5.1 Conduit and Tubing Types & Installation Rules

5.1 Conduit and Tubing Types & Installation Rules

Quick Reference:

• Support Spacing (EMT): Secure within 3 ft of termination; support every 10 ft (NEC 358.30). Can extend to 5 ft where structural members do not permit 3 ft.
• Support Spacing (RMC / IMC): Secure within 3 ft; support straight runs per Table 344.30(B)(2) (10 ft to 20 ft based on trade size).
• Support Spacing (PVC): Secure within 3 ft; support per Table 352.30 (3 ft to 8 ft based on trade size).
• Support Spacing (FMC / LFMC): Secure within 12 inches of box; support every 4.5 ft (NEC 348.30, 350.30). Exceptions: 6 ft luminaire whips, 3 ft for terminal flexibility.
• Maximum Bends: 360° total (the equivalent of four 90° bends) between pull points (boxes, conduit bodies, fittings).
• Field Reaming: Mandatory on all field-cut conduit ends to remove sharp burrs (NEC 300.4(G), 358.28).
• Insulating Bushings: Required for conductors 4 AWG and larger entering enclosures (NEC 300.4(G)).
• PVC Physical Damage: Schedule 80 is listed for severe physical damage; Schedule 40 is prohibited where subject to physical damage.
• PVC Expansion Fittings: Mandatory if thermal expansion/contraction $\ge 0.25\text{ inches}$ (6 mm) per Table 352.44.

Raceway systems serve as the structural backbone of commercial, industrial, and residential electrical installations. Under the National Electrical Code (NEC), raceways provide mechanical protection against impact, shield wiring from moisture and corrosive atmospheres, establish equipment grounding paths (for metallic systems), and ensure a safe, serviceable pathway for conductor replacement or upgrading. On the Connecticut E-2 Unlimited Journeyperson exam, mastery of raceway rules requires understanding the exact statutory boundaries between permitted uses, support intervals, bending limitations, and mechanical preparation.

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1. Overview of Raceway Classifications and Code Scope

An electrical raceway is defined in NEC Article 100 as an enclosed channel designed expressly for holding wires, cables, or busbars. Chapter 3 of the NEC groups raceways into distinct articles based on material composition and physical characteristics:

• Metallic Conduits & Tubing: Electrical Metallic Tubing (EMT, Art. 358), Intermediate Metal Conduit (IMC, Art. 342), Rigid Metal Conduit (RMC, Art. 344), and Flexible Metal Conduit (FMC, Art. 348).
• Liquidtight Flexible Conduits: Liquidtight Flexible Metal Conduit (LFMC, Art. 350) and Liquidtight Flexible Nonmetallic Conduit (LFNC, Art. 356).
• Rigid Nonmetallic Conduits: Rigid Polyvinyl Chloride Conduit (PVC, Art. 352).

Every raceway type is engineered for specific structural demands. Installing a raceway in an unapproved application—such as installing EMT where exposed to severe physical damage or installing Schedule 40 PVC where subject to vehicle impact—is a direct code violation.

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2. Metallic Raceways: Characteristics, Permitted Uses & Prohibitions

Electrical Metallic Tubing (EMT) — Article 358

Electrical Metallic Tubing is a lightweight, unthreaded steel or aluminum raceway commonly known in the trade as "thin-wall." Because EMT is unthreaded, sections are joined using set-screw or compression fittings.

• Uses Permitted (358.10): Exposed and concealed work; dry, damp, and wet locations (provided listed wet-location fittings are installed); embedded in poured concrete (with concrete-tight compression fittings); installed in hazardous locations only as explicitly permitted in Chapter 5.
• Uses Not Permitted (358.12): Where subject to severe physical damage; where protected solely by enamel against corrosion; in cinder concrete or cinder fill where subject to permanent moisture unless encased in at least 2 inches of non-cinder concrete.
• Grounding Path: EMT is recognized as an effective equipment grounding conductor (EGC) under NEC 250.118(4) when installed with listed fittings that maintain electrical continuity.

Intermediate Metal Conduit (IMC) — Article 342

Intermediate Metal Conduit is a circular metal raceway with a wall thickness intermediate between EMT and heavy-wall RMC. IMC offers high mechanical impact resistance at a lower weight than RMC. It features threaded ends and standard tapered pipe threads.

