•351-1. Scope. This article covers liquidtight flexible metal conduit and liquidtight flexible nonmetallic conduit.

A. Liquidtight Flexible Metal Conduit

•351-2. Definition. Liquidtight flexible metal conduit is a raceway of circular cross section having an outer liquidtight, nonmetallic, sunlight-resistant jacket over an inner flexible metal core with associated couplings, connectors, and fittings and approved for the installation of electric conductors.

•351-3. Other Articles. Installations of liquidtight flexible metal conduit shall comply with the applicable provisions of Article 300 and with the specific sections of Articles 350, 501, 502, 503, and 553 referenced below.

(FPN): For marking requirements, see Section 110-21.

•351-4. Use.

(a) Permitted. Listed liquidtight flexible metal conduit shall be permitted to be used in exposed or concealed locations:

1. Where conditions of installation, operation, or maintenance require flexibility or protection from liquids, vapors, or solids.
2. As permitted by Sections 501-4(b), 502-4, 503-3, and 504-20 and in other hazardous (classified) locations where specifically approved, and by Section 553-7(b).
3. For direct burial where listed and marked for the purpose.

(b) Not Permitted. Liquidtight flexible metal conduit shall not be used:

1. Where subject to physical damage.
2. Where any combination of ambient and conductor temperature will produce an operating temperature in excess of that for which the material is approved.

•351-5. Size.

(a) Minimum. Liquidtight flexible metal conduit smaller than 1/2-in. electrical trade size shall not be used.

Exception: 3/8-in. size shall be permitted as covered in Section 350-10(a).

(b) Maximum. The maximum size of liquidtight flexible metal conduit shall be the 4-in. trade size.

•351-6. Number of Conductors.

(a) Single Conduit. The number of conductors permitted in a single conduit, 1/2-in. through 4-in. trade sizes, shall not exceed the percentage of fill specified in Table 1, Chapter 9.

(b) 3/8-In. Liquidtight Flexible Metal Conduit. The number of conductors permitted in 3/8-in. liquidtight flexible metal conduit shall not exceed that permitted in Table 350-12.

•351-7. Fittings. Liquidtight flexible metal conduit shall be used only with approved terminal fittings. Angle connectors shall not be used for concealed raceway installations.

•351-8. Supports. Liquidtight flexible metal conduit shall be securely fastened in place by an approved means within 12 in. (305 mm) of each box, cabinet, conduit body, or other conduit termination and shall be supported and secured at intervals not to exceed 4 1/2 ft (1.37 m).

Exception No. 1: Where liquidtight flexible metal conduit is fished.

Exception No. 2: Lengths not exceeding 3 ft (914 mm) at terminals where flexibility is necessary.

Exception No. 3: Lengths not exceeding 6 ft (1.83 m) from a fixture terminal connection for tap conductors to lighting fixtures as permitted in Section 410-67(c). Horizontal runs of liquidtight flexible metal conduit supported by openings through framing members at intervals not greater than 4 1/2 ft (1.37 m) and securely fastened within 12 in. (305 mm) of termination points shall be permitted.

•351-9. Grounding. Liquidtight flexible metal conduit shall be permitted as a grounding means as covered in Section 250-91(b). Where an equipment bonding jumper is required around liquidtight flexible metal conduit, it shall be installed in accordance with Section 250-79.

Exception: Listed liquidtight flexible metal conduit shall be permitted as a grounding means in the 1 1/4-in. and smaller trade sizes if the total length of all liquidtight flexible metal conduit in any ground return path is 6 ft (1.83 m) or less, the conduit is terminated in fittings listed for grounding, and the circuit conductors contained therein are protected by overcurrent devices rated at 20 amperes or less for 3/8-in. and 1/2-in. trade sizes and 60 amperes or less for 3/4-in. through 1 1/4-in. trade sizes. Where used to connect equipment where flexibility is required, an equipment grounding conductor shall be installed.

(FPN): See Sections 501-16(b), 502-16(b), and 503-16(b) for types of equipment grounding conductors.

•351-10. Bends — Number in One Run. There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, e.g., conduit bodies and boxes.

•351-11. Splices and Taps. Splices and taps shall be made in accordance with Section 300-15. See Article 370 for rules on the installation and use of boxes and conduit bodies.

