12.1 Service Conductors, Clearances & Service-Entrance Requirements
Key Takeaways
- A service point is the point of connection between the serving utility and the premises wiring, and it divides utility-owned conductors from the premises service conductors the NEC regulates.
- NEC 230.9 requires overhead service conductors to maintain a 3-foot clearance from windows designed to be opened, doors, porches, balconies, ladders, stairs, fire escapes, and similar locations, but conductors run above the top level of a window are not required to be kept 3 feet away.
- NEC 230.24(B) sets vertical clearance for overhead service conductors above finished grade at 10 feet at the electrical entrance to a building and above areas accessible only to pedestrians, 12 feet over residential property and driveways, 15 feet over areas subject to truck traffic other than public streets, and 18 feet over public streets, alleys, roads, and parking areas subject to truck traffic.
- NEC 230.24(A) generally requires overhead service conductors to clear a roof by 8 feet, with reduced clearances permitted for low-slope conditions and a 18-inch allowance where not more than 4 feet of conductor passes over the roof overhang for a service mast.
- A building or structure is generally supplied by only one service under NEC 230.2, with specific permitted exceptions for special conditions, special occupancies, capacity requirements, and different characteristics.
12.1 Service Conductors, Clearances & Service-Entrance Requirements
Exam Focus: The service point, categories of service conductors, 230.2 one-service rule, 230.9 clearance from openings, 230.24 vertical clearances above roofs and grade, 230.42 sizing, and service masts.
The PSI subject list names services as a tested topic in its own right. Article 230 is where the premises wiring system begins, and it contains a set of clearance numbers that are pure memorization-and-lookup material — perfect open-book exam fodder.
Where the NEC's Jurisdiction Begins: The Service Point
The service point is the point of connection between the facilities of the serving utility and the premises wiring. Everything on the utility side is outside the NEC's scope under 90.2(B); everything on the premises side is Article 230's business.
+-------------------------------------------------------------------------+
| ANATOMY OF A SERVICE |
| |
| Utility transformer |
| | |
| | <-- SERVICE DROP / SERVICE LATERAL (utility-owned) |
| v |
| [ SERVICE POINT ] <== the NEC's jurisdiction starts here |
| | |
| | <-- OVERHEAD or UNDERGROUND SERVICE CONDUCTORS |
| v |
| [ Meter enclosure ] |
| | |
| | <-- SERVICE-ENTRANCE CONDUCTORS |
| v |
| [ SERVICE DISCONNECTING MEANS ] <== end of Article 230 |
| | |
| v |
| Feeders (Article 215) and branch circuits (Article 210) |
+-------------------------------------------------------------------------+
| Term | Meaning |
|---|---|
| Service drop | Overhead conductors from the utility to the service point |
| Service lateral | Underground conductors from the utility to the service point |
| Overhead service conductors | Premises overhead conductors from the service point to the service equipment |
| Underground service conductors | Premises underground conductors from the service point to the service equipment |
| Service-entrance conductors | Conductors from the service point or the last termination outside to the service disconnecting means |
One Service Per Building — NEC 230.2
A building or other structure served must be supplied by only one service, unless one of the permitted conditions applies:
- Special conditions — fire pumps, emergency systems, legally required standby, optional standby, parallel power production, and systems designed for connection to multiple sources.
- Special occupancies — by special permission, multiple-occupancy buildings where there is no space available for service equipment accessible to all occupants, or a single building large enough that two or more suppliers are needed.
- Capacity requirements — where the capacity requirements exceed 2,000 amperes at 1,000 V or less, where the load requires a capacity greater than the utility normally supplies through one service, or by special permission.
- Different characteristics — different voltages, frequencies, phases, or uses such as different rate schedules.
Where a building is supplied by more than one service, or by any combination of services, feeders, and branch circuits, a permanent plaque or directory must be installed at each service or feeder disconnect location denoting all other services, feeders, and branch circuits supplying that building and the area served by each (230.2(E)).
Clearance From Building Openings — NEC 230.9
Overhead service conductors must maintain a clearance of not less than 900 mm (3 ft) from:
- Windows that are designed to be opened
- Doors, porches, balconies, ladders, stairs, fire escapes, and similar locations
Two qualifiers decide most exam items:
- Conductors run above the top level of a window are not required to be maintained 3 ft away. A fixed picture window imposes no clearance at all, because it is not designed to be opened.
