater in a straight run between securely mounted items."
According to NEC Table 352.44, rigid PVC conduit expands or contracts by approximately 0.406 inches per 100 feet for every 10°F temperature change.
PVC CONDUIT THERMAL EXPANSION CALCULATION
Example: 120-Foot Exterior PVC Run on a Colorado Building Wall
Winter Low: -20°F | Summer High: 100°F | Temperature Delta: 120°F
Step 1: Determine Temperature Change (ΔT)
ΔT = 100°F - (-20°F) = 120°F
Step 2: Apply Table 352.44 Formula
ΔL = 120 ft × (120°F / 10°F) × (0.406 in. / 100 ft)
ΔL = 1.20 × 12 × 0.406 in. = 5.85 inches of thermal movement!
Conclusion: Over 5.8 inches of movement occurs! Multiple expansion
fittings must be installed to prevent conduit bowing or pulled boxes.
PVC Support Spacing (NEC Table 352.30)
Unlike steel conduit which can span 10 feet, PVC requires closer support spacing because of its flexibility:
| Conduit Trade Size (Inches) | Maximum Support Spacing for Rigid PVC (Table 352.30) |
|---|---|
| 1/2" through 1" | 3 feet |
| 1-1/4" through 2" | 5 feet |
| 2-1/2" through 3" | 6 feet |
| 3-1/2" through 5" | 7 feet |
| 6" | 8 feet |
All sizes must be supported within 3 feet of every box or termination fitting.
Flexible Metal Conduits: FMC & LFMC (Articles 348 & 350)
Flexible conduits isolate vibration from rotating machinery (motors, pumps, transformers) and provide flexible routing in tight spaces.
Types & Construction
- Flexible Metal Conduit (FMC / "Greenfield" — Art. 348): Helically wound, interlocking galvanized steel or aluminum armor. Permitted only in dry locations.
- Liquidtight Flexible Metal Conduit (LFMC / "Sealtite" — Art. 350): Flexible metal core with an outer sunlight-resistant, liquidtight thermoplastic jacket. Permitted in wet, outdoor, and corrosive locations.
Securing and Supporting (NEC 348.30 & 350.30)
- Supported within 12 inches (300 mm) of every box, cabinet, or fitting.
- Supported at intervals not exceeding 4.5 feet (1.4 m).
- The 6-Foot Luminaire/Equipment Exception: Flexible conduit runs up to 6 feet (1.8 m) from an enclosure termination are permitted without intermediate support for fixture whips to luminaires or connections to vibrating motors.
Grounding Rules: The 6-Foot / 20-Ampere Limit (NEC 250.118(5) & (6))
A heavily tested exam concept is whether FMC or LFMC can serve as the equipment grounding conductor:
NEC 250.118 Grounding Limitation: FMC and LFMC are permitted to serve as the sole equipment grounding conductor ONLY IF ALL of the following conditions are met:
- The total length in any ground return path does not exceed 6 feet (1.8 m).
- The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
- The conduit terminates in fittings listed for grounding.
If the run exceeds 6 feet OR the circuit breaker exceeds 20 amperes, a separate wire-type equipment grounding conductor (copper or aluminum) must be installed inside or outside the conduit.
Comprehensive Conduit Selection & Support Matrix
| Raceway Type | Article | Max Support Interval | Support from Box | Max Bends | Permitted as Sole EGC? |
|---|---|---|---|---|---|
| RMC (Rigid Metal) | Art. 344 | 10 ft (up to 20 ft Table 344.30(B)(2)) | 3 ft (5 ft exception) | 360° | YES (All sizes) |
| IMC (Intermediate Metal) | Art. 342 | 10 ft (up to 20 ft Table 344.30(B)(2)) | 3 ft (5 ft exception) | 360° | YES (All sizes) |
| EMT (Thinwall) | Art. 358 | 10 ft | 3 ft (5 ft unbroken) | 360° | YES (All sizes) |
| PVC (Sch 40/80) | Art. 352 | 3 ft to 8 ft (Table 352.30) | 3 ft | 360° | NO (Wire EGC required) |
| FMC (Greenfield) | Art. 348 | 4.5 ft (6 ft whip exception) | 12 in. | 360° | Only if ≤ 6 ft AND ≤ 20A |
| LFMC (Liquidtight) | Art. 350 | 4.5 ft (6 ft whip exception) | 12 in. | 360° | Only if ≤ 6 ft AND ≤ 20A |
Practical Jobsite Scenarios & Common Exam Traps
| Jobsite Scenario | Technical Reality & Code Rule | Common PSI Exam Trap |
|---|---|---|
| Outdoor EMT Set-Screw Fittings: An installer runs EMT on the exterior wall of a car wash using zinc die-cast set-screw fittings. | Violation of 358.42 & 314.15: Wet and outdoor locations mandate listed raintight compression fittings. Set-screw fittings allow water intrusion and rapid short circuits. | Assuming set-screw fittings are permitted outdoors if protected by a roof overhang. |
| 50A Rooftop AC Whip: An HVAC installer connects a 50A rooftop air handler using a 5-foot whip of LFMC without pulling a green grounding wire, relying on the metallic armor. | Violation of 250.118(6): Although the whip is under 6 feet, the overcurrent device is 50A (exceeds 20A limit). A separate wire-type EGC is strictly required. | Forgetting that BOTH conditions (≤ 6 feet AND ≤ 20 amperes) must be met simultaneously. |
| Schedule 40 PVC at Warehouse Dock: An electrician installs Schedule 40 PVC conduit exposed along a warehouse truck loading dock wall. | Violation of 352.10(F): Areas subject to vehicular traffic or forklift impact are subject to physical damage. Schedule 80 PVC or RMC is mandatory. | Assuming Schedule 40 PVC is permitted anywhere outdoors or in commercial facilities. |
| Conduit Run with Five 90° Bends: An apprentice installs a 1-inch EMT run between two junction boxes containing five 90-degree bends ($5 \times 90^{\circ} = 450^{\circ}$). | Violation of 358.26: The maximum total bends cannot exceed 360 degrees between pull points. An intermediate pull box or conduit body (LB) must be installed. | Believing 360 degrees applies per 100 feet of pipe rather than between pull points. |
An electrician is installing a straight horizontal run of 2-inch Rigid Metal Conduit (RMC) along a warehouse structural steel beam using threaded couplings. According to NEC Table 344.30(B)(2), what is the maximum permitted distance between conduit supports?
What is the maximum total degrees of bends permitted between pull points (pull boxes, junction boxes, or conduit bodies) in an Electrical Metallic Tubing (EMT) run under NEC 358.26?
When installing a 5-foot length of Liquidtight Flexible Metal Conduit (LFMC) to connect an exterior 40-ampere commercial air conditioning condensing unit, what is the equipment grounding requirement?