5.2 Outside Branch Circuits, Feeders & Overhead/Underground Clearances
Key Takeaways
- NEC Article 225 governs outside branch circuits and feeders run on or between buildings, structures, or poles on the premises.
- Overhead span clearances under NEC 225.18 and 230.24 require 10 ft above pedestrian walkways (<=150V to ground), 12 ft over residential driveways (<=300V to ground), 15 ft over driveways exceeding 300V, 18 ft over public roads and commercial truck traffic areas, and 24.5 ft over railroad tracks.
- Open conductors must maintain a minimum clearance of 3 feet from windows that are designed to open, doors, porches, balconies, ladders, stairs, and fire escapes per NEC 225.19(D) and 230.9(A).
- Overhead conductors passing over roofs must maintain 8 ft vertical clearance over flat roofs (225.19(A)), reduced to 3 ft for roof pitches 4:12 or steeper, and reduced to 18 inches for through-the-roof mast installations terminating within 6 ft of the roof edge.
- NEC Table 300.5 establishes mandatory minimum underground cover depths: 24 inches for direct burial cables (Column 1), 6 inches for Rigid Metal Conduit (Column 2), 18 inches for PVC Schedule 40/80 (Column 3), and 12 inches for residential 120V GFCI circuits (Column 4).
5.2 Outside Branch Circuits, Feeders & Overhead/Underground Clearances
Wiring installed outdoors between separate buildings or structures—such as running power from a residential dwelling to a detached shop, or distributing three-phase feeders across a commercial campus—is governed by NEC Article 225 (Outside Branch Circuits and Feeders), Article 230 (Services), and Article 300 (Wiring Methods).
Because outdoor wiring is exposed to severe environmental stressors, mechanical impacts, vehicle traffic, snow accumulation, and structural maintenance hazards, the National Electrical Code enforces strict vertical, horizontal, roof, and underground cover clearances.
1. Outside Branch Circuits & Outbuilding Disconnects (NEC Article 225)
When a feeder or branch circuit supplies a detached building or structure (such as a garage, agricultural barn, pump house, or commercial storage facility), specific disconnect and grounding rules apply.
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| OUTBUILDING FEEDER DISCONNECT REQUIREMENTS |
| |
| [DISCONNECTING MEANS (NEC 225.31 & 225.32)] |
| * A disconnecting means must be provided at the detached structure to |
| disconnect all ungrounded conductors simultaneously. |
| * Location: Readily accessible at the point of entrance (interior or ext).|
| |
| [NUMBER OF SUPPLIES PERMITTED (NEC 225.30)] |
| * General Rule: A building or structure can only be served by ONE feeder |
| or branch circuit from another building. |
| * Exceptions: Fire pumps, emergency systems, multi-voltage requirements, |
| or large capacity installations by special permission. |
| |
| [GROUNDING AT DETACHED BUILDING (NEC 250.32)] |
| * Feeder MUST include an Equipment Grounding Conductor (4-wire feeder). |
| * Detached building must have its OWN grounding electrode system (e.g., |
| ground rods or concrete-encased electrode). |
| * Neutral and Ground busbars MUST REMAIN ISOLATED (Floating neutral). |
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2. Overhead Span Clearances Above Grade (NEC 225.18 & 230.24(B))
Open overhead conductors and multi-conductor cables operating at 600V nominal or less must maintain minimum vertical clearances above finished grade, sidewalks, platforms, and roadways.
