5.3 Raceways: EMT, RMC, IMC, PVC & Flexible Conduits
Key Takeaways
- Electrical Metallic Tubing (EMT - NEC Article 358) is a lightweight unthreaded steel or aluminum raceway supported within 3 ft of boxes and every 10 ft along runs; it is prohibited where subject to severe physical damage, and requires compression fittings with gaskets in wet locations.
- Rigid Metal Conduit (RMC - Article 344) and Intermediate Metal Conduit (IMC - Article 342) are heavy-wall threaded raceways approved for severe physical damage and Class I Division 1 hazardous locations, with minimum burial depth of 6 inches under Table 300.5.
- Rigid Polyvinyl Chloride Conduit (PVC - Article 352) requires Schedule 80 where subject to physical damage per NEC 352.10(F); because PVC expands at ~4.06 inches per 100 ft per 100°F temperature shift, thermal expansion fittings are required when length changes exceed 0.25 inches under NEC 352.44.
- Under the 2023 NEC exam reference, listed FMC and LFMC can serve as an equipment grounding path only when all applicable 250.118 conditions are met, including the 6-foot combined-path and 20-ampere limits; candidates assigned the 2026 NEC must use its revised provisions.
- The 360-degree bend mandate across all NEC raceway articles restricts total bends between pull points (boxes, conduit bodies) to not more than four 90-degree quarter bends to prevent conductor insulation damage during pulling.
5.3 Raceways: EMT, RMC, IMC, PVC & Flexible Conduits
Quick Answer: Raceways provide an engineered, enclosed channel for pulling and protecting electrical conductors. EMT (Article 358) is a thin-wall unthreaded raceway generally secured within 3 feet of terminations and supported every 10 feet; fittings in wet locations must be listed for that environment. RMC (Article 344) and IMC (Article 342) are heavy-wall threaded conduits permitted in hazardous environments and locations subject to severe physical damage, offering the lowest direct burial depth (6 inches). Rigid PVC (Article 352) requires Schedule 80 where subject to physical damage, and necessitates expansion fittings when thermal movement equals or exceeds 0.25 inches. Under the 2023 NEC exam reference, listed FMC or LFMC may serve as an equipment grounding path only when every condition in 250.118 is met; the applicable raceway combination is limited to 6 feet and the older rule limits the circuit to 20 amperes. All raceways are restricted to a maximum of 360 degrees of total bends between pull points.
1. Electrical Metallic Tubing (EMT - NEC Article 358)
Electrical Metallic Tubing (EMT), often called "thin-wall," is the most common commercial raceway. Because EMT has a relatively thin wall, it cannot be threaded; all connections must be made with specialized clamp-on or set-screw fittings.
ELECTRICAL METALLIC TUBING (EMT)
=================================================================
(Thin-wall unthreaded steel or aluminum circular raceway)
[Box] <---- 3 ft max ----> [Strap] <------- 10 ft max -------> [Strap]
(5 ft if no framing) (NEC 358.30)
Uses Permitted & Prohibited (NEC 358.10 & 358.12)
- Permitted (358.10): Exposed and concealed installations; dry, damp, and wet locations (using listed wet-location fittings); embedded in concrete; direct burial in the earth where corrosion protection is provided.
- Prohibited (358.12): Where subject to severe physical damage (such as vehicle pathways or loading docks); in cinder concrete or fill where subject to permanent moisture unless protected by a 2-inch layer of ordinary concrete; in hazardous (classified) locations except where specifically permitted by Chapter 5.
Securing and Supporting (NEC 358.30)
- Distance to Terminations (358.30(A)): EMT must be securely fastened within 3 feet (900 mm) of each outlet box, junction box, device box, cabinet, conduit body, or other tubing termination.
- Unbroken Structural Spacing Exception: Where structural members do not permit fastening within 3 feet, the distance to the first support may be increased to not more than 5 feet (1.5 m).
- Support Along the Run: EMT must be supported at intervals not exceeding 10 feet (3.0 m).
Fittings: Set-Screw vs. Compression
- Set-Screw and Compression Fittings: Both styles are available with different listings; the connection method alone does not establish a wet-location rating.
- Wet-Location Fittings: NEC 358.42 requires couplings and connectors used with EMT to be made up tight, and installations in wet locations must comply with 314.15. Use fittings specifically listed and marked for the wet environment; do not infer suitability merely because a fitting is compression style.
2. Heavy-Wall Metal Raceways: RMC & IMC (Articles 344 & 342)
When electrical conductors require maximum mechanical shielding against catastrophic impact, crushing forces, or volatile atmospheric gases, heavy-wall threaded conduits are mandated.
+-------------------------------------------------------------------------+
| RMC vs. IMC vs. EMT WALL THICKNESS |
| |
| (O) RMC: Heavy-wall threaded galvanized steel (Article 344) |
| * Maximum mechanical protection; Class I, Div. 1 hazardous |
| |
| (o) IMC: Intermediate-wall threaded steel (Article 342) |
| * Lighter than RMC; ~30% lighter weight; same interior capacity |
| |
| ( ) EMT: Thin-wall unthreaded steel (Article 358) |
| * Cannot be threaded; set-screw or compression fittings only |
+-------------------------------------------------------------------------+
Rigid Metal Conduit (Type RMC - Article 344)
- Construction: Heavy-wall threaded conduit of galvanized steel, stainless steel, or aluminum.