• Uses Permitted (342.10): Permitted in all atmospheric conditions and occupancies; areas subject to severe physical damage; direct burial in the earth; embedded in concrete; installed in all hazardous (classified) locations as permitted in Chapter 5.
• Corrosion Protection (342.10(B)): When installed in concrete or in direct contact with the earth, IMC must have approved corrosion protection (such as factory galvanizing or supplementary nonmetallic coatings).

Rigid Metal Conduit (RMC) — Article 344

Rigid Metal Conduit is the heaviest, thickest-wall metallic raceway in standard electrical construction. It is manufactured from galvanized steel, stainless steel, or aluminum and utilizes standard National Pipe Taper (NPT) threaded couplings.

• Uses Permitted (344.10): All occupancies and atmospheric conditions; areas subject to severe physical damage (loading docks, industrial plants, mechanical rooms); embedded in masonry or earth; all hazardous (classified) locations.
• Grounding Path: Both RMC and IMC are fully recognized equipment grounding conductors under NEC 250.118(2) and (3).

Flexible Metal Conduit (FMC) — Article 348

Commonly referred to as "Greenfield," FMC is formed from a helically wound, interlocked metallic strip (steel or aluminum), providing flexibility for equipment connections subject to vibration or physical adjustment.

• Uses Permitted (348.10): Exposed and concealed locations; luminaire supply whips; motor and machinery terminal connections.
• Uses Not Permitted (348.12): In wet locations; in hoists or hoistways; in storage battery rooms; where exposed to oil, gasoline, or other materials having a deteriorating effect; underground or embedded in poured concrete; where subject to physical damage.
• Grounding Path: FMC is permitted to serve as an EGC only under strict conditions: fittings must be listed for grounding, overcurrent protection must not exceed 20 amperes, trade size must not exceed 1-1/4 inches, and total length of the ground return path cannot exceed 6 feet (NEC 250.118(5)). Otherwise, an equipment bonding jumper or separate copper EGC is mandatory.

Liquidtight Flexible Metal Conduit (LFMC) — Article 350

Commonly known as "Sealtite," LFMC consists of an interlocked flexible metal core covered by an outer liquidtight, sunlight-resistant thermoplastic jacket.

• Uses Permitted (350.10): Exposed or concealed work where flexibility or protection from liquids, vapors, or solids is required; wet locations; outdoor equipment connections (air-conditioning condensing units, pool equipment, outdoor transformers); direct burial where listed and marked.
• Uses Not Permitted (350.12): Where subject to physical damage; where operating temperatures exceed the temperature rating of the jacket material.

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3. Nonmetallic Raceways: PVC & LFNC

Rigid Polyvinyl Chloride Conduit (PVC) — Article 352

Rigid PVC conduit is a nonconductive, noncorrosive raceway widely used in underground, chemical, and outdoor installations. It is solvent-welded using approved PVC primer and cement. The NEC recognizes two primary wall-thickness schedules:

               SCHEDULE 40 PVC vs. SCHEDULE 80 PVC
  =================================================================
  Specification       Schedule 40 PVC          Schedule 80 PVC
  -----------------------------------------------------------------
  Wall Thickness      Standard Wall            Extra Heavy Wall
  Physical Damage     NOT permitted where      SPECIFICALLY LISTED for
                      subject to physical      areas subject to severe
                      damage (352.10(F))       physical damage
  Internal Area       Larger usable area       Smaller usable area
                      (Standard fill Table 4)  (Thicker wall reduces ID!)
  Typical Use         Underground trenching,   Pole risers, parking
                      encased in concrete,     garages, driveways, exposed
                      dry/wet walls            industrial traffic areas
  -----------------------------------------------------------------

• Uses Permitted (352.10): Underground in direct burial or concrete encasement; in walls, floors, and ceilings; in locations subject to severe corrosive influences (wastewater treatment, coastal atmospheres, chemical plants); wet and damp locations.
• Uses Not Permitted (352.12): In hazardous (classified) locations except as permitted in Articles 501–515; for the support of luminaires or other equipment; where subject to ambient temperatures exceeding 50°C (122°F); where conductor insulation temperature ratings exceed the conduit rating.
• Grounding Requirement: Because PVC is a nonmetallic insulator, it cannot function as an EGC. An insulated or bare copper equipment grounding conductor must always be pulled in PVC raceways to bond all metal enclosures and terminations (NEC 352.60).

