SER, SEU, URD AND USE-2: SERVICE AND UNDERGROUND CABLE

Four Names, Four Jobs

SEU and SER are service entrance cable, Article 338. SEU is the older form, most often two insulated conductors with a bare concentric neutral wrapped around them. SER is the round version with three insulated conductors plus a ground. Both are 600 volt assemblies used for service drops, panel feeders and subpanels.

URD and USE-2 are underground. URD is a utility term rather than an NEC cable type: it describes a direct-buried secondary assembly, usually two or three XLPE-insulated aluminum conductors twisted around a bare neutral. USE-2 is a single conductor rated for underground service entrance, 90 degrees wet or dry, sunlight resistant, and permitted in direct contact with the earth.

• SEU: two insulated conductors with a bare concentric neutral, for single-phase service and heavy appliance feeds.
• SER: three insulated conductors plus ground, round, for subpanels and interior feeders.
• URD: triplex or quadruplex direct-burial assembly, almost always aluminum, for yard and pedestal work.
• USE-2: single conductor, direct burial, also used on service laterals and solar DC circuits.
• Mobile home feeder: four-wire direct-burial assembly from 2-2-2-4 through 4/0-4/0-4/0-2/0.

How SEU and SER Are Built

SEU carries its neutral outside the insulated cores, wrapped helically as bare strands. That is why SEU is compact and why the neutral is obvious at a glance. SER puts the same conductors in a round jacket with the ground bare or insulated, which is what a subpanel feed requires.

Both are made with XHHW or THHN conductors under a sunlight-resistant jacket, and both are limited by termination temperature. Where SE cable is installed in thermal insulation, the ampacity must come from the 60 degree column, and that is the rule that catches people who read the 90 degree print and size to it.

URD, USE-2 and Direct Burial

URD feeds a pedestal, a shop, a well house or a yard light from a transformer. The conductors are XLPE-insulated aluminum twisted around a bare neutral, and the assembly is built to go into the ground without conduit. Neutral sizes run full, two-thirds or one-third of the phase conductor, and that ratio is on the tag.

USE-2 is the single-conductor version of the same idea: 90 degrees wet or dry, sunlight resistant, suitable for direct burial. That is why it appears on service laterals, irrigation pumps and solar arrays. It is not a substitute for building wire inside a wall, and it is not rated for continuous submersion in a wet well.

• Aluminum is normal for URD and for large USE-2; copper appears in smaller services and retrofit work.
• Bare aluminum is not installed in contact with earth or masonry, which is why every underground aluminum conductor is insulated.
• Aluminum terminations need a listed AL/CU connector, an antioxidant where the manufacturer requires one, and the specified torque.
• A direct-burial assembly still needs the jacket intact; a nick at the reel flange is where moisture starts.

Ampacity and the Neutral

The neutral is where these cables differ from ordinary feeders. A reduced neutral is legitimate for a service carrying mostly line-to-line load, and it is the wrong choice for a panel full of 120 volt branch circuits. Read the neutral size on the tag before valuing the reel.

Ampacity comes from the conductor insulation and the termination, not from the jacket. A 4/0 aluminum URD assembly carries 180 amperes in the 75 degree column; the same conductor reads 205 in the 90 degree column, which a 75 degree lug cannot use. Small sizes are also limited by the service and feeder rules rather than by the insulation alone.

Photograph and Note

The tag on a URD reel states conductor size, neutral size, voltage class and footage. On SEU and SER the print is on the jacket and the neutral is visible at the cut end. Two photographs cover most of what a quote needs, and a third showing the flange settles whether the reel can be handled on site.

Wire and cable is the whole offer. Transformers, switchgear, breakers, motor control centers, generators, chillers and machinery are not bought, however close they sit to the cable. Aluminum service and underground stock is covered further under aluminum wire, interior branch-circuit stock under building wire, and the quote form takes either.

• The reel tag, showing phase size, neutral size and footage.
• A cut end, where the conductor count and the bare neutral are visible.
• The jacket print, showing the type and temperature rating.
• The reel flange, since a 5,000-foot URD reel needs equipment to move.
• Whether the cable has been buried and pulled back out, and for how long.