9.2 Metal Raceways — EMT, RMC, IMC & FMC

9.2 Metal Raceways — EMT, RMC, IMC & FMC

Metallic raceways form the backbone of commercial and industrial electrical distribution. They provide unparalleled mechanical protection against severe impact, crush hazards, and environmental wear, while simultaneously serving as an equipment grounding conductor (EGC) under specified National Electrical Code (NEC) conditions. To excel on the Kentucky Journeyman Electrician examination, candidates must master the unique physical properties, installation limits, support spacings, and fitting requirements for the four primary metallic raceway families:

1. Electrical Metallic Tubing (EMT) — NEC Article 358
2. Intermediate Metal Conduit (IMC) — NEC Article 342
3. Rigid Metal Conduit (RMC) — NEC Article 344
4. Flexible Metal Conduit (FMC) — NEC Article 348

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1. Overview & Mechanical Comparison of Metallic Raceways

Metallic raceways differ primarily in wall thickness, weight, tensile strength, whether they can be cut and field-threaded with standard NPT dies, and the severity of physical damage they are legally permitted to withstand.

+---------------------------------------------------------------------------------------------------+
|                             METALLIC RACEWAY TAXONOMY & CHARACTERISTICS                           |
|                                                                                                   |
|  Raceway Type │ Code Art. │ Wall Thickness │ Threading? │ Max Support │ Securing from Enclosure   |
|  ─────────────┼───────────┼────────────────┼────────────┼─────────────┼───────────────────────────│
|  EMT          │ Art. 358  │ Thin-wall      │ No (Thread-│ 10 Feet     │ Within 3 Feet (900 mm)    │
|               │           │ (Lightweight)  │ less only) │ (3.0 m)     │ (5 ft exception)          |
|  ─────────────┼───────────┼────────────────┼────────────┼─────────────┼───────────────────────────│
|  IMC          │ Art. 342  │ Intermediate   │ Yes (NPT   │ 10 Feet     │ Within 3 Feet (900 mm)    │
|               │           │ (High-strength)│ Threads)   │ (Up to 20') │ (Straight run extensions) │
|  ─────────────┼───────────┼────────────────┼────────────┼─────────────┼───────────────────────────│
|  RMC          │ Art. 344  │ Heavy-wall     │ Yes (NPT   │ 10 Feet     │ Within 3 Feet (900 mm)    │
|               │           │ (Maximum mass) │ Threads)   │ (Up to 20') │ (Straight run extensions) │
|  ─────────────┼───────────┼────────────────┼────────────┼─────────────┼───────────────────────────│
|  FMC          │ Art. 348  │ Helical strip  │ No (Clamp/ │ 4.5 Feet    │ Within 12 Inches (300 mm) │
|  ("Greenfield")│          │ (Flexible)     │ Squeeze)   │ (1.4 m)     │ (Unsupported exceptions)  |
+---------------------------------------------------------------------------------------------------+

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2. Electrical Metallic Tubing (EMT, NEC Article 358)

Electrical Metallic Tubing, commonly referred to in the trade as "thin-wall," is an unthreaded steel or aluminum raceway of circular cross section. Because it is lightweight, cost-effective, and easy to bend with a manual hand bender, EMT is the most widely installed raceway in commercial office buildings, retail facilities, and institutional construction.

Uses Permitted (NEC 358.10)

EMT is permitted for:

• Exposed and Concealed Work: In dry, damp, and wet locations (when installed with listed wet-location fittings).
• In Concrete or Direct Burial: Permitted in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and approved as suitable for the condition.

Uses Not Permitted (NEC 358.12)

EMT shall not be used under the following conditions:

1. Severe Physical Damage: Where, during installation or afterward, it will be subject to severe physical damage (such as low mounting in industrial loading docks subject to forklift impact).
2. Support of Luminaires or Equipment: EMT cannot be used to support luminaires or other equipment boxes, except conduit bodies no larger than the largest trade size of the tubing (NEC 314.23).
3. Hazardous (Classified) Locations: Not permitted unless a Chapter 5 article specifically permits it.

