5.1 Service Clearances, Overhead & Underground Service-Drop Requirements (230.9, 230.24 & 230.30)
Key Takeaways
- NEC 230.2 establishes the general rule that a building or structure shall be supplied by only one service, with specific exceptions for fire pumps, emergency systems, capacity exceeding 2,000A, different voltages/phases, or special occupancies.
- NEC 230.9 mandates a minimum 3-foot clearance between open service conductors and windows that open, doors, porches, balconies, stairs, and fire escapes, with an exemption for conductors installed above the top level of a window.
- Under NEC 230.24(B), overhead service-drop vertical clearances above grade are 10 ft for pedestrian-only areas, 12 ft over residential driveways, 18 ft over public streets/alleys/truck areas, and 24.5 ft over railroad tracks.
- Overhead roof clearances under NEC 230.24(A) require 8 ft general clearance, reducible to 3 ft for roof pitch ≥ 4/12 (≤ 300V), and 18 inches for through-the-roof service masts located within 4 ft of the roof edge with max 6 ft of conductor over the roof.
- Underground service lateral raceways entering buildings must be sealed per NEC 230.8 and 300.5(G), and emerging conductors must be protected against physical damage using RMC, IMC, or Schedule 80 PVC from 18 in. below grade to at least 8 ft above grade.
Service Clearances, Overhead & Underground Service-Drop Requirements (NEC Article 230)
Article 230 of the National Electrical Code governs the installation, conductors, and equipment for control and protection of services—the vital link where utility-supplied power enters a customer's electrical distribution system. Because service conductors operate ahead of the main overcurrent protective device (OCPD), they carry immense available fault current and represent a unique fire and shock hazard. On the Oklahoma Journeyman Electrician examination, mastering service supply rules, clearance envelopes, structural mast limitations, and underground burial depths is essential for code compliance and public safety.
1. Number of Services Permitted (NEC 230.2)
As a foundational safety standard, NEC 230.2 establishes the general rule:
"A building or other structure served shall be supplied by only one service unless permitted in 230.2(A) through (D)."
Limiting a structure to a single service ensures that emergency first responders (such as firefighters) and service electricians can completely de-energize the entire building from a single concentrated location without leaving energized backfeeds.
+-----------------------------------------------------------------------------+
| EXCEPTIONS TO THE "ONE SERVICE" RULE (NEC 230.2) |
| |
| [A. Special Conditions] [B. Special Occupancies] |
| - Fire pumps - Multiple-occupancy buildings with no |
| - Emergency / Standby systems common equipment access (special perm) |
| - Parallel solar PV / energy - Exceptionally large buildings |
| storage power sources [D. Different Characteristics] |
| [C. Capacity Requirements] - Different voltages (480V vs 208V) |
| - Calculated load > 2,000A - Different frequencies or phases |
| - Single-phase utility limits - Different utility billing rate tariffs |
+-----------------------------------------------------------------------------+
Mandatory Identification Plaques (NEC 230.2(E))
Where a building is supplied by more than one service, or by a combination of feeders, branch circuits, and services, a permanent plaque or directory must be installed at each service disconnect location denoting all other services, feeders, and branch circuits supplying that building and the specific area or equipment served by each.
2. Clearances from Building Openings (NEC 230.9)
Service conductors installed as open conductors, multiconductor cables without an overall outer jacket, or cabled service-drop conductors must maintain safe physical separation from building openings where occupants could reach out and contact energized lines.
+-----------------------------------------------------------------------------+
| CLEARANCE FROM BUILDING OPENINGS (NEC 230.9) |
| |
| +------------------------+ |
| | Conductors Above Top | <-- EXEMPT from 3-ft rule |
| | Level of Window Frame | (Permitted overhead) |
| +------------------------+ |
| | |
| +--------------------+ |
| | OPERABLE WINDOW | |
| <--- 3 Feet ---> | OR DOOR OPENING | <--- 3 Feet ---> |
| Clearance Required +--------------------+ Clearance Required |
| | |
| <--- 3 Feet Below ---> |
+-----------------------------------------------------------------------------+
Key Clearance Rules under NEC 230.9:
- 3-Foot Minimum Clearance (230.9(A)): Service conductors must have a clearance of not less than 3 feet (900 mm) from windows that open, doors, porches, balconies, ladders, stairs, fire escapes, or similar locations.
