Article 230 of the National Electrical Code (NEC) explains the installation of service conductors and service equipment that brings electrical power from the utility supply to a building or structure.

The scope of the article includes electrical requirements related to:

• Overhead and underground service conductors

• Service entrance conductor types, sizes, and protection

• Service disconnect locations and grouping

• Overcurrent protection at the point of entry

• Emergency disconnects for residential dwellings

Below is a complete overview of the most important rules and real-world cable considerations found in NEC Article 230:

General Requirements

NEC 230.2 limits buildings to one service, but multiple services may be permitted under specific conditions. For example:

• Large commercial or industrial buildings that have multiple occupancies (like a mall or multi-tenant warehouse)

• Buildings with firewalls that divide the structure into separate fire areas

• Facilities where different voltage systems are needed for separate functions (e.g., 208Y/120V and 480Y/277V)

In these cases, each service must be justified and separately marked, and meet spacing and grouping rules set by the NEC.

Service Conductors

Service conductors include:

• Overhead service drops installed by the utility

• Underground service laterals are where the utility installs conductors underground to the building

• Service entrance conductors, installed by the customer from the meter base to the service disconnecting means

NEC Article 230 regulates all of these.

Conductor Type and Rating

The type of cable used for service entrance depends heavily on installation conditions:

• THHN/THWN-2 is a common choice for service entrance conductors installed in conduit. It's suitable for both indoor and outdoor use with conduits like RMC, IMC, EMT, or PVC. THHN/THWN has a temperature rating of 90°C in dry locations and 75°C when wet. It is versatile and widely approved.

• XHHW-2 is the best choice for environments prone to high humidity or temperature fluctuations, such as rooftop solar setups or installations in coastal regions. Like THHN, it must be installed in conduit but provides enhanced moisture resistance and thermal stability with a 90°C rating in dry and wet environments.

• SER and SEU cables are specifically service entrance conductors. They are used in above-ground residential applications to run from the meter to the main service panel. SER includes individual conductors for hot, neutral, and ground, while SEU combines the neutral and ground. These cables are not allowed in underground or wet environments and are typically installed indoors or along exterior walls in dry conditions in conduit.

• USE-2 and XLP cables are used for underground service laterals. They’re designed for direct burial or conduit use and can withstand constant exposure to moisture. However, unless dual-rated as RHH or RHW-2, these cables cannot be used inside buildings. For residential applications, utilities often use URD cable, a configuration of USE-2-rated conductors twisted together in triplex or quadruplex assemblies for efficient underground power delivery. URD cables are convenient because they are preassembled for 1-phase power.

• All conductor types must be marked and listed for service entrance use.

As a general rule for cables used for service entrance:

• Use THHN/XHHW-2 for overhead or indoor service entrances in conduit.

• Use SER cable for above-ground residential service (panel to meter).

• Use USE-2 or URD for underground lateral service (direct burial or in conduit).

• Always size aluminum conductors up one or more levels compared to copper for the same ampacity.

Conductor Size

Conductor ampacity must be calculated using NEC 310.16 or NEC 310.12 (for dwellings), with adjustments for ambient temperature and conduit fill. Sizing is based on connected load, calculated per NEC Article 220.

For residential dwellings with 100–400 amps, NEC 310.12 allows reduced sizing due to typical load diversity — for example, 1/0 AWG copper or 4/0 AWG aluminum for a 200A service. For non-dwelling units, the standard ampacity tables in NEC 310.16 apply.

 NEC 310.12 – Dwelling Services (100–400A)

NEC 310.16 – General Applications (75°C Column)

When using aluminum, conductors must be upsized compared to copper due to lower conductivity. Aluminum also requires an antioxidant paste at terminations and lugs rated for aluminum use. It’s commonly used in underground laterals (e.g., URD cable) to save costs on direct burial.

Conductor Routing

Because service entrance conductors are unprotected until the main disconnect, NEC requires that they be routed to minimize exposure and physical damage.

They must be installed in rigid metal conduit (RMC) or intermediate metal conduit (IMC) where exposed.

• RMC is thicker and used in areas prone to mechanical damage, such as outdoor service masts or commercial yards.

• IMC is lighter but still suitable for interior walls or protected runs.

• Schedule 80 PVC may be used in areas subject to light impact (f.e. garage routing) or underground risers.

For underground service laterals, USE-2 or URD cable is either directly buried (typically 24" deep) or installed in conduit (minimum 18" depth when protected). SE cables are not permitted underground.

Service Equipment

Service equipment includes the main disconnecting means, metering enclosure, and any associated grounding and bonding connections. This equipment must be listed, securely mounted, and clearly marked. It must enclose all service entrance conductors and provide safe, fast access for maintenance or emergency shutdown.

Disconnecting Means

Per NEC 230.70:

• A means must be provided to disconnect all ungrounded conductors.

• It must be located outside or immediately inside the building at the point of entrance.

• It must be readily accessible without tools and clearly labeled.

Overcurrent Protection

• NEC 230.91 requires that overcurrent protection be installed at the main service disconnect, not in line with the service entrance conductors. These unprotected conductors must be kept short, straight, and physically protected.

• The overcurrent protection device (OCPD) must be rated to handle the available fault current in the event of a short circuit or overload.

Maximum Number of Disconnects

NEC 230.71(A) allows:

• Up to six service disconnects per service entrance, provided they are grouped in one location.

• A single main breaker panel is standard for residential applications.

As of the 2023 NEC, new installations are generally required to use a single disconnect for improved coordination and simplicity.

Emergency Disconnects

NEC 230.85 requires a clearly labeled emergency disconnect for all one- and two-family dwellings. This disconnect must be located outside the building, clearly labeled as “EMERGENCY DISCONNECT,” and readily accessible to first responders in case of fire or disaster.

Marking and Identification

All conductors must be properly identified:

• Neutral (grounded) conductors must be white or gray (NEC 200.6)

• Grounding conductors must be green or bare

• Re-identification (e.g., sleeving a black conductor to indicate neutral) is permitted only under NEC guidelines

• Cable jackets must display the voltage rating, conductor type, and ampacity