6.5 Service-Entrance Cable (Types SE & USE), Other Cables & Flexible Cords

6.5 Service-Entrance Cable (Types SE & USE), Other Cables & Flexible Cords

Conductors and Cables is tied for the largest subject area on the Alabama Electrical Journeyman exam at ten questions, and it does not stop at Romex and MC. Service-entrance cable and flexible cord are both everyday products with restrictive, heavily tested rules, and both are places where an experienced electrician's habits can be out of step with the Code.

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Service-Entrance Cable (NEC Article 338)

Article 338 covers a single-conductor or multiconductor assembly, with or without an overall covering, primarily used for services. Three type letters matter:

The USE Trap

The distinction that shows up on exams is that Type USE has no flame-retardant covering, so it is restricted to underground use and is not permitted for interior wiring. Type USE is not a general-purpose replacement for Type SE, and single-conductor USE is not permitted for interior branch circuits. Conversely, Type SE without a listed moisture- and sunlight-resistant marking is not a direct-burial product. Read the print legend on the jacket rather than the color of the reel.

Uses Permitted (NEC 338.10)

• As service-entrance conductors, installed per Article 230 with the applicable overhead or underground rules.
• As branch circuits or feeders, provided all circuit conductors of the cable are of the insulated type. A common legacy installation used the bare conductor of Type SE cable as the grounded conductor for a range or dryer; that is no longer permitted for new work, and today's 4-wire SER feeds a separate insulated grounded conductor and a separate equipment grounding conductor.
• Interior installations must follow the Part II installation requirements of Article 334 (the Type NM rules) for support, securing, protection from physical damage, and boring through framing.

The 60 Degrees C Ampacity Rule (NEC 338.10(B)(4)(a))

This is the single most tested Article 338 detail. Where Type SE cable is used for interior branch circuits or feeders and the cable is installed in thermal insulation, the ampacity must be taken from the 60 degrees C column of Table 310.16, not the 75 degrees C column and certainly not 90 degrees C.

Worked consequence: a 4 AWG copper SER feeder is 85 A in the 75 degrees C column but only 70 A in the 60 degrees C column. Run that same cable through a wall cavity packed with batt insulation to a detached garage panel and it will no longer support a 90 A calculated load. As with Type NM, the higher 90 degrees C conductor rating may still be used as the starting point for derating calculations, but the final result cannot exceed the 60 degrees C value.

Uses Not Permitted (NEC 338.12)

Type SE cable must not be used underground with or without a raceway, and Type USE must not be used for interior wiring, above ground except where protected, or as aerial cable unless it is a listed multiconductor cable.

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Flexible Cords and Cables (NEC Article 400)

Flexible cord is the product electricians reach for when equipment must move, vibrate, or be disconnected frequently. Article 400 exists because it is also the product people misuse as permanent wiring.

Common Type Letters (Table 400.4)

The letters decode the same way conductor insulation letters do:

• S - extra-hard usage, 600V
• SJ - junior hard service, 300V
• O - oil-resistant outer jacket; OO - oil-resistant jacket and oil-resistant insulation
• W - suitable for outdoor and wet locations
• T - thermoplastic; no letter T implies thermoset rubber

So SOOW is extra-hard usage, 600 volts, oil-resistant jacket and insulation, rated for wet and outdoor use, which is why it is the standard choice for portable industrial equipment and temporary power.

Uses Permitted (NEC 400.10)

Flexible cord is permitted for pendants, wiring of luminaires, connection of portable equipment, elevator cables, cranes and hoists, connection of utilization equipment to facilitate frequent interchange, prevention of the transmission of noise or vibration, and appliances designed to be moved for maintenance and repair where the appliance has an identified attachment-plug connection.

Uses Not Permitted (NEC 400.12)

Commit this list to memory. Flexible cords and cables shall not be used:

1. As a substitute for the fixed wiring of a structure.
2. Where run through holes in walls, structural ceilings, suspended or dropped ceilings, or floors.
3. Where run through doorways, windows, or similar openings.
4. Where attached to building surfaces (an exception permits cord attachment to a building surface in accordance with 368.56(B) for busway tap devices).
5. Where concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings.
6. Where installed in raceways, except as specifically permitted elsewhere in the Code.
7. Where subject to physical damage.

The practical translation: a cord stapled up a stud bay and fished through a top plate to feed a new receptacle is a violation on three separate counts, and it is one of the most common findings on a rough-in inspection.

Ampacity and Protection

• Ampacity comes from Table 400.5(A)(1) and Table 400.5(A)(2), never from Table 310.16. Note that the tables distinguish between cords with two current-carrying conductors and cords with three, and that the equipment grounding conductor is not counted as current-carrying.
• Overcurrent protection for flexible cord follows NEC 240.5, which permits cord supplied as part of a listed appliance or listed extension cord set to be protected by the branch-circuit device.
• Strain relief (NEC 400.14): flexible cord must be connected so that tension is not transmitted to the terminals, using a strain-relief fitting, a knot in the cord, or a listed cord grip.
• Equipment grounding (NEC 250.138): cord-and-plug-connected equipment is grounded through an equipment grounding conductor in the cord terminated on the grounding pole of the attachment plug.

Fixture Wire (Article 402)

Article 402 covers the small conductors inside and immediately supplying luminaires. The sizes run from 18 AWG through 10 AWG, and overcurrent protection follows the small-conductor limits of 240.4(D): 7 amperes for 18 AWG and 10 amperes for 16 AWG copper. Fixture wire is not building wire and may not be substituted for branch-circuit conductors.