Securing and Supporting Rules (NEC 358.30)

Proper support maintains mechanical integrity and ensures EMT acts as an effective equipment grounding conductor:

• Distance from Enclosures (358.30(A)): EMT must be securely fastened in place within 3 feet (900 mm) of each outlet box, junction box, device box, cabinet, conduit body, or other tubing termination.
• Support Intervals (358.30(A)): EMT must be supported at intervals not exceeding 10 feet (3.0 m) along the run.
• Exception No. 1 (Structural Framing Constraint): Where structural members do not permit securement within 3 feet, an unbroken length of EMT is permitted to be secured up to 5 feet (1.5 m) from a box or cabinet termination.
• Exception No. 2 (Horizontal Runs): Horizontal runs of EMT through framing members (such as metal or wood joists/studs) at intervals not exceeding 10 feet are considered supported, provided the tubing is securely fastened within 3 feet of each termination.

Fitting Classifications: Set-Screw vs. Compression

Because EMT cannot be threaded, all connections rely on threadless couplings and connectors:

• Set-Screw Fittings: Utilize steel set screws that tighten down against the tubing wall. Generally used in dry locations. Some are listed as concrete-tight, but set-screw fittings are not raintight.
• Compression Fittings: Utilize an external hex nut that compresses a split tapered metal ring around the tubing circumference. Standard compression fittings are rated concrete-tight.
• Raintight Wet-Location Fittings: For wet locations (such as outdoors or car washes), EMT compression fittings must be explicitly listed and marked "Raintight" with an internal elastomeric sealing gland to prevent water ingress.

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3. Rigid Metal Conduit (RMC, Article 344) & Intermediate Metal Conduit (IMC, Article 342)

Where electrical wiring requires absolute maximum mechanical protection against crushing, vehicular impact, or destructive industrial processes, engineers specify Rigid Metal Conduit (RMC) or Intermediate Metal Conduit (IMC).

Mechanical Differences: RMC vs. IMC

• Rigid Metal Conduit (RMC, Article 344): The heaviest and thickest metallic conduit manufactured. Made from galvanized steel, stainless steel, or aluminum. It features standard National Pipe Taper (NPT) threads with a 3/4-inch taper per foot.
• Intermediate Metal Conduit (IMC, Article 342): Developed in the 1970s as a modern, high-strength alternative to RMC. IMC utilizes a lighter, thinner wall of higher-yield-strength alloy steel. It weighs approximately 30% less than RMC while offering virtually equivalent physical protection and impact resistance. IMC utilizes the exact same threading tools, dies, fittings, and couplings as RMC.

Uses Permitted (NEC 342.10 & 344.10)

Both RMC and IMC are permitted in all atmospheric conditions and occupancies:

• Permitted where exposed to severe physical damage.
• Permitted in all Class I, Class II, and Class III hazardous (classified) locations (NEC Articles 500 through 506).
• Permitted direct-buried in earth trenches (requiring only 6 inches of cover under Table 300.5).
• Permitted embedded in poured concrete or installed under corrosive cinder fills (when encased in at least 2 inches of concrete).

Securing and Supporting (NEC 342.30 & 344.30)

Like EMT, RMC and IMC must be securely fastened within 3 feet (900 mm) of each box, cabinet, or fitting, and supported at intervals not exceeding 10 feet (3.0 m).

However, because of the rigid mechanical strength of threaded pipe couplings, the NEC provides a major labor-saving exception for fixed straight runs.

Extended Support Spacing Table (NEC Table 344.30(B)(2) / Table 342.30(B)(2))

For straight runs made up with threaded couplings, maximum support spacing is permitted to be increased based on trade diameter:

Exam Trap Alert: To utilize the extended support intervals in Table 344.30(B)(2), three strict criteria must be satisfied: (1) the run must be straight, (2) the pipe must be joined with threaded couplings (threadless compression couplings do not qualify), and (3) the supports must prevent transmission of stresses to the terminations where the conduit deflects between supports. If these conditions are met, a 3-inch or 4-inch RMC feeder rack requires supports only every 20 feet.

Vertical Risers from Industrial Machinery (NEC 344.30(B)(3))

Where RMC or IMC rises vertically from industrial machinery or fixed equipment, support intervals are permitted to be extended up to 20 feet (6.0 m), provided the conduit is equipped with threaded couplings, firmly supported at the top and bottom of the riser, and has no other intermediate supports available.