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| OVERHEAD SPAN CLEARANCE MATRIX (NEC 225.18) |
| |
| 24.5 FT =========================================> Railroad Tracks |
| |
| 18.0 FT =========================================> Public Roads, Streets,|
| Alleys, Commercial |
| Truck Parking |
| |
| 15.0 FT =========================================> Residential Driveways |
| Exceeding 300V to Gnd |
| |
| 12.0 FT =========================================> Residential Driveways |
| & Commercial Areas |
| Max 300V to Ground |
| |
| 10.0 FT =========================================> Pedestrian Walkways & |
| Platforms Only |
| Max 150V to Ground |
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Detailed Clearance Applications
| Elevation Requirement | Applicable Ground Condition / Surface Area | Voltage Restriction | Code Citation |
|---|---|---|---|
| 10 Feet (3.0 m) | Above finished grade, sidewalks, platforms, or any projection from which conductors might be reached by pedestrians only. | Max 150V to ground | NEC 225.18(1) & 230.24(B)(1) |
| 12 Feet (3.7 m) | Over residential property and driveways, and commercial areas not subject to commercial truck traffic. | Max 300V to ground | NEC 225.18(2) & 230.24(B)(2) |
| 15 Feet (4.5 m) | Over residential property, driveways, and commercial areas where voltage exceeds 300V to ground. | Exceeding 300V to ground | NEC 225.18(3) & 230.24(B)(3) |
| 18 Feet (5.5 m) | Over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and cultivated/grazing agricultural fields. | Max 600V nominal | NEC 225.18(4) & 230.24(B)(4) |
| 24.5 Feet (7.5 m) | Track clearance over railroad tracks. | Max 600V nominal | NEC 225.18(5) & 230.24(B)(5) |
[!IMPORTANT] Exam Distinction Between 12 Ft and 15 Ft Driveway Rules: For a standard 120/240V single-phase residential driveway (where voltage to ground is 120V), the minimum overhead clearance is 12 feet. If the overhead span is a 480V 3-phase delta or 277/480V wye feeder crossing a driveway, the voltage to ground exceeds 300V (or is 277V/480V), requiring a minimum clearance of 15 feet.
3. Clearances from Building Openings & Surfaces (NEC 225.19 & 230.9)
Conductors installed outdoors must be kept away from building openings to protect occupants and maintenance personnel from accidental contact.
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| CLEARANCE FROM BUILDING OPENINGS (NEC 230.9) |
| |
| +------------------------+ |
| | WINDOW (OPENS) | |
| | | |
| +------------------------+ |
| | | |
| <-- 3 FT -->| |<-- 3 FT --> |
| [CLEARANCE] [CLEARANCE] |
| |
| * Minimum 3 Feet (900 mm) clearance required from windows that OPEN, |
| doors, porches, balconies, stairs, ladders, and fire escapes. |
| * EXCEPTION: Conductors run ABOVE the top level of a window are permitted |
| to be less than 3 feet from the window. |
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Key Building Opening Rules
- 3-Foot Envelope: Overhead conductors must maintain at least 3 feet (900 mm) of clearance from windows that open, doors, porches, balconies, stairs, and fire escapes.
- Top of Window Exception: Conductors installed above the upper trim level of a window are not within reach of persons leaning out and therefore are exempt from the 3-foot horizontal clearance.
- Vegetation / Trees (NEC 225.26 & 230.10): Vegetation such as live trees shall never be used for the support of overhead conductor spans or outdoor lighting fixtures (except for temporary decorative holiday lighting under Article 590).
4. Overhead Clearances Over Roofs (NEC 225.19(A) & 230.24(A))
Conductors passing over roofs must maintain adequate clearance based on roof slope and installation geometry.