- Physical Protection: Approved for all locations, including where subject to severe physical damage.
- Hazardous Locations: Fully approved for Class I, Division 1 and Division 2 locations (chemical plants, oil refineries, spray paint booths) where explosion-proof integrity is required.
Intermediate Metal Conduit (Type IMC - Article 342)
- Construction: Engineered in the 1970s as a high-strength, lighter-weight alternative to RMC. IMC wall thickness is approximately 30% thinner than RMC, but utilizes higher-strength steel alloy to provide equivalent mechanical impact resistance.
- Threading: Uses standard National Pipe Taper (NPT) 3/4-inch-per-foot pipe dies, fully interchangeable with RMC fittings and threaded hubs.
Support Distances for RMC & IMC (Table 344.30(B)(2))
While the baseline support interval is 10 feet, straight horizontal runs of threaded RMC and IMC made up with threaded couplings are permitted extended support spans based on trade size:
| Conduit Trade Size | Standard Baseline Support | Extended Straight Run Support (Table 344.30(B)(2)) |
|---|---|---|
| 1/2 in. and 3/4 in. | 10 feet (3.0 m) | 10 feet (3.0 m) |
| 1 in. | 10 feet (3.0 m) | 12 feet (3.7 m) |
| 1-1/4 in. and 1-1/2 in. | 10 feet (3.0 m) | 14 feet (4.3 m) |
| 2 in. and 2-1/2 in. | 10 feet (3.0 m) | 16 feet (4.9 m) |
| 3 in. and larger | 10 feet (3.0 m) | 20 feet (6.1 m) |
3. Rigid Polyvinyl Chloride Conduit (Type PVC - Article 352)
Rigid PVC conduit is a nonmetallic raceway prized for total corrosion immunity, chemical resistance, and watertight solvent-cemented joints.
Schedule 40 vs. Schedule 80 PVC
- Schedule 40 PVC: Standard wall thickness. Suitable for underground direct burial, concrete encasement, wet locations, and concealed interior walls.
- Schedule 80 PVC (NEC 352.10(F)): Extra-heavy wall thickness. Mandatory wherever PVC conduit is exposed to physical damage. Common examples include where underground conduit stubs up through grade along an exterior wall, near vehicular traffic, or on industrial equipment skids.
SCHEDULE 40 vs. SCHEDULE 80 PVC CONDUIT
Schedule 40 (Standard) Schedule 80 (Heavy-Wall)
+--------------------+ +--------------------+
| Thin Wall PVC | | Thick Wall PVC |
| (Larger ID Area) | | (Smaller ID Area) |
+--------------------+ +--------------------+
* Standard underground runs * Where subject to physical
* Lower impact resistance damage (NEC 352.10(F))
* Higher conductor capacity * Reduced wire fill area!
Exam Trap — Schedule 80 Reduced Wire Fill: Because Schedule 80 PVC has a much thicker pipe wall, its internal cross-sectional area is significantly smaller than Schedule 40 PVC of the same trade size! When performing conduit fill calculations for Schedule 80 PVC, candidates must look up Chapter 9 Table 4 specifically under Rigid PVC Conduit, Schedule 80, rather than Schedule 40.
PVC Thermal Expansion & Calculation (NEC 352.44)
Polyvinyl chloride has a high thermal expansion coefficient—nearly five times that of steel conduit. In environments with wide temperature swings (such as outdoor building facades or unconditioned attics in Massachusetts), PVC expands and contracts dramatically.
The Expansion Fitting Rule (NEC 352.44): An expansion fitting must be installed in any run of PVC conduit where the total expected temperature variation will result in a length change of 0.25 inch (6 mm) or more between securely mounted boxes or fixed anchor points.
Per NEC Table 352.44, PVC expands at approximately 4.06 inches per 100 feet per 100°F temperature change (or $0.0406\text{ in/ft/100}^\circ\text{F}$).
Step-by-Step PVC Expansion Example:
An electrical contractor installs a 150-foot run of Schedule 40 PVC conduit along an exterior south-facing brick wall in Boston, MA. The winter minimum design temperature is -10°F, and direct solar summer radiation heats the conduit to 110°F.
- Calculate Temperature Differential ($\Delta T$):
- Determine Expansion Rate per 100 ft:
- Calculate Total Expansion for 150 ft:
- Conclusion: Since $7.308\text{ inches} \ge 0.25\text{ inches}$, an expansion fitting is required. Select a listed fitting with sufficient movement range and set it for the installation temperature in accordance with the manufacturer's instructions.
4. Flexible Raceways: FMC & LFMC (Articles 348 & 350)
Flexible conduits isolate mechanical vibration, facilitate routing around architectural obstructions, and provide flexible whips for motor connections and luminaire disconnects.
2023 NEC FLEXIBLE-RACEWAY GROUNDING MATRIX
(250.118(5), 250.118(6), 348.60, 350.60)
Is the flex length 6 feet or less? ───────────► NO ──► PULL SEPARATE EGC!