Conductor Protection at Terminations (NEC 300.4(G))

Where raceways contain conductors of size 4 AWG or larger, the conduit must be equipped with an identified insulating bushing or fitting featuring an integral smoothly rounded insulating throat.

• Sharp metallic conduit threads will slice through conductor insulation when heavy conductors are pulled under thousands of pounds of mechanical winch tension.
• Installing an insulated grounding bushing fulfills both the conductor insulation protection requirement of 300.4(G) and the equipment bonding mandate of Article 250.

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4. Flexible Metal Conduit (FMC, NEC Article 348)

Flexible Metal Conduit (FMC), universally known by electricians as "Greenfield" (named after inventor Harry Greenfield), is constructed from a continuously wound, interlocking spiral strip of galvanized steel or aluminum.

               FLEXIBLE METAL CONDUIT SUPPORT (NEC 348.30)

       Box / Enclosure
      ┌───────────────┐
      │               │
      └───┬───────┬───┘
          │       │
          │   *   │ <── MAXIMUM 12 INCHES (300 mm) TO FIRST STRAP
          └───┬───┘
              O <────── PIPE STRAP
              │
              │
              │   * <── MAXIMUM 4.5 FEET (1.4 m) BETWEEN STRAPS
              │
              O <────── PIPE STRAP
              │
              ▼

Trade Size Limitations (NEC 348.20)

• Minimum Size: The minimum trade size for FMC is trade size 1/2 (metric designator 16).
• Trade Size 3/8 Exception (348.20(A)): Trade size 3/8 FMC is permitted for lengths not exceeding 6 feet (1.8 m) for:
  1. Luminaire tap connections to recessed lighting fixtures (NEC 410.117(C)).
  2. Enclosure connections for equipment or motor leads.
  3. Utilization equipment manufactured with integral wiring leads.

Uses Permitted and Prohibited (NEC 348.10 & 348.12)

• Permitted: In exposed and concealed locations where flexibility is required during operation, maintenance, or equipment alignment.
• Prohibited:
  • In wet locations (FMC has an open interlocking spiral seam that freely admits water).
  • In hoistways (except short elevator control spans).
  • In storage battery rooms.
  • In hazardous (classified) locations (except specific Class I/II Div 2 exceptions).
  • Where subject to physical damage.

Securing and Supporting Spacing (NEC 348.30)

• General Rule (348.30(A)): FMC must be securely fastened within 12 inches (300 mm) of each box, cabinet, conduit body, or fitting, and supported at intervals not exceeding 4.5 feet (1.4 m).
• Unsupported Exceptions (348.30(A) Exceptions): FMC is permitted without securing or support in these cases:
  • Where fished between access points through concealed spaces in finished buildings.
  • Lengths not exceeding 3 feet (trade sizes 1/2 through 1-1/4), 4 feet (1-1/2 through 2), or 5 feet (2-1/2 and larger) where flexibility is necessary after installation.
  • Lengths not exceeding 6 feet (1.8 m) from the last point where the raceway is securely fastened, for connections to luminaires or other equipment within an accessible ceiling.
  • Lengths not exceeding 6 feet from a luminaire terminal connection for tap conductors to luminaires (410.117(C)).
• Horizontal Runs (348.30(B)): Horizontal runs supported by openings through framing members at intervals not over 4.5 feet, and secured within 12 inches of terminations, are considered supported.

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5. The Universal 360-Degree Bend Rule

One of the most heavily tested provisions across all raceway articles in the NEC is the limitation on total bends between pull points.

                   THE 360-DEGREE BEND RULE

  Box A ───[ 90° ]──────[ 45° ]──────[ 45° ]──────[ 90° ]──────[ 90° ]─── Box B
             │            │            │            │            │
            90°    +     45°    +     45°    +     90°    +     90°  = 360° TOTAL

  * MAXIMUM 360° equivalent bends (four 90° quarter-bends) permitted!
  * To add any further bends, a PULL POINT (J-box or conduit body) must be added.