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| OVERHEAD CLEARANCE OVER ROOFS MATRIX |
| |
| [STANDARD FLAT ROOF / LOW-PITCH ROOF (< 4:12 PITCH)] |
| * Minimum Vertical Clearance = 8.0 Feet (2.4 m) |
| * Must be maintained for at least 3.0 Feet in all directions from edge. |
| |
| [STEEP-PITCHED ROOF (>= 4:12 PITCH - 4" RISE PER 12" RUN)] |
| * Minimum Vertical Clearance = 3.0 Feet (900 mm) |
| * Applicable where voltage between conductors does not exceed 300V. |
| |
| [OVERHANG / SERVICE MAST PENETRATION EXCEPTION (230.24(A) Ex. 3)] |
| * Minimum Vertical Clearance = 18 Inches (450 mm) |
| * Permitted ONLY IF conductors pass over not more than 4 Feet of roof |
| overhang AND terminate at a through-the-roof service mast located |
| within 6 Feet of the roof edge. |
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Comparison Table: Roof Clearances
| Roof Profile & Condition | Minimum Vertical Clearance | Conditions & Restrictions |
|---|---|---|
| Flat Roofs & Low Slopes (< 4:12) | 8.0 Feet (2.4 m) | Standard rule; pedestrian walkable roofs subject to 8 ft clearance |
| Roofs with Pitch >= 4:12 | 3.0 Feet (900 mm) | Voltage between conductors must not exceed 300V; steep slope prevents walking |
| Mast Penetration over Overhang | 18 Inches (450 mm) | Max 4 ft conductor span over overhang; mast within 6 ft of roof edge |
| Conductively Attached Overhang | 12 Inches (300 mm) | Conductors terminate at side of building; passes over eaves only |
5. Underground Minimum Cover Depths (NEC Table 300.5)
Underground wiring methods must be buried deep enough to prevent mechanical damage from excavation, vehicular traffic, and landscaping equipment. Cover is defined as the shortest distance measured between the top surface of any direct-buried conductor, cable, or raceway and the finished grade or concrete surface.
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| NEC TABLE 300.5 BURIAL DEPTH OVERVIEW |
| |
| Grade Level ----------------------------------------------------------- |
| | |
| |-- 6" ---> Column 2: Rigid Metal Conduit (RMC) / IMC |
| | |
| |-- 12" ---> Column 4: Residential 120V GFCI Branch Circuit (<=20A) |
| | |
| |-- 18" ---> Column 3: Nonmetallic Raceway (PVC Sch 40/80, RTRC) |
| | |
| |-- 24" ---> Column 1: Direct Burial Cable (Type UF, USE) |
+-----------------------------------------------------------------------------+
Master Underground Cover Requirements (Table 300.5)
| Location / Surface Condition | Col. 1: Direct Burial (UF, USE) | Col. 2: Rigid Metal Conduit (RMC/IMC) | Col. 3: Rigid PVC (Sch 40/80) | Col. 4: Residential 120V GFCI (<= 20A) | Col. 5: In/Under 4" Concrete Slab |
|---|---|---|---|---|---|
| All locations not specified below (Standard Earth / Soil) | 24 in. | 6 in. | 18 in. | 12 in. | 18 in. |
| In trench below 2-in. thick concrete or equivalent pad | 18 in. | 6 in. | 12 in. | 6 in. | 6 in. |
| Under minimum 4-in. thick concrete slab (no vehicular traffic) | 18 in. | 4 in. (or 0" in slab) | 4 in. (or 0" in slab) | 6 in. | 4 in. |
| Under commercial driveways, parking lots, & alleys | 24 in. | 24 in. | 24 in. | 24 in. | 24 in. |
| Under streets, highways, roads, and public thoroughfares | 24 in. | 24 in. | 24 in. | 24 in. | 24 in. |
| Airport runways and adjacent vehicular areas | 18 in. | 18 in. | 18 in. | 18 in. | 18 in. |
[!NOTE] Critical Underground Rules to Memorize:
- Under Commercial Driveways & Streets: ALL wiring methods in Table 300.5—whether direct burial, PVC, or RMC—require a minimum of 24 inches of cover when subjected to vehicular traffic.
- Warning Ribbon (NEC 300.5(D)(3)): Underground direct-buried service lateral conductors not encased in concrete must have a continuous, durable warning ribbon placed in the trench at least 12 inches (300 mm) above the conductors.
- Raceway Seals (NEC 300.5(G)): Conduits that enter an interior conditioned building from underground exterior locations must be sealed with an approved duct seal compound to prevent moisture condensation and hazardous