NEC 338 Service-Entrance Cable for the Journeyman Exam: SE, SER, SEU and USE, Feeders, Insulation and Bending

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.

SE, SER, SEU and USE: what the names mean

Name	What it is	Typical job
SE	Service-entrance cable, the type in Article 338	Service entrance, feeders and branch circuits under the conditions in 338.10
SER	Trade name for round SE cable, usually with insulated phase conductors, an insulated neutral and a bare conductor	Feeders to subpanels and ranges
SEU	Trade name for flat SE cable, usually two insulated conductors with a bare concentric conductor wrapped around them	Service entrance from the meter
USE	Underground service-entrance cable	Underground service conductors; limits on above-ground and interior use are in 338.12

SER and SEU are names used on site and in catalogs. The NEC rules are written for Types SE and USE, so on the exam, read what the cable actually contains: how many conductors are insulated, and whether one is bare.

The bare conductor rule for feeders and branch circuits

This is the point that trips people up. On a service, the bare conductor in SEU cable can be the grounded (neutral) conductor. On a feeder or branch circuit, SE cable needs all of its circuit conductors insulated, and a bare conductor may serve only as an equipment grounding conductor (NEC 338.10(B)). The section has an exception for certain existing installations where the bare neutral starts in service equipment; check its exact conditions in 338.10(B) before you apply it.

So a new feeder to a subpanel in SE cable is usually four-wire SER: two insulated phase conductors, an insulated neutral and a bare equipment grounding conductor. A three-wire SEU with a bare neutral is the service-entrance case, not a new feeder.

Interior use and thermal insulation

Used for interior wiring, SE cable follows the installation requirements of Part II of Article 334, the NM cable rules, excluding 334.80 (NEC 338.10(B)(4)). The ampacity rule that matters most applies to SE cable with ungrounded conductors 10 AWG and smaller installed in thermal insulation: its ampacity is based on the 60°C conductor temperature rating. The higher insulation rating may be used as the starting point for ampacity correction and adjustment, as long as the final result is not more than the 60°C value (NEC 338.10(B)(4)). Larger SE cable follows the normal ampacity rules, including the terminal temperature limits. For outdoor runs, the section sends you to the outside branch circuit and feeder rules in Article 225.

Where it may not go

SE cable is not permitted underground, with or without a raceway, and it is not used where subject to physical damage unless it is protected. Outdoors, check the sunlight resistance rules for the cable you have. USE cable has its own limits for interior wiring and above-ground use (NEC 338.12(A)) (NEC 338.12(B)). Read both lists in your 2026 NEC; questions often describe the location and ask whether the cable type fits.

Worked example: SE cable in attic insulation

A 10 AWG copper SE cable with 90°C insulation runs through attic thermal insulation to a 25 A load. Assume the ambient temperature calls for a correction factor of 0.87 from the 90°C column (check the factor for your ambient in the NEC correction table), and no adjustment for more than three current-carrying conductors. Use Table 310.16 values for 10 AWG copper: 30 A at 60°C, 35 A at 75°C, 40 A at 90°C. Check these values in your 2026 NEC. At 10 AWG, the 60°C rule for SE cable in thermal insulation applies.

1. Start at 90°C for correction. 40 A × 0.87 = 34.8 A.
2. Apply the 60°C cap for thermal insulation. The final ampacity may not exceed the 60°C value of 30 A.
3. Result. 30 A allowable ampacity, which is more than the 25 A load. A full circuit check also looks at continuous load, overcurrent protection and terminal temperature limits. Without the cap you might have answered 34.8 A.

If the same attic run used 6 AWG SE cable, this 60°C rule would not apply, because it covers only 10 AWG and smaller; the normal ampacity and terminal limits would.

Worked example: bending radius

A round SE cable has an outside diameter of 1.2 inches. The minimum radius, measured along the inner edge of the bend, is 5 × 1.2 = 6 inches (NEC 338.24).

Check yourself

1. In a new SE cable feeder to a subpanel, may the bare conductor be the neutral? No, apart from the existing-installation exception; a bare conductor may serve only as the equipment grounding conductor.
2. 10 AWG SE cable in thermal insulation: which temperature column caps the final ampacity? 60°C. The rule covers ungrounded conductors 10 AWG and smaller.
3. A round SE cable is 0.8 inches in diameter. What is the minimum bending radius? 4 inches.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 310, 334 and 338