- The Above-Window Exemption: Service conductors installed above the top level of a window are exempt from the 3-foot requirement because occupants cannot lean or reach upward out of an open window sash to contact overhead conductors.
- Prohibition Below Openings (230.9(C)): Service conductors shall NOT be installed underneath openings through which materials might be moved (e.g., commercial second-story delivery doors, hayloft openings in agricultural structures) and shall not obstruct entrance to these building openings.
3. Overhead Service-Drop & Conductor Clearances (NEC 230.24)
Overhead service spans must maintain strict vertical and horizontal clearances above ground, roads, driveways, platforms, and roofs to prevent contact with vehicles, pedestrians, and maintenance personnel.
Vertical Clearance Above Grade (NEC 230.24(B))
| Location / Surface Classification | Minimum Vertical Clearance | Voltage & Application Limits |
|---|---|---|
| Pedestrian walkways, sidewalks, finished grade, decks | 10 feet (3.0 m) | For conductors rated ≤ 150V to ground (e.g., 120/240V 1-ph, 208Y/120V 3-ph) |
| Residential driveways & commercial areas (no truck traffic) | 12 feet (3.7 m) | Voltage ≤ 300V to ground |
| Residential driveways & commercial areas (no truck traffic) | 15 feet (4.5 m) | Voltage > 300V to ground (e.g., 480Y/277V systems) |
| Public streets, alleys, roads, parking areas (truck traffic) | 18 feet (5.5 m) | All public roadways, commercial driveways, and agricultural cultivated land |
| Railroad track rails | 24.5 feet (7.5 m) | Overhead spans crossing railroad tracks |
+-----------------------------------------------------------------------------+
| OVERHEAD SERVICE CLEARANCE ENVELOPE (NEC 230.24) |
| |
| [24.5 ft] ============================================= Railroad Tracks |
| [18.0 ft] ============================================= Public Streets |
| [15.0 ft] ============================================= Driveway (>300V) |
| [12.0 ft] ============================================= Driveway (≤300V) |
| [10.0 ft] ============================================= Sidewalk / Grade |
+-----------------------------------------------------------------------------+
Clearances Over Roofs (NEC 230.24(A))
- General Rule: Overhead service conductors must maintain a vertical clearance of not less than 8 feet (2.5 m) above the roof surface. This clearance must be maintained for a distance of at least 3 feet (900 mm) in all directions from the edge of the roof.
- Exception 1 (Sloped Roofs ≥ 4/12 Pitch): Where the voltage between conductors does not exceed 300 volts, and the roof has a slope of not less than 4 inches in 12 inches (4/12 pitch or 33.3% slope), the clearance may be reduced to 3 feet (900 mm).
- Exception 2 (Through-the-Roof Service Mast / Overhang): Where conductors pass over only the overhanging portion of the roof (eaves/overhang), the clearance may be reduced to 18 inches (450 mm) provided:
- Voltage between conductors does not exceed 300 volts.
- Conductors pass over not more than 4 feet (1.2 m) of the roof overhang measured horizontally.
- Conductors terminate at a through-the-roof service raceway (mast) or approved support located within 4 feet of the roof edge.
- A maximum of 6 feet (1.8 m) of conductor length passes over the roof surface.
- Roofs Subject to Pedestrian Traffic: Where roofs, balconies, or decks are accessible to pedestrian traffic, service conductors must maintain a vertical clearance of at least 10 feet (3.0 m) per 230.24(B).
