9.3 Metallic & Nonmetallic Raceway Systems (EMT, PVC, RMC, FMC)

9.3 Metallic & Nonmetallic Raceway Systems (EMT, PVC, RMC, FMC)

Conduit and raceway systems provide enclosed physical channels designed to route, protect, and ground electrical conductors. The National Electrical Code governs distinct metallic and nonmetallic raceway types, each engineered for specific structural environments, atmospheric hazards, and mechanical stresses.

On the Idaho Journeyman Electrician exam, raceway questions test support intervals, maximum degrees of bends between pull points, fittings for wet vs dry locations, PVC expansion calculations, and equipment grounding conductor (EGC) rules.

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1. Electrical Metallic Tubing (EMT - Article 358)

Electrical Metallic Tubing (EMT), often called thin-wall conduit, is an unthreaded steel or aluminum raceway of circular cross-section.

+-----------------------------------------------------------------------------+
|                        EMT INSTALLATION RULES (ARTICLE 358)                 |
|                                                                             |
|   [SUPPORT RULES - NEC 358.30]                                              |
|   * Fastened within 3 FEET (900 mm) of every box, cabinet, or conduit body. |
|     (Extended to 5 ft where structural members do not permit 3 ft).         |
|   * Supported at intervals not exceeding 10 FEET (3.0 m) along run.         |
|                                                                             |
|   [BENDING RULES - NEC 358.26 (THE 360° RULE)]                              |
|   * Total bends between pull points CANNOT EXCEED 360 DEGREES               |
|     (e.g., maximum of four 90° bends, or two 90° and four 45° bends).       |
|                                                                             |
|   [FITTINGS - DRY VS WET LOCATIONS]                                         |
|   * Set-Screw Fittings ---> Permitted for DRY locations only.               |
|   * Compression Fittings -> Permitted for dry, damp, & wet (if "Raintight").|
+-----------------------------------------------------------------------------+

Uses Permitted & Prohibited (NEC 358.10, 358.12)

• Permitted: Exposed and concealed in dry, damp, and wet locations (using listed raintight fittings); embedded in poured concrete; in direct contact with earth (if protected by approved corrosion protection).
• Prohibited: Where subject to severe physical damage; in hazardous (classified) locations except where specifically permitted by Chapter 5; for the support of luminaires or equipment (except conduit bodies no larger than the tubing size).

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2. Rigid Metal Conduit (RMC - Article 344) & Intermediate Metal Conduit (IMC - Article 342)

Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC) are heavy-wall threaded ferrous or nonferrous metal raceways that provide the highest level of mechanical protection available.

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|                  RMC & IMC MAXIMUM SUPPORT SPANS (TABLE 344.30(B)(2))       |
|                                                                             |
|   CONDUIT TRADE SIZE              MAXIMUM DISTANCE BETWEEN SUPPORTS         |
|   -----------------------------------------------------------------------   |
|   1/2 in.  to  3/4 in.   ---------> 10 FEET (3.0 m)                         |
|   1 in. trade size       ---------> 12 FEET (3.7 m)                         |
|   1-1/4 in. to 1-1/2 in. ---------> 14 FEET (4.3 m)                         |
|   2 in.  to  2-1/2 in.   ---------> 16 FEET (4.9 m)                         |
|   3 in. and larger       ---------> 20 FEET (6.0 m)                         |
|                                                                             |
|   VERTICAL RISERS FROM INDUSTRIAL MACHINERY: Up to 20 ft between supports   |
|   if threaded couplings are used and fixed firmly top and bottom.           |
+-----------------------------------------------------------------------------+

RMC & IMC Key Code Mandates

1. Threading & Reaming (NEC 344.28 / 342.28): All field-cut conduit ends must be reamed with a reamer or deburring tool to remove sharp burrs that could damage conductor insulation during pulling. Threads must have a standard 3/4-inch taper per foot (1-in-16 taper, NPT).
2. Support Near Enclosures (344.30(A)): RMC/IMC must be securely fastened within 3 feet (900 mm) of each box, cabinet, or fitting (extended to 5 ft for unbroken conduit lengths).
3. Corrosion Protection: When buried directly in earth or cinder concrete, RMC/IMC must have supplemental nonmetallic coatings (e.g., PVC-coated rigid conduit) unless soil resistivity testing indicates low corrosiveness.

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3. Rigid Polyvinyl Chloride Conduit (PVC - Article 352)

Rigid PVC Conduit is a nonmetallic, corrosion-resistant raceway widely used for underground, wet, chemical, and slab installations.