- The vertical clearance requirement of 230.9(B) applies to the area below the conductors where they pass above a platform, projection, or surface from which they might be reached — 3 ft.
Vertical Clearances Above Grade — NEC 230.24(B)
Overhead service conductors are not permitted to be readily accessible and must comply with these minimum clearances, measured from final grade or the surface below:
| Clearance | Condition |
|---|---|
| 3.0 m (10 ft) | At the electrical service entrance to a building, at the lowest point of the drip loop, and above areas or sidewalks accessible only to pedestrians — for conductors 150 V or less to ground |
| 3.7 m (12 ft) | Over residential property and driveways, and over commercial areas not subject to truck traffic — for conductors 300 V or less to ground |
| 4.5 m (15 ft) | Same areas as the 12 ft category, where the voltage exceeds 300 V to ground |
| 5.5 m (18 ft) | Over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other land traversed by vehicles such as cultivated, grazing, forest, and orchard land |
[!CAUTION] Read the surface, then read the voltage. The 12 ft and 15 ft rows describe the same surfaces; the only variable is whether the conductors exceed 300 volts to ground. A 120/240 V residential drop over a driveway needs 12 ft; a 277/480 V drop over the same driveway needs 15 ft.
Clearance Above Roofs — NEC 230.24(A)
The general rule is a clearance of not less than 2.5 m (8 ft) above the roof surface, maintained for a distance of at least 900 mm (3 ft) in all directions from the edge of the roof. Three relaxations are commonly tested:
- Not readily walkable / steep roof: where the roof has a slope of not less than 100 mm in 300 mm (4 in in 12 in), the clearance reduces to 900 mm (3 ft).
- Short pass over an overhang: where not more than 1.8 m (6 ft) of overhead service conductor passes over not more than 1.2 m (4 ft) of roof overhang and terminates at a through-the-roof raceway or approved support, the clearance reduces to 450 mm (18 in). This is the service mast allowance.
- Voltage-limited case: the reduction applies where the voltage between conductors does not exceed 300 V.
Where the service mast is used as support, 230.28 requires it to be of adequate strength or to be supported by braces or guys, and only power service-drop or overhead service conductors may be attached to it — no communications or CATV drops.
Sizing Service-Entrance Conductors — NEC 230.42
Service-entrance conductors must have an ampacity not less than the maximum load to be served, calculated as:
Additional minimums:
- 230.79 sets the minimum rating of the service disconnecting means: 15 A for a one-circuit installation, 30 A for a two-circuit installation, 100 A, 3-wire, for a one-family dwelling, and 60 A for all other installations.
- 230.31(B) and 230.23(B) set a floor on conductor size — generally not smaller than 8 AWG copper or 6 AWG aluminum, with a 12 AWG copper allowance for limited loads of a single branch circuit.
Michigan note: For emergency disconnects on one- and two-family dwellings and for the surge protection requirement at 230.67, apply the Michigan-amended rules covered in Section 10.4 rather than the unamended NFPA text.
Details Inspectors Check First
- Drip loop. Service-entrance conductors must be arranged so water will not enter service equipment; a drip loop below the point of attachment is the standard method (230.54(F)).
- Service head above the point of attachment (230.54(C)), unless it is impracticable, in which case the head may be within 600 mm (24 in) of the attachment.
- Raintight service head on a service mast (230.54(A)).
- No conductors passing through the interior of another building or structure except as permitted by 230.6, which describes conductors considered outside a building — encased in at least 50 mm (2 in) of concrete or brick, or in a raceway under at least 450 mm (18 in) of earth.
An overhead 120/240-volt service drop crosses a residential driveway. What is the minimum vertical clearance above the driveway required by NEC 230.24(B)?
Overhead service conductors pass beside a fixed picture window that cannot be opened, and above the top level of a second window that can be opened. What clearance does NEC 230.9 require?
A service mast supports an overhead service where 5 feet of service conductor passes over 3 feet of roof overhang and terminates at a through-the-roof raceway. What minimum clearance above the roof overhang does NEC 230.24(A) permit?
Under NEC 230.79, what is the minimum rating of the service disconnecting means for a one-family dwelling?