NEC Article 230 Underground Service Conductors and Service-Entrance Conductors for the Journeyman Exam: Size, Wiring Methods and Protection

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

Supply side of the disconnect

The service disconnect, the six-disconnect rule and overcurrent protection are on the services and overcurrent protection page. Here we stay on the supply side of the disconnect, from the utility’s underground lateral to the conductors that land in the panel.

The names, in order

Conductor	Runs from	Runs to
Service lateral	Utility’s underground distribution system	Service point. The utility generally controls this section, and the serving utility sets the arrangement
Underground service conductors	Service point	First point of connection to the service-entrance conductors
Service-entrance conductors	End of the underground service conductors or service drop	Service disconnect

Where the service point is, and who owns which section, is set by the serving utility. Ask the utility before you start.

Underground service conductors (230.30 to 230.33)

• Insulation. Underground service conductors are insulated for the voltage. A grounded conductor can be uninsulated when it is copper in a raceway or part of a cable identified for underground use, and in the other cases the section permits.
• Minimum size. Not smaller than 8 AWG copper or 6 AWG aluminum or copper-clad aluminum. Smaller conductors are allowed only for the limited loads the section describes (for example, a single branch circuit for a small polyphase power load or a controlled water heater), and the exception sets the minimum size.
• Grounded conductor. For its minimum size, go to 250.24, which sends you to Table 250.102(C)(1).
• Spliced and tapped. Underground service conductors can be spliced or tapped by the methods the section allows.

One set per service (230.40)

Each service drop, set of overhead service conductors, set of underground service conductors or service lateral supplies only one set of service-entrance conductors. The exceptions include:

• Multiple occupancies. A building with more than one occupancy can have one set of service-entrance conductors for each occupancy or group of occupancies.
• Separate enclosures. Where two to six service disconnects in separate enclosures are grouped at one location, one set of service-entrance conductors can supply each or several of them.
• A few others, including certain arrangements with a single-family dwelling and a separate structure.

Sizing service-entrance conductors (230.42)

1. Load calculation. Start with the calculated load. In the 2026 NEC, those calculations are in Article 120 instead of Article 220.
2. 125 percent of continuous load. Size the conductors for the noncontinuous load plus 125 percent of the continuous load. An exception allows 100 percent of both when the conductors end at an overcurrent device, and that device and its assembly are both listed for 100 percent operation.
3. Not less than the disconnect. The ampacity is also not less than the minimum disconnect rating in 230.79 that applies. That is not always the nameplate of the disconnect you install; a dwelling can still use the 310.12 allowance, as the example below shows. The 230.79 minimums are covered on the services and overcurrent protection page.

Dwelling shortcut (310.12). For a qualifying 120/240 V single-phase dwelling service rated 100 through 400 A, the ungrounded service conductors can have an ampacity of at least 83 percent of the service rating, subject to the conditions in that section (NEC 310.12).

Wiring methods (230.43)

Section 230.43 tells you which wiring methods you can use for service-entrance conductors. They include Type SE cable, Type MC cable, Type MI cable, Type IGS cable, rigid metal conduit, intermediate metal conduit, electrical metallic tubing, electrical nonmetallic tubing, PVC conduit, RTRC, HDPE conduit, nonmetallic underground conduit with conductors (NUCC), wireways, busways, auxiliary gutters, cablebus, open wiring on insulators, and certain flexible raceways under added conditions. Check the full list in your 2026 NEC before you answer a wiring method question.

Exam trap. Type NM cable is not on the list. Neither is Type AC cable. If a question offers NM as a service-entrance wiring method, it is wrong.

Cable tray (230.44). Cable trays that hold service-entrance conductors are identified with permanently affixed warning labels marked “Service-Entrance Conductors,” spaced at intervals not more than 10 ft. Such a tray generally holds only service-entrance conductors. Where the exception lets other conductors share the tray, a solid fixed barrier separates them from the service-entrance conductors.

Protection and support (230.50, 230.51)

Physical damage. Underground service-entrance conductors are protected under the underground installation rules in 300.5. Above grade, service-entrance cables that are subject to physical damage are protected by one of these: rigid metal conduit, intermediate metal conduit, Schedule 80 PVC conduit, electrical metallic tubing, Type RTRC-XW conduit, or other approved means.

Supporting SE cable. Service-entrance cables are supported by straps or other approved means within 12 in. of every service head, gooseneck or connection to a raceway or enclosure, and at intervals not exceeding 30 in.

Worked example 1: strapping SE cable

A 108 in. vertical run of SE cable goes from the meter base down to the service disconnect.

1. First strap: within 12 in. of the meter base.
2. Last strap: within 12 in. of the disconnect enclosure.
3. Between them: at most 30 in. apart.
4. Count: allow 12 in. at each end, which leaves 84 in. between the end straps. 84 ÷ 30 = 2.8, so that takes three spaces of 30 in. or less, which means two straps in the middle. Four straps altogether.

Worked example 2: a 200 A dwelling service

A one-family dwelling has a 120/240 V, 200 A single-phase service. Assume the 310.12 dwelling allowance applies, no ampacity adjustment or correction is needed, and the terminals are rated 75°C.

1. 83 percent rule: 200 × 0.83 = 166 A.
2. Copper: Table 310.16, 75°C column. 2/0 copper = 175 A, which is 166 A or more. 2/0 copper.
3. Aluminum: 3/0 aluminum = 155 A (too small). 4/0 aluminum = 180 A. 4/0 aluminum.

Five quick questions

1. Minimum underground service conductor size, copper, general rule? 8 AWG.
2. Can Type NM cable be used for service-entrance conductors? No.
3. Maximum spacing between straps on SE cable? 30 in.
4. Maximum spacing of the “Service-Entrance Conductors” labels on a cable tray? 10 ft.
5. 200 A dwelling service, 83 percent rule: minimum ampacity? 166 A.

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 120, 230, 250 and 310