Electrical Service Entrance: How Power Enters Your Building
The electrical service entrance is the point where the utility grid's power supply transitions into a building's privately owned electrical system. This page covers the physical components, code classifications, installation scenarios, and key decision thresholds that define how service entrance systems are specified, permitted, and inspected across residential, commercial, and industrial contexts in the United States. Understanding this infrastructure matters because failures or undersized installations at the service entrance affect every downstream circuit in the building.
Definition and scope
The service entrance is the assembly of conductors and equipment through which electrical supply from the utility is delivered to the building's distribution system. Under NFPA 70 (National Electrical Code), the NEC defines service conductors as those between the utility service point and the service disconnecting means — the main breaker or disconnect switch. The utility company owns and maintains the infrastructure up to the service point (typically the meter base or transformer connection); everything downstream falls under the building owner's responsibility and is subject to NEC compliance and local authority having jurisdiction (AHJ) enforcement.
Service entrance scope includes:
- Service drop or lateral — overhead conductors from the utility pole (service drop) or underground conductors from the transformer vault (service lateral)
- Service entrance conductors — the wires running from the service drop/lateral attachment point to the meter
- Meter base and metering enclosure — utility-installed equipment measuring consumption
- Service entrance cable (SEC) or conduit run — conductors between meter and main disconnect
- Main disconnect / service panel — the first means of disconnecting all ungrounded conductors per NEC Article 230
- Grounding electrode system — rods, plates, or concrete-encased electrodes bonding the system to earth per NEC Article 250
The electrical service entrance explained resource on this site expands on each component's physical characteristics and code references.
How it works
Utility power arrives at the service drop or lateral at the voltage level the transformer has stepped down for the customer class — typically 120/240V single-phase for residential occupancies and 120/208V or 277/480V three-phase for commercial or industrial users. For a detailed breakdown of voltage classification, see 240V vs 120V Circuits Explained.
From the utility attachment point, ungrounded (hot) and grounded (neutral) conductors travel to the meter base. The meter base is owned by the utility but must be installed in compliance with both utility specifications and NEC Article 230. After metering, service entrance conductors continue to the main disconnect. At the main panel, the neutral conductor bonds to the grounding electrode system — a connection permitted at the service entrance only, not repeated at subpanels (NEC 250.24).
The main disconnect must interrupt all ungrounded conductors simultaneously. NEC 230.71 limits the number of service disconnects to 6 for a given service, with exceptions for specific occupancy types. Overcurrent protection (the main breaker) is sized to match the service conductor ampacity, which determines the building's total available capacity.
Common scenarios
Residential overhead service (100–200A, single-phase)
The most common residential installation uses a 200A, 120/240V single-phase three-wire service. Conductors span from the utility pole to a weatherhead mounted on the structure, then run through service entrance cable (Type SE or SER per NEC Article 338) to the meter base and main panel. The residential electrical systems overview covers panel sizing and load considerations in detail.
Residential underground service lateral
Underground service replaces the overhead drop with conductors in conduit buried to the utility's specified depth — typically 24 inches for rigid metal conduit or 24 inches for direct-burial cable per NEC Table 300.5. The utility trench may require inspection before backfill.
Commercial 120/208V three-phase service
Light commercial buildings typically receive a 120/208V wye-configured three-phase service, supporting both 120V single-phase loads (lighting, receptacles) and 208V three-phase loads (HVAC, equipment). The commercial electrical systems overview addresses demand calculations and service sizing for retail and office contexts. Three-phase service architecture is covered in depth at Three-Phase Electrical Systems US.
Industrial 480V service
Heavy industrial facilities receive 480V three-phase service, typically stepped down internally by distribution transformers for 120V branch circuits. Service conductors at this voltage level require arc flash hazard labeling per NFPA 70E.
Overhead vs. underground contrast
Overhead services cost less to install but carry higher exposure to storm damage and falling-conductor hazards. Underground laterals require trenching but eliminate most weather-related outage risk and avoid National Electrical Safety Code (NESC) clearance requirements for conductors above driveways and walkways. Many jurisdictions and utilities now require underground service for new construction subdivisions.
Decision boundaries
Ampacity and service sizing
Service entrance conductors must be sized per NEC Table 310.12 for single-family dwellings or NEC Article 220 load calculations for all other occupancy types. A 100A service is the minimum permitted for a new single-family dwelling (NEC 230.79). A 200A service is the practical standard for homes above 1,500 square feet or those with electric heat, EV charging, or heat pump systems. See Electrical System Capacity and Amperage Ratings for a structured sizing framework.
Permitting and inspection
Service entrance work triggers a permit requirement in effectively all U.S. jurisdictions because it constitutes a modification to the service conductors or main disconnect — both regulated under the NEC and adopted state codes. Many AHJs require a separate utility notification before reconnection. The electrical permit requirements by project type page maps permit thresholds across project categories. Inspection occurs in two phases for new installations: rough-in inspection before walls are closed and final inspection before the meter is set.
Code compliance checkpoints
NEC Article 230 governs service entrance installation in full. Key compliance checkpoints include: conductor clearance heights above grade and rooflines (NEC 230.24), service entrance cable weatherproofing at the weatherhead, proper mast height for overhead service attachment, and grounding electrode conductor sizing (NEC Table 250.66). Local amendments adopted by the AHJ may impose stricter requirements than the base NEC. Consulting NEC Code Compliance for Electrical Systems provides a code-section reference framework for these requirements.
References
- NFPA 70: National Electrical Code (NEC) — Primary standard governing service entrance installation, conductor sizing, disconnect requirements, and grounding (Articles 230, 250, 310, 338)
- NFPA 70E: Standard for Electrical Safety in the Workplace — Arc flash and electrical safety requirements applicable to commercial and industrial service entrance equipment
- ANSI C2: National Electrical Safety Code (NESC) — IEEE/ANSI standard governing utility-side conductor clearances and overhead service drop requirements
- U.S. Consumer Product Safety Commission (CPSC) — Federal agency publishing electrical hazard data relevant to service entrance failures and aging infrastructure
- National Fire Protection Association (NFPA) — Publisher of NEC and NFPA 70E; maintains code update cycles and AHJ adoption tracking