9.3 Nonmetallic Raceways & Flexible Conduits

9.3 Nonmetallic Raceways & Flexible Conduits

Nonmetallic raceways and liquidtight flexible conduits are essential for installations exposed to severe corrosion, moisture, earth burial, and equipment vibration. While metallic conduits provide formidable structural rigidity, nonmetallic raceways excel in acidic soils, chemical processing plants, coastal marine environments, and underground trenches where metallic conduits eventually corrode. Furthermore, outdoor HVAC compressors, industrial chillers, and electric motors require flexible, weather-sealed conduits capable of absorbing sustained vibration without cracking or admitting rainwater.

On the Kentucky Journeyman Electrician examination, questions in this domain emphasize Rigid Polyvinyl Chloride Conduit (PVC, Article 352), Liquidtight Flexible Metal Conduit (LFMC, Article 350), and Liquidtight Flexible Nonmetallic Conduit (LFNC, Article 356). Candidates must be fully prepared to perform thermal expansion calculations, select between Schedule 40 and Schedule 80 PVC, determine support intervals from code tables, and apply equipment bonding jumper rules.

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1. Rigid Polyvinyl Chloride Conduit (PVC, NEC Article 352)

Rigid PVC conduit is a nonmetallic, nonconductive raceway composed of polyvinyl chloride polymer. It is flame-retardant, impervious to most chemical agents, and resistant to atmospheric oxidation.

+-----------------------------------------------------------------------------------+
|                         SCHEDULE 40 VS. SCHEDULE 80 PVC                           |
|                                                                                   |
|  Feature                  Schedule 40 PVC                 Schedule 80 PVC         |
|  ───────────────────────────────────────────────────────────────────────────────  |
|  Wall Thickness           Standard                        Heavy Wall (+30% to 50%)|
|  Internal Cross-Section   Larger (Standard wire fill)     Smaller (Reduced fill)  |
|  Physical Damage Rating   Not approved for physical damage Approved for physical  |
|                           (NEC 352.12(C))                 damage (NEC 352.10(F))  |
|  Common Applications      Underground trenches, inside    Pole risers, parking    |
|                           walls, encased in concrete      garages, loading docks  |
+-----------------------------------------------------------------------------------+

Schedule 40 vs. Schedule 80 PVC Selection Criteria

• Schedule 40 PVC: Features standard wall thickness. It is listed for underground direct burial, concrete encasement, and exposed indoor/outdoor locations not subject to physical damage.
• Schedule 80 PVC: Manufactured with an exceptionally thick wall capable of resisting heavy mechanical impacts. Under NEC 352.10(F), Schedule 80 PVC is specifically listed and identified for use where subject to physical damage.
  • Conductor Fill Impact: Because Schedule 80 has the same outer diameter as Schedule 40 (to accept standard couplings) but a substantially thicker wall, its internal diameter and usable cross-sectional area are significantly smaller. When sizing conductors in Schedule 80 PVC, electricians must use the dimensions from NEC Chapter 9, Table 4 (Rigid PVC Conduit, Schedule 80) rather than standard tables.

Uses Permitted (NEC 352.10)

PVC conduit is permitted:

• Concealed in walls, floors, and ceilings.
• In direct contact with the earth (direct burial in trenches under Table 300.5).
• In concrete walls, floors, and slabs.
• In wet, damp, and severely corrosive locations (e.g., sewage treatment plants, indoor swimming pool equipment rooms, agricultural livestock barns under Article 547).

Uses Not Permitted (NEC 352.12)

PVC conduit shall not be used:

1. Hazardous (Classified) Locations: Prohibited in hazardous locations, except where specifically permitted in Chapter 5 (such as underground portions in service stations under Article 514).
2. Support of Luminaires or Equipment: PVC cannot be used to support lighting fixtures or other equipment enclosures (NEC 352.12(B)).
3. Physical Damage (for Schedule 40): Schedule 40 is prohibited where subject to physical damage (NEC 352.12(C)).
4. Ambient Temperature Limits: Prohibited where ambient temperatures exceed 50°C (122°F) or for conductors whose insulation temperature rating exceeds the temperature rating of the conduit listing.