Liquidtight Flexible Nonmetallic Conduit (LFNC) — Article 356

LFNC is a nonmetallic flexible raceway manufactured in three types: LFNC-A (smooth interior with reinforcement), LFNC-B (smooth interior with continuous rigid nylon/PVC spiral), and LFNC-C (corrugated interior and exterior). LFNC-B is the most common commercial type.

• Uses Permitted (356.10): Wet locations; outdoor equipment whips up to 6 ft; direct burial where listed; corrosive atmospheres.
• Uses Not Permitted (356.12): Where subject to physical damage; in lengths exceeding 6 feet unless specifically approved; where ambient temperatures exceed the material rating.

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4. Comprehensive Raceway Comparison Matrix

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5. Mechanical Execution: Cutting, Reaming & Threading

Field Reaming Mandatory (NEC 300.4(G) & 358.28)

Cutting conduit with a pipe cutter, hacksaw, or reciprocating saw inherently forces metal inward, creating a razor-sharp circumferential burr. When conductors are pulled into the raceway under high pulling tension, these burrs act like blades, stripping the thermoplastic insulation and creating catastrophic ground faults.

• NEC 358.28 (EMT): All cut ends of EMT shall be reamed or otherwise finished to remove rough edges.
• NEC 342.28 & 344.28 (IMC & RMC): All cut ends shall be reamed to remove rough edges. Where conduit is threaded in the field, a standard cutting die with a 3/4-inch taper per foot (1 in 16) shall be used. Field-cut threads must be coated with an approved electrically conductive, corrosion-resistant compound (such as zinc-rich cold galvanizing paint) per NEC 300.6.

       CONDUIT CUTTING & REAMING HAZARD
   [ Conduit Wall ] ===============\   <-- Razor-sharp inward burr
   ----------------                 \      from pipe cutter wheel
   [ Interior Bore]                  | <-- Conductors pulled against burr
   ----------------                 /      will suffer torn insulation!
   [ Conduit Wall ] ===============/   <-- MUST BE REAMED SMOOTH

Insulating Bushings for Conductors 4 AWG and Larger (NEC 300.4(G))

Where raceways contain ungrounded conductors of 4 AWG or larger that enter a cabinet, box, auxiliary gutter, or raceway enclosure, the conductors must be protected by a substantial fitting providing a smoothly rounded insulating surface (such as an insulating bushing or a listed conduit fitting with an integral insulated throat).

• The bushing prevents conductor insulation from cracking or abrading against the sharp metallic locknut or raceway collar when wires are bent under tight termination angles.

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6. Securing and Supporting Intervals

Proper raceway support ensures structural rigidity, prevents fitting separation under pulling stress, and maintains ground-fault current return path continuity.

Support Spacing Requirements by Raceway Type

                         RACEWAY SUPPORT SPANS
   =========================================================================
   Raceway Type        Max Distance from Box        Max Distance Between Supports
   -------------------------------------------------------------------------
   EMT (358.30)        3 feet (900 mm) *             10 feet (3.0 m)
   RMC / IMC           3 feet (900 mm)               Table 344.30(B)(2) (10–20 ft)
   PVC Conduit         3 feet (900 mm)               Table 352.30 (3–8 ft)
   FMC / LFMC          12 inches (300 mm) **         4.5 feet (1.4 m)
   LFNC (356.30)       12 inches (300 mm)            3 feet (≤ 1") / 4.5 feet (> 1")
   =========================================================================
   * EMT Exception: Support permitted up to 5 ft from box where structural
     framing members do not permit securing within 3 ft.
   ** FMC/LFMC Exceptions: Up to 6 ft unsupported for luminaire whips; up to
      3 ft unsupported at terminals where flexibility is required.