│
YES
│
▼
Is the circuit breaker 20A or less? ──────────► NO ──► PULL SEPARATE EGC!
│
YES
│
▼
Are fittings listed for grounding? ───────────► NO ──► PULL SEPARATE EGC!
│
YES
│
▼
FLEX ARMOR PERMITTED AS GROUNDING PATH!
Flexible Metal Conduit (Type FMC - Article 348)
- Known in the trade as "Greenfield." Helically wound, interlocking flexible galvanized steel or aluminum strip.
- Permitted in exposed and concealed dry locations only. Prohibited in wet locations.
- Supported within 12 inches (300 mm) of boxes and every 4.5 feet (1.4 m) along runs.
Liquidtight Flexible Metal Conduit (Type LFMC - Article 350)
- Built with an internal interlocking flexible metal core covered by an overall extruded liquidtight, sunlight-resistant thermoplastic jacket.
- Permitted in outdoor, wet, oily, and corrosive locations (e.g., rooftop air conditioning condensing units, industrial coolant pumps).
Equipment Grounding Limitations (2023 NEC 250.118(5) & (6))
For the 2023 NEC edition named in the current exam reference list, FMC and LFMC in the allowed trade-size range can serve as an equipment grounding path only if every applicable condition is met:
- The total length of the flexible conduit ground path does not exceed 6 feet (1.8 m).
- The circuit conductors contained within the flex are protected by an overcurrent device rated at 20 amperes or less.
- The termination fittings are explicitly listed for grounding.
- Where post-installation flexibility is needed to reduce vibration or permit equipment movement, install the wire-type EGC or bonding jumper required by the applicable raceway rule; stainless-steel FMC also requires the added conductor or jumper.
Under that edition, if the combined flex path is longer than 6 feet or the OCPD is 30 amperes, the raceway does not qualify as the sole equipment-grounding path; install a wire-type equipment grounding conductor with the circuit conductors. If the exam authorization lists the 2026 NEC after the Massachusetts item-bank transition, use that edition because 250.118 was reorganized and its flexible-raceway conditions changed.
5. The Maximum Total Bends Rule (360° Rule)
Across all raceway articles in Chapter 3:
- EMT: NEC 358.26
- RMC: NEC 344.26
- IMC: NEC 342.26
- PVC: NEC 352.26
- FMC: NEC 348.26
- LFMC: NEC 350.26
The Universal 360-Degree Rule: "There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, conduit bodies and boxes."
THE 360-DEGREE BEND RULE
[Box A] ──── (90°) ───────── (90°) ───────── (90°) ───────── (90°) ────► [Box B]
Bend 1 Bend 2 Bend 3 Bend 4
TOTAL = 90° + 90° + 90° + 90° = 360° (MAXIMUM PERMITTED!)
If a fifth bend (even a 15° kick) is added, a pull point
(conduit body like a Type C or LB, or junction box) is MANDATORY!
Every degree of bend increases the mechanical friction and sidewall bearing pressure during conductor pulling. Installing more than 360 degrees between boxes crushes conductor insulation against the inside radius of the bend, causing short circuits during initial energization.
6. Underground Installation Depths (NEC Table 300.5)
Underground burial depth is defined as the shortest distance measured between finished grade and the top exterior surface of the buried raceway or cable:
+-------------------------------------------------------------------------------------------------+
| MINIMUM COVER REQUIREMENTS PER NEC TABLE 300.5 |
+-----------------------+----------------------+---------------------+----------------------------+
| Wiring Method | Direct Burial Grade | Under 4 in. Concrete| Under Commercial Roads |
+-----------------------+----------------------+---------------------+----------------------------+
| Rigid Metal Conduit | **6 inches** | **4 inches** | 6 inches |
| Intermediate Metal | **6 inches** | **4 inches** | 6 inches |
| Listed nonmetallic raceway | **18 inches** | **4 inches** (slab) | 18 inches |
| Direct Buried Cable | **24 inches** | **18 inches** | 24 inches |
| Residential 120V GFCI | **12 inches** | **6 inches** | 24 inches |
+-----------------------+----------------------+---------------------+----------------------------+
Key Exam Insight: Because RMC and IMC possess exceptional mechanical strength, their required burial depth in open earth is only 6 inches, compared to 18 inches for PVC conduit and 24 inches for direct-buried UF cable.
An electrical contractor is installing Rigid Polyvinyl Chloride (PVC) conduit on the exterior facade of a commercial warehouse where the conduit is subject to physical damage from forklift traffic. According to NEC 352.10(F), which type of PVC conduit is required?
Under the 2023 NEC, an electrician installs a 5-foot length of 3/4-inch Liquidtight Flexible Metal Conduit (LFMC) to connect a rooftop air conditioning compressor motor. The circuit is protected by a 30-ampere circuit breaker. Under NEC 250.118(6) and 350.60, which statement is correct?
Under NEC 358.26, what is the maximum total degrees of bends permitted between pull points (such as conduit bodies or junction boxes) in a run of Electrical Metallic Tubing (EMT)?