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|                   SCHEDULE 40 VS. SCHEDULE 80 PVC (NEC 352.10)              |
|                                                                             |
|   [SCHEDULE 40 PVC]                                                         |
|   * Standard wall thickness.                                                |
|   * Permitted underground, embedded in concrete, walls, floors, & ceilings. |
|   * PROHIBITED where subject to physical damage!                            |
|                                                                             |
|   [SCHEDULE 80 PVC]                                                         |
|   * Heavy wall thickness (smaller interior cross-sectional area).           |
|   * MANDATORY where subject to PHYSICAL DAMAGE (e.g., pole risers, exposed  |
|     commercial traffic areas, driveways).                                   |
+-----------------------------------------------------------------------------+

Support Spacing for Rigid PVC (NEC Table 352.30)

PVC must be securely fastened within 3 feet (900 mm) of every box/cabinet and supported along runs per Table 352.30:

Thermal Expansion & Expansion Fittings (NEC 352.44 & Table 352.44)

PVC expands and contracts significantly with temperature changes (approximately 5 times more than steel). NEC 352.44 mandates that expansion fittings must be installed whenever the anticipated temperature change will cause a length change of 0.25 inches (6 mm) or more in a straight conduit run.

ΔL = L(inches) x ΔT x 0.0000338 — or equivalently — ΔL = L(feet) x ΔT x 0.000406

Where:

• ΔL = Change in length (inches)
• L = Total length of straight conduit run (use INCHES with the 0.0000338 coefficient, or FEET with the 0.000406 coefficient)
• ΔT = Expected temperature differential between extreme summer heat and winter cold (°F)

[!WARNING] Watch the units. The NEC coefficient of thermal expansion for PVC is 3.38 x 10⁻⁵ in./in./°F — inches of growth per inch of conduit per degree. Plugging a run length expressed in feet straight into that coefficient understates the answer by a factor of 12. Table 352.44 sidesteps the problem by tabulating length change per 100 feet of conduit: a 100°F temperature change yields 4.06 inches per 100 ft.

Example Calculation: A 100-foot run of outdoor PVC in Boise, Idaho, experiences a temperature swing of 100°F (from 0°F winter to 100°F summer).

• Convert to inches: 100 ft x 12 = 1,200 in. → ΔL = 1,200 x 100 x 0.0000338 = 4.06 inches.
• Cross-check against Table 352.44: at a 100°F change the table lists 4.06 in. per 100 ft. Match.

Because 4.06 in. far exceeds the 0.25 in. threshold, an expansion fitting is mandatory — and note that a fitting offering only 2 inches of travel would be inadequate for this run.

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4. Flexible Conduits: FMC (Article 348) & LFMC (Article 350)

Flexible Metal Conduit (FMC) and Liquidtight Flexible Metal Conduit (LFMC) provide vibration isolation, mechanical flexibility, and ease of routing to motors, transformers, and luminaires.

+-----------------------------------------------------------------------------+
|                      FLEXIBLE CONDUIT SUPPORT & EGC RULES                   |
|                                                                             |
|   [SUPPORT SPACING (348.30 / 350.30)]                                       |
|   * Fastened within 12 INCHES (300 mm) of every box/fitting.                |
|   * Supported at intervals not exceeding 4.5 FEET (1.4 m) along run.        |
|                                                                             |
|   [SUPPORT EXCEPTIONS]                                                      |
|   * Lengths up to 3 FEET at terminals where flexibility is required.        |
|   * Lengths up to 6 FEET from luminaire terminal connection (fixture whip). |
|                                                                             |
|   [EQUIPMENT GROUNDING RESTRICTIONS (NEC 250.118(5) & (6))]                 |
|   FMC / LFMC may serve as the sole EGC ONLY IF:                             |
|   1. The total length in the ground return path DOES NOT EXCEED 6 FEET.     |
|   2. The circuit is protected by an OCPD rated <= 20A (for 3/8"-1/2")   |
|      or <= 60A (for 3/4" through 1-1/4").                                |
|   3. Listed grounding fittings are installed.                               |
|   If run > 6 ft or circuit > 60A: MUST pull a separate wire-type EGC!       |
+-----------------------------------------------------------------------------+

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5. Master Raceway Comparison & Bend Budget Matrix

+-----------------------------------------------------------------------------+
|                  THE 360-DEGREE BEND BUDGET CALCULATION                     |
|                                                                             |
|   [PULL BOX A]                                              [PULL BOX B]    |
|        |                                                         |          |
|        +-----( 90° )=======( 90° )=======( 90° )=======( 90° )---+          |
|                 1             2             3             4                 |
|                                                                             |
|   Total Bend Degrees = 90° + 90° + 90° + 90° = 360° (MAXIMUM PERMITTED!)    |
|   If a 5th bend (even 15°) is added, a pull point or conduit body (C, LB)   |
|   MUST be inserted!                                                         |
+-----------------------------------------------------------------------------+