RMC and IMC Support in Straight Runs (NEC Table 344.30(B)(2))

For straight runs of RMC or IMC made up with threaded couplings, the distance between supports may be extended beyond 10 feet based on the rigidity of the pipe diameter:

• 1/2 in. and 3/4 in.: 10 feet
• 1 in.: 12 feet
• 1-1/4 in. and 1-1/2 in.: 14 feet
• 2 in. and 2-1/2 in.: 16 feet
• 3 in. and larger: 20 feet

PVC Support Intervals (NEC Table 352.30)

Because thermoplastic materials soften and sag under ambient heat, PVC support intervals are significantly shorter:

• 1/2 in. through 1 in.: 3 feet
• 1-1/4 in. through 2 in.: 5 feet
• 2-1/2 in. through 3 in.: 6 feet
• 3-1/2 in. through 5 in.: 7 feet
• 6 in.: 8 feet

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7. Bending Rules & The 360-Degree Limitation

Across every raceway article in the NEC (342.26 for IMC, 344.26 for RMC, 348.26 for FMC, 350.26 for LFMC, 352.26 for PVC, 358.26 for EMT), the Code enforces an absolute statutory limit on field bends:

$\mathbf{\text{Total Bends Between Pull Points} \le 360^\circ \quad \text{(Four } 90^\circ \text{ Quarter-Bends)}}$

• Definition of Pull Points: Bends must be counted cumulatively between conduit terminations at boxes, conduit bodies (such as Type LB, C, LL, LR, or T bodies), cabinets, or handhole enclosures. Standard couplings do not constitute pull points.
• Why 360 Degrees? Each degree of bend increases sidewall bearing pressure (SWBP) on conductor insulation during pulling. Exceeding 360 degrees causes exponential increases in pulling tension, resulting in stretched copper, torn insulation, and pulled-apart conduit couplings.
• Offsets and Kicks: Electricians frequently fail inspections by overlooking kicks and box offsets. Two 45° offset bends equal 90°. If a conduit run contains three 90° bends and two 45° offsets, total bends equal $90 + 90 + 90 + 45 + 45 = 360^\circ$. Any additional bend requires installing a conduit body or pull box!

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8. PVC Thermal Expansion & Contraction Calculations

Polyvinyl chloride has a high thermal coefficient of expansion—expanding or contracting approximately four to five times more than steel. Under NEC 352.44, expansion fittings must be installed whenever the total expected longitudinal movement in a straight run equals or exceeds 0.25 inches (6 mm).

Expansion Formula

$\mathbf{\Delta L = \left( \frac{\text{Run Length}}{100} \right) \times \text{Expansion per 100 ft (from Table 352.44)}}$

Where:

• $\Delta L$ = Total length change (inches)
• $\Delta T = T_{\text{max}} - T_{\text{min}}$ (Anticipated seasonal temperature swing in °F)

High-Yield Table 352.44 Expansion Values (Inches per 100 ft of PVC)

• $\Delta T = 50^\circ\text{F} \rightarrow 2.03\text{ inches}$
• $\Delta T = 80^\circ\text{F} \rightarrow 3.25\text{ inches}$
• $\Delta T = 100^\circ\text{F} \rightarrow 4.06\text{ inches}$
• $\Delta T = 120^\circ\text{F} \rightarrow 4.87\text{ inches}$

Worked Example: Outdoor Commercial PVC Expansion Fitting Sizing

Problem: A 160-foot straight run of Schedule 40 PVC conduit is installed on an exposed exterior southern wall of a commercial warehouse in Connecticut. The design temperature extremes range from a winter low of $-10^\circ\text{F}$ to a direct-sun summer rooftop high of $110^\circ\text{F}$. Calculate the total expected movement and determine if expansion fittings are required.

Step 1: Calculate the temperature differential ($\Delta T$). $\Delta T = 110^\circ\text{F} - (-10^\circ\text{F}) = 120^\circ\text{F}$

Step 2: Find the expansion factor per 100 ft from Table 352.44. For $\Delta T = 120^\circ\text{F}$, Table 352.44 indicates an expansion rate of 4.87 inches per 100 feet of PVC.

Step 3: Calculate the total thermal change in length. $\Delta L = \left( \frac{160\text{ ft}}{100\text{ ft}} \right) \times 4.87\text{ in} = 1.60 \times 4.87 = 7.79\text{ inches}$

Step 4: Determine compliance with NEC 352.44. Because $7.79\text{ inches} \ge 0.25\text{ inches}$, expansion fittings are legally mandatory. Furthermore, because a typical single PVC expansion fitting accommodates 2 to 4 inches of piston travel, this 160-ft installation requires two 4-inch expansion barrels anchored at midpoints with proper guide brackets to prevent buckling.