B. Liquidtight Flexible Nonmetallic Conduit

•351-22. Definition. Liquidtight flexible nonmetallic conduit is a raceway of circular cross section of various types

1. A smooth seamless inner core and cover bonded together and having one or more reinforcement layers between the core and cover.
2. A smooth inner surface with integral reinforcement within the conduit wall.
3. A corrugated internal and external surface without integral reinforcement within the conduit wall. This conduit is flame-resistant and, with fittings, is approved for the installation of electrical conductors.

•351-23. Use.

(a) Permitted. Listed liquidtight flexible nonmetallic conduit shall be permitted to be used in exposed or concealed locations:

(FPN): Extreme cold may cause some types of nonmetallic conduits to become brittle and therefore more susceptible to damage from physical contact.

1. Where flexibility is required for installation, peration, or maintenance;
   
2. Where protection of the contained conductors is required from vapors, liquids, or solids;
3. For outdoor locations where listed and marked as suitable for the purpose.
   (FPN): For marking requirements, see Section 110-21.
4. For direct burial where listed and marked for the purpose.

(b) Not Permitted. Liquidtight flexible nonmetallic conduit shall not be used:

1. Where subject to physical damage;
2. Where any combination of ambient and conductor temperatures is in excess of that for which the liquidtight flexible nonmetallic conduit is approved;
3. In lengths longer than 6 ft (1.83 m).
   
   Exception No. 1: Liquidtight flexible nonmetallic conduit as defined in Section 351-22(2) shall be permitted to be installed in lengths longer than 6 ft (1.83 m) where secured in accordance with Section 351-27.
   
   Exception No. 2: Where longer length is essential for a required degree of flexibility.
   
   
4. (4) Where voltage of the contained conductors is in excess of 600 volts, nominal.
   Exception: As permitted in Section 600-32(a), Exception, for electric signs over 600 volts.

•351-24. Size. The sizes of liquidtight flexible nonmetallic conduit shall be electrical trade sizes 1/2 in. to 4 in. inclusive.

Exception No. 1: 3/8-in. size for enclosing the leads of motors as permitted in Section 430,145(b).

Exception No. 2: 3/8-in. trade size shall be permitted in lengths not in excess of 6 ft (1.83 m) as a part of a listed assembly, for tap connections to lighting fixtures as required in Section 410-67(c), or for utilization equipment.

Exception No. 3: 3/8-in. trade size for electric sign conductors on insulators in accordance with Section 600-32(a).

•351-25. Number of Conductors. The number of conductors permitted in a single conduit shall be in accordance with the percentage fill specified in Table 1, Chapter 9.

•351-26. Fittings. Liquidtight flexible nonmetallic conduit shall be used only with terminal fittings identified for such use. Angle connectors shall not be used for concealed raceway installations.

•351-27. Supports. Liquidtight flexible nonmetallic conduit, as defined in Section 351-22(2), shall be securely fastened at intervals not greater than 3 ft (914 mm) and within 12 in. (305 mm) on each side of every outlet box, junction box, cabinet, or fitting.

Exception No. 1: Where liquidtight flexible nonmetallic conduit is fished.

Exception No. 2: Lengths not exceeding 3 ft (914 mm) at terminals where flexibility is necessary.

Exception No. 3: Lengths not exceeding 6 ft (1.83 m) from a fixture terminal connection for tap conductors to lighting fixtures as permitted in Section 410-67(c). Horizontal runs of liquidtight flexible nonmetallic conduit supported by openings through framing members at intervals not greater than 3 ft (914 mm) and securely fastened within 12 in. (305 mm) of termination points shall be permitted.

•351-28. Equipment Grounding. Where an equipment grounding conductor is required for the circuits installed in liquidtight flexible non-metallic conduit, it shall be permitted to be installed on the inside or outside of the conduit. Where in-stalled on the outside, the length of the equipment grounding conductor shall not exceed 6 ft (1.83 m) and shall be routed with the raceway or enclosure. Fittings and boxes shall be bonded or grounded in accordance with Article 250.

•351-29. Splices and Taps. Splices and taps shall be made in accordance with Section 300-15. See Article 370 for rules on the installation and use of boxes and conduit bodies.

•351-30. Bends — Number in One Run. There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, e.g., conduit bodies and boxes.