Field Trimming and Reaming (NEC 352.28)

All field-cut ends of PVC conduit must be cut square and carefully trimmed inside and outside with a reamer or pocket knife to remove all rough burrs. If rough PVC edges are left unreamed, pulling high-tension conductors will strip insulation, initiating high-impedance arcing faults.

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2. Thermal Expansion and Contraction Calculations (NEC 352.44 & Table 352.44)

One of the most mathematically rigorous topics tested on the Journeyman exam is the sizing and placement of PVC Expansion Fittings.

Why PVC Moves: The Physics of Thermal Expansion

Polyvinyl chloride has an exceptionally high coefficient of linear thermal expansion:

• Linear expansion coefficient: approximately $3.38 \times 10^{-5} \text{ in/in/}^\circ\text{F}$ ($6.08 \times 10^{-5} \text{ mm/mm/}^\circ\text{C}$), the value behind Table 352.44.
• By comparison, steel has an expansion coefficient of only $0.65 \times 10^{-5} \text{ in/in/}^\circ\text{F}$.
• PVC expands and contracts at 5 to 6 times the rate of steel!

The Code Trigger (NEC 352.44)

NEC 352.44 Expansion Fittings: 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 confirmed to be 0.25 inches (6 mm) or greater in a straight run between securely mounted boxes, cabinets, elbows, or other conduit terminations.

                 PVC CONDUIT EXPANSION JOINT OPERATION

    Fixed Anchor Box                                          Fixed Anchor Box
    ┌──────────────┐                                          ┌──────────────┐
    │              │                                          │              │
    └───┬──────────┘                                          └──────────┬───┘
        │                                                                │
        ▼                                                                ▼
  ═════════════════════[ EXPANSION FITTING ]═══════════════════════════════════
                        ┌──────────┬────────┐
                        │  BARREL  │ PISTON │
                        └──────────┴────────┘
                         <── Travel Stroke ──>

  * In Summer (High Temp): Piston pushes IN as conduit expands.
  * In Winter (Low Temp):  Piston pulls OUT as conduit contracts.

NEC Table 352.44 Expansion Values

NEC Table 352.44 establishes the standard baseline for PVC expansion:

• PVC conduit expands or contracts 4.06 inches per 100 feet of conduit for every 100°F temperature change.

Mathematical Expansion Formula

$\Delta L = \left( \frac{L}{100} \right) \times \left( \frac{\Delta T}{100} \right) \times 4.06\text{ inches}$ Where:

• $\Delta L$ = Total calculated length change (inches)
• $L$ = Total length of the straight PVC conduit run (feet)
• $\Delta T$ = Temperature differential between the maximum expected summer temperature and minimum expected winter temperature ($T_{\text{max}} - T_{\text{min}}$) in degrees Fahrenheit

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Step-by-Step Worked Exam Calculation Example

Exam Scenario: An electrician installs a straight run of 150 feet of 2-inch Schedule 40 PVC conduit exposed along a commercial building parapet wall in Louisville, Kentucky. Engineering design data establishes that the minimum winter ambient temperature drops to $-10^\circ\text{F}$, and the maximum summer direct sunlight temperature reaches $130^\circ\text{F}$.

Question: What is the total length change of this PVC conduit run, and is an expansion fitting required under NEC 352.44?

Step 1: Calculate the Total Temperature Differential ($\Delta T$) $\Delta T = T_{\text{max}} - T_{\text{min}} = 130^\circ\text{F} - (-10^\circ\text{F}) = 130 + 10 = 140^\circ\text{F}$

Step 2: Apply the NEC Table 352.44 Expansion Formula $\Delta L = \left( \frac{150\text{ ft}}{100} \right) \times \left( \frac{140^\circ\text{F}}{100} \right) \times 4.06\text{ inches}$ $\Delta L = 1.5 \times 1.4 \times 4.06\text{ inches}$ $\Delta L = 2.1 \times 4.06\text{ inches} = 8.526\text{ inches}$

Step 3: Evaluate Code Mandate

• The calculated length change is 8.53 inches.
• Because 8.53 inches is greater than or equal to 0.25 inches, an expansion fitting is legally mandatory.
• Check the stroke rating of the expansion fitting you choose. If one fitting's travel is less than the 8.5-inch change, install additional fittings along the run, with guide straps that let the conduit slide.

