SER vs SEU cable: service entrance or feeder
Published Sep 30, 2026
SER and SEU are both Type SE service-entrance cable under NEC Article 338, and both are usually aluminum. The difference is the neutral. SEU has two insulated conductors and a bare concentric conductor, so it belongs on the service between the meter and the service disconnect. SER is round, and its neutral is insulated, so it is the one you use for a feeder to a subpanel.
How they are built
SEU is flat. Two insulated conductors lie side by side, and a bare conductor is spiral-wrapped around them under the outer jacket. On a service, that bare concentric conductor is the neutral. It is common on the run from the weatherhead or meter base to the main panel.
SER is round. The conductors are cabled together, and the size string on the jacket tells you what is inside. In a four-conductor SER such as 4/0-4/0-4/0-2/0 aluminum or 2-2-2-4 copper, the first three are insulated (two ungrounded conductors and an insulated neutral) and the last, smaller one is a bare equipment grounding conductor.
Side by side
| SEU | SER | |
|---|---|---|
| NEC article | 338 | 338 |
| Shape | Flat | Round |
| Neutral | Bare, concentric around the insulated conductors | Insulated |
| Separate equipment ground | No | Yes, bare, in four-conductor types |
| Typical use | Service entrance, meter to service disconnect | Service entrance and feeders to subpanels |
| New feeder that needs a neutral | No | Yes |
| 240 V load with no neutral | Yes, with the bare conductor as the ground (338.10(B)(2)) | Yes |
| Underground, with or without raceway | No (338.12(A)) | No (338.12(A)) |
Why the neutral matters after the service
At the service, the neutral and the grounding system are bonded together, so a bare neutral is fine there. Past the service disconnect, the NEC keeps neutral current off grounding paths (250.24(A)(5) and 250.142(B)). A feeder to a subpanel therefore needs an insulated neutral and a separate equipment grounding conductor. SER has both. SEU does not.
338.10(B) spells out what SE cable may do as a branch circuit or feeder:
- Where all circuit conductors are insulated, it can be used as a branch circuit or feeder. That is four-conductor SER.
- Where the bare conductor is used only for equipment grounding, it can be used too. That lets SEU feed a 240 V load such as a condenser or a water heater with the bare wrap as the ground.
- An exception covers existing range and dryer circuits that used the bare conductor as the neutral. It does not allow new ones (see 250.140).
Installing it inside
SE cable run inside a building follows Part II of Article 334 (the NM-B installation rules), except 334.80, per 338.10(B)(4). When it runs through thermal insulation, its ampacity is held to the 60 °C column. The conductors' 90 °C rating can be used for correction and adjustment, but the result cannot exceed the 60 °C value. Outside, as a feeder or branch circuit, it follows Part I of Article 225.
Sizing: 310.12 and 310.16
For a one-family dwelling, or one dwelling unit of a two- or multifamily building, at 120/240 V single-phase, 310.12 lets the service conductors have an ampacity of not less than 83 percent of the service rating (310.15(B)(7) in the 2017 edition). That same rule covers a feeder that carries the whole dwelling load. For a 200 A service, that is an ampacity of at least 166 A, which works out to the familiar 4/0 aluminum or 2/0 copper from the 75 °C column of Table 310.16.
Every other feeder, including a subpanel feeder, is sized from Table 310.16 at the terminal temperature allowed by 110.14(C), usually 75 °C, and from the 60 °C column when the cable passes through thermal insulation. The 83 percent rule does not apply to a subpanel. For a side-by-side of metals, see aluminum vs copper feeders, and check long runs with the voltage drop calculator.
Common mistakes
- Using three-wire SEU to feed a subpanel in new work, with the bare wrap as the neutral.
- Burying SER or pulling it into underground conduit. Type SE is not permitted underground, even in a raceway. Use USE-2, URD or wet-rated conductors in conduit.
- Sizing a subpanel feeder with the 83 percent rule.
- Stapling SER through an insulated wall at its 75 °C ampacity.
- Forgetting anti-oxidant compound or the torque value the lug maker lists for aluminum conductors.
Shop SE cable
- 3/0-3/0-3/0-1/0 SER aluminum, 4-4-4-6 SER aluminum and 4/0-4/0-4/0-2/0 SER copper
- 2-2-2-4 SER copper and 6-6-6-6 SER copper for smaller feeders
- 2-2-2 SEU aluminum and 4/0-4/0-4/0 SEU aluminum for service entrance
- More sizes in aluminum building wire
Verify with your local code. Service rules, the 83 percent allowance and utility requirements for the meter-to-panel run vary by NEC edition and by jurisdiction. Ask your inspector and your utility before you set the meter base.