4.4 Flexible Raceways, ENT, Wireways, Cable Trays, Specialty Cables & Plenum Rules

Why this section matters

Articles 300 through 398 carry about 10 of the 60 NEC and theory questions, the most of any area on the PSI outline. Sections 4.1 through 4.3 covered Article 300 basics, common cables, rigid raceways, conduit fill, and boxes. This section covers the other wiring methods that show up in lookup questions.

Support and securing intervals at a glance

Flexible metal conduit (Article 348)

Not permitted (348.12):

• in wet locations;
• in hoistways (except as Article 620 allows) or storage battery rooms;
• in hazardous locations, except as 501.10(B) allows;
• where exposed to materials that damage the conductors;
• underground or embedded in poured concrete or aggregate;
• where subject to physical damage.

Securing (348.30): within 12 inches of each box, cabinet, conduit body, or fitting, and at intervals no greater than 4-1/2 feet. The exceptions include fished runs and short unsupported lengths where flexibility is needed after installation:

• not over 3 ft for trade sizes 1/2 through 1-1/4;
• 4 ft for 1-1/2 through 2;
• 5 ft for 2-1/2 and larger;
• luminaire whips up to 6 ft in accessible ceilings.

Bends: no more than 360 degrees total between pull points, the same limit as rigid raceways (348.26).

As an equipment grounding conductor (250.118(5)): listed FMC qualifies only when all of these are true:

• it is terminated in listed fittings;
• the circuit conductors are protected at 20 A or less;
• the combined length of flexible metal conduit, flexible metallic tubing, and liquidtight flexible metal conduit in the same ground-fault return path is 6 ft or less;
• it is not installed for flexibility after installation (to minimize vibration, for example). When flexibility is needed, a wire-type EGC is required.

Liquidtight flexible metal and nonmetallic conduit (Articles 350, 356)

• LFMC is used where flexibility or protection from liquids, vapors, or solids is needed. It is not permitted where subject to physical damage. It is secured like FMC (12 inches and 4-1/2 feet, 350.30).
• As an EGC (250.118(6)): listed fittings; trade sizes 3/8 through 1/2 protected at 20 A or less; trade sizes 3/4 through 1-1/4 at 60 A or less; combined flexible length 6 ft or less; and a wire EGC when flexibility after installation is required.
• LFNC is not permitted where subject to physical damage. It is generally limited to 6-ft lengths, except where the article allows longer secured runs or a longer length is approved as essential for flexibility (356.12). LFNC never serves as an EGC; a wire-type EGC is pulled in.

Electrical nonmetallic tubing (Article 362)

• Permitted: in buildings not over three floors above grade, exposed or concealed. In taller buildings it must be concealed in walls, floors, and ceilings behind a thermal barrier with at least a 15-minute finish rating (or where fully sprinklered, as the article allows). Also permitted in wet locations and encased in concrete with approved fittings (362.10).
• Not permitted: in hazardous locations (except as permitted elsewhere), to support luminaires, where ambient temperatures exceed the tubing's listing, for direct burial, where exposed to direct sunlight unless identified for it, or where subject to physical damage (362.12).
• Securing: within 3 ft of each box and at intervals of no more than 3 ft (362.30).

Auxiliary gutters and wireways (Articles 366, 376)

Cable trays (Article 392)

A cable tray is a support system, not a raceway. Only the wiring methods listed in Table 392.10(A) may be installed in it. In industrial establishments where only qualified persons will service the installation, single conductors 1/0 AWG and larger and certain other methods are permitted (392.10(B)). Metal cable trays may serve as equipment grounding conductors under the conditions in 392.60. Ampacity comes from 392.80, which replaces Table 310.16 adjustments for many tray installations.

Specialty cables

Article 300 rules that cut across every method

• Corrosion (300.6): metal raceways and fittings must be protected against corrosion suitable for the environment.
• Raceways between temperature zones (300.7(A)): where a raceway passes from a warm to a cold area (into a walk-in cooler, for example), seal it to prevent circulation of warm air and condensation.
• Wet locations above grade (300.9): the interior of raceways in wet locations above grade is itself a wet location, so the conductors must be listed for wet locations (such as THWN-2).
• Independent support (300.11): raceways, cables, and boxes must be securely fastened in place. Ceiling grid wires may support them only if the wires are identified for the purpose and installed per 300.11(B). Raceways generally may not support other raceways or cables; one exception lets Class 2 cables controlling equipment be supported by that equipment's power raceway.
• Firestopping (300.21): penetrations of fire-resistance-rated walls, floors, and ceilings must be firestopped with approved methods that keep the fire rating.
• Ducts and plenums (300.22):
  • (A) no wiring at all in ducts that carry dust, loose stock, or flammable vapors;
  • (B) in ducts built to carry environmental air, only limited metal methods: MI cable without a nonmetallic cover, smooth or corrugated impervious-sheath MC cable, and EMT, IMC, or RMC without a nonmetallic cover (flexible metal conduit only in short lengths for equipment connections);
  • (C) in other spaces used for environmental air (plenum ceilings), metal raceways, MI, MC, and AC cable without nonmetallic coverings, and cables specifically listed for plenum use.