Setting the Expansion Fitting Barrel During Installation

When installing an expansion fitting, the piston must be positioned based on the ambient temperature at the time of installation ($T_{\text{install}}$): $\text{Piston Setting} = \text{Total Stroke} \times \left( \frac{T_{\text{max}} - T_{\text{install}}}{T_{\text{max}} - T_{\text{min}}} \right)$

• On a freezing winter day, the conduit is already contracted, so the piston is installed nearly fully extended (ready to collapse inward as weather warms).
• On a sweltering summer day, the conduit is already expanded, so the piston is installed nearly fully seated inside the barrel.

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3. PVC Securing and Supporting (NEC Table 352.30)

Under NEC 352.30, PVC conduit must be securely fastened within 3 feet (900 mm) of each box, cabinet, conduit body, or other termination.

Because PVC lacks the structural stiffness of steel, support intervals are significantly closer than metallic raceways. NEC Table 352.30 mandates the following maximum support spacings:

Proper Strapping with Expansion Joints

Where expansion fittings are installed, conduit straps must not clamp tightly against the outside wall of the PVC pipe. Installers must use loose-fitting guide straps that allow the conduit to slide longitudinally back and forth through the strap. If pipe straps are clamped rigidly, thermal expansion forces will bow the conduit into a wavy snake pattern, break glue joints, and rip terminal boxes from walls.

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4. Liquidtight Flexible Metal Conduit (LFMC, NEC Article 350)

Liquidtight Flexible Metal Conduit (LFMC) consists of an internal flexible, interlocking spiral wound metal core (similar to FMC) enclosed beneath an outer extruded, seamless, sunlight-resistant, liquidtight thermoplastic jacket.

               LIQUIDTIGHT FLEXIBLE CONDUIT CROSS-SECTION

            Outer Liquidtight Thermoplastic Jacket
            (Resistant to sunlight, water, cutting oils, coolant)
            ┌──────────────────────────────────────────┐
            │ ┌──────────────────────────────────────┐ │
            │ │ Helically Wound Galvanized Steel     │ │
            │ │ Interlocking Metallic Core           │ │
            │ │ (Provides crush & tensile strength)  │ │
            │ └──────────────────────────────────────┘ │
            └──────────────────────────────────────────┘

Uses Permitted (NEC 350.10)

LFMC is permitted in:

• Exposed or concealed locations.
• Wet locations where exposed to rain, water spray, or machine oils.
• Direct burial in earth trenches where explicitly listed and marked for direct burial.
• Flexible connections to motors, pumps, commercial kitchen equipment, and rooftop air-conditioning condensing units.

Uses Not Permitted (NEC 350.12)

LFMC shall not be used:

• Where subject to physical damage.
• Where operating ambient temperatures or conductor insulation temperatures exceed the rating of the conduit listing.

Securing and Supporting (NEC 350.30)

• Must be securely fastened within 12 inches (300 mm) of each box, cabinet, or fitting.
• Supported at intervals not exceeding 4.5 feet (1.4 m).
• Unsupported Exceptions: Support is not required for lengths not exceeding 3 feet at terminals where flexibility is required, or lengths not exceeding 6 feet for luminaire tap connections in accessible ceilings.

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5. Liquidtight Flexible Nonmetallic Conduit (LFNC, NEC Article 356)

Liquidtight Flexible Nonmetallic Conduit (LFNC)—frequently known in the field as "sealtite" or "carflex"—contains no internal metal core. It is constructed entirely from nonmetallic polymeric materials.

Three Types of LFNC (NEC 356.2)

1. LFNC-A: Features a smooth seamless inner core with internal nonmetallic reinforcing layers within the conduit wall.
2. LFNC-B: Features a smooth inner surface with an integral, rigid nonmetallic spiral reinforcement embedded directly within the conduit wall (the most widely installed commercial product).
3. LFNC-C: Features a corrugated internal and external wall surface without reinforcing layers.