Code Citations

• Intermediate Metal Conduit (IMC): NEC 342.26
• Rigid Metal Conduit (RMC): NEC 344.26
• Flexible Metal Conduit (FMC): NEC 348.26
• Electrical Metallic Tubing (EMT): NEC 358.26
• Rigid Polyvinyl Chloride Conduit (PVC): NEC 352.26

The Rule Statement

Number of Bends (Between Pull Points): There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, conduit bodies and boxes.

Physics and Engineering Rationale

Every degree of bend increases the mechanical pulling tension required to draw conductors through a raceway. Conductor pulling friction is governed by the capstan equation: $T_{\text{out}} = T_{\text{in}} \times e^{(\mu \times \theta)}$ Where:

• $T_{\text{out}}$ = pulling tension leaving the bend
• $T_{\text{in}}$ = tension entering the bend
• $e$ = base of natural logarithms (2.718)
• $\mu$ = coefficient of friction between wire insulation and conduit wall
• $\theta$ = total angle of bend in radians (360 degrees = 2π ≈ 6.28 radians)

Because bend angles enter as an exponent, exceeding 360 degrees causes pulling tension to skyrocket. Conductor insulation experiences crushing sidewall bearing pressure (SWBP) against the inside radius of the conduit elbows, which strips thermoplastic jackets, elongates copper conductors beyond their tensile elastic limit, and snaps pulling ropes.

What Counts as a Bend?

Every directional deviation in the conduit run counts toward the 360-degree limit:

• A 90-degree standard factory elbow = 90 degrees
• A 45-degree offset = two 45-degree bends = 90 degrees
• A three-bend saddle = one 45-degree center bend plus two 22.5-degree return bends = 90 degrees
• A four-bend saddle = four 22.5-degree bends = 90 degrees
• A box kick = 15 to 30 degrees

Pull Points Resetting the Count

To reset the bend count to zero, electricians must install a recognized pull point:

• An accessible junction box or pull box with a removable cover.
• A conduit body equipped with a removable cover (such as an LB, LL, LR, C, or T body).
• Important: A standard conduit coupling does not reset the bend count; only fittings where conductors can be physically pulled and manipulated qualify.

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6. Dissimilar Metals, Corrosion Protection & Grounding Continuity (NEC 300.6 & 250.118)

Galvanic Corrosion (NEC 300.6)

Where dissimilar metals come into contact in the presence of an electrolyte (such as rainwater, humidity, or airborne salts), galvanic action occurs. The more active galvanic metal acts as an anode and rapidly corrodes.

• Dissimilar Metals (344.14, 358.14): Where practicable, avoid dissimilar metals in contact anywhere in the raceway system. Stainless steel and aluminum fittings are permitted with galvanized steel RMC, and galvanized steel fittings with aluminum RMC, where not subject to severe corrosive influences. Aluminum raceways encased in concrete or in direct contact with the earth need supplementary corrosion protection (300.6(B)). Copper conductors may be installed in aluminum raceways; the concern is metal-to-metal contact between raceway parts.
• Corrosion-Resistant Coatings: Ferrous raceways (RMC, IMC, EMT) are hot-dip galvanized with a protective zinc coating. In harsh chemical environments (wastewater treatment plants, fertilizer facilities), PVC-coated rigid conduit ("rob-roy") must be utilized.

Metal Raceways as Equipment Grounding Conductors (EGC)

Under NEC 250.118, the following metallic raceways are recognized as legal Equipment Grounding Conductors:

1. EMT (250.118(A)(4)): Permitted as sole EGC when installed with listed fittings tightened securely.
2. IMC (250.118(A)(3)) & RMC (250.118(A)(2)): Permitted as sole EGC throughout their entire installation range.
3. Flexible Metal Conduit (FMC, 250.118(A)(5)): Permitted to serve as the EGC only if all the following conditions are met:
   • The conduit is terminated in listed fittings.
   • The circuit overcurrent protective device is rated 20 amperes or less.
   • The combined length of FMC and LFMC in the ground return path does not exceed 6 feet (1.8 m).
   • Flexibility Rule: Where FMC connects equipment where flexibility is necessary to minimize vibration or to allow movement after installation (such as motors or vibrating screens), a wire-type equipment grounding conductor must be installed (250.118(A)(5)(d)), regardless of circuit rating or length.