+-----------------------------------------------------------------------------+
| ROOF OVERHANG 18-INCH CLEARANCE EXCEPTION |
| |
| Service Drop Conductor |
| ========================\ |
| \ Min. 18 in. Clearance |
| \ |
| +------------------[Weatherhead] |
| | | | |
| | | | Service Mast |
| | Roof Overhang | | (Within 4 ft of edge) |
| +==================+===+===== |
| | <--- Max 4 ft ---> | |
| | (Max 6 ft wire) | Building Wall |
+-----------------------------------------------------------------------------+
4. Service Mast Structural Support (NEC 230.28)
Where a conduit raceway serves as a through-the-roof service mast to support overhead service-drop or service conductors, it is subjected to severe mechanical cantilever forces from conductor weight, wind drag, and winter ice accumulation.
NEC 230.28 Requirements:
- Mechanical Strength: Service masts must have adequate mechanical strength or be supported by braces or guy wires to withstand the mechanical strain imposed by the service drop.
- Permitted Raceway Types: Only rigid metal conduit (RMC) or intermediate metal conduit (IMC) are typically approved by utility specifications and AHJs as structural service masts. Electrical Metallic Tubing (EMT) and PVC conduit lack structural strength and are strictly prohibited from serving as mast supports.
- Prohibition on Foreign Attachments (NEC 230.28(B)): Service masts shall NOT be used to attach or support other non-electrical systems. Attaching telephone, cable television (CATV), broadband data drops, satellite dishes, or radio antennas to an electrical service mast is a direct code violation.
5. Underground Service Lateral Requirements (NEC 230.30–230.32)
Underground service conductors (service laterals) are installed below grade from the utility distribution transformer or handhole to the building meter base or main service panel.
Permitted Wiring Methods (NEC 230.30):
- Direct-burial underground service-entrance cable (Type USE or USE-2).
- Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC).
- Rigid Polyvinyl Chloride (PVC) Conduit (Schedule 40 or Schedule 80).
- High-Density Polyethylene (HDPE) Conduit and Reinforced Thermosetting Resin Conduit (RTRC).
Minimum Burial Depths (NEC Table 300.5(A)):
- Direct-Burial Cable (Type USE): 24 inches minimum cover.
- Rigid PVC Conduit (Schedule 40/80): 18 inches minimum cover (reduced to 4 inches if encased beneath a 4-inch concrete slab).
- Rigid Metal Conduit (RMC) / Intermediate Metal Conduit (IMC): 6 inches minimum cover.
- Under Public Roads, Streets, & Commercial Driveways: 24 inches minimum cover for all wiring methods.
Protection from Physical Damage & Building Entry Sealing:
- Mechanical Protection (NEC 230.32 & 300.5(D)): Conductors emerging from the ground must be protected in listed raceways (RMC, IMC, or Schedule 80 PVC) starting at least 18 inches below finished grade up to their point of termination or at least 8 feet above grade.
- Raceway Sealing (NEC 230.8 & 300.5(G)): Underground raceways entering a building from below grade must be sealed with a listed duct-seal compound or expanding foam plug at the building entry point to prevent moisture, subterranean water, radon, and sewer/combustible gases from migrating into the electrical equipment.
A residential 120/240V overhead service drop terminates at a through-the-roof rigid metal conduit service mast. The service mast is positioned 3 feet from the roof edge, and the conductors pass over 3 feet of the roof overhang with a total conductor length of 4.5 feet over the roof surface. What is the minimum allowable vertical clearance required between the service conductors and the roof overhang?
An electrician is running 120/240V open service-drop conductors along the exterior side wall of a two-story residence. The route passes near a second-story operable bedroom window. How must these service conductors be positioned relative to the window under NEC 230.9?
An overhead 120/240V single-phase service drop is being installed across a commercial alleyway and truck loading dock driveway behind a retail shopping center in Oklahoma. What is the minimum vertical clearance required above finished grade for these conductors under NEC 230.24(B)?
A telecommunications contractor arrives at a residential job site and plans to secure a cable television (CATV) coaxial drop cable and an internet fiber optic bracket directly to the customer's 2-inch rigid metal conduit electrical service mast. How does the National Electrical Code address this installation?