Installation Rules & Length Limitations (NEC 356.10 & 356.12)

• Uses Permitted: Suitable for wet locations, outdoor sunlight exposure, direct burial (where listed), and corrosive environments.
• The 6-Foot Length Limitation (356.12): LFNC is not permitted in lengths longer than 6 feet (1.8 m), except where a longer length is approved as essential for a required degree of flexibility, or where Type LFNC-B is secured as required by 356.30 (permitted by 356.10).
• Securing and Supporting (356.30): Where LFNC-B is installed in lengths longer than 6 feet, it must be securely fastened at intervals not exceeding 3 feet (900 mm) and within 12 inches (300 mm) on each side of every box, cabinet, or fitting.

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6. Grounding and Equipment Bonding for Flexible Raceways (NEC Article 250)

Grounding rules for flexible raceways represent a major focus on electrical licensing examinations because improper grounding creates fatal shock hazards on vibrating equipment.

              FLEXIBLE CONDUIT GROUNDING & BONDING MATRIX

  Raceway Type    Recognized as sole EGC?      Bonding Jumper Required?
  ─────────────────────────────────────────────────────────────────────────
  Rigid PVC       NO (100% Nonmetallic)        ALWAYS (Wire EGC required inside)
  LFNC            NO (100% Nonmetallic)        ALWAYS (Wire EGC required inside)
  FMC             YES, if all apply:           YES, if OCPD > 20 A,
                  - Listed fittings            OR total length > 6 ft,
                  - OCPD 20 A or less          OR used for flexibility
                  - Total flex length 6 ft max
  LFMC            YES, if all apply:           YES, if outside the limits
                  - Listed fittings              at left, OR used for
                  - 3/8-1/2: OCPD 20 A max       flexibility after
                  - 3/4-1-1/4: OCPD 60 A max     installation
                  - Total flex length 6 ft max

Grounding Rules for Nonmetallic Conduits (PVC & LFNC)

Because PVC, LFNC, and other nonmetallic raceways are electrical insulators, they have zero ground-fault carrying capability. A separate equipment grounding conductor (EGC) sized according to NEC Table 250.122 must always be installed inside the raceway.

LFMC Equipment Grounding Limitations (NEC 250.118(A)(6))

LFMC is permitted to serve as the equipment grounding conductor only if all of the following conditions are met:

1. The conduit is terminated in listed fittings.
2. For trade sizes 3/8 and 1/2, the circuit overcurrent device is rated 20 amperes or less.
3. For trade sizes 3/4 through 1-1/4, the circuit overcurrent device is rated 60 amperes or less, and no FMC, FMT, or LFMC in trade sizes 3/8 or 1/2 is in the same ground-fault path.
4. The total combined length of flexible conduit in the ground-fault return path does not exceed 6 feet (1.8 m).
5. The conduit is not installed to provide flexibility after installation (see the rule below).

Exam Trap Alert: A 30-ampere circuit to an air-conditioning condenser through 5 feet of 3/4-inch LFMC passes the size and ampere tests, because the 3/4-inch size allows up to 60 amperes. But if the LFMC is there to absorb compressor vibration, it is providing flexibility, and 250.118(A)(6)(e) then requires a separate wire-type equipment grounding conductor anyway.

Flexibility After Installation (NEC 250.118(A)(5)(d) and (A)(6)(e))

Vibration & Flexibility Rule: Where FMC or LFMC connects equipment where flexibility is necessary to minimize the transmission of vibration, or to allow movement after installation (such as an electric motor, pump, or compressor), an equipment grounding conductor must be installed, regardless of the circuit ampere rating or conduit length.

Continual motor vibration loosens the spiral convolutions of the flexible metal core, creating high-resistance gaps that fail to clear a ground fault. An external or internal bonding jumper sized per NEC 250.102(D) ensures positive fault-clearing safety.

Bonding Jumper Routing (NEC 250.102(E))

An equipment bonding jumper is permitted to be installed inside or outside the raceway:

• If installed outside, the bonding jumper length is limited to a maximum of 6 feet (1.8 m).
• The external bonding jumper must be routed with the raceway and bonded to the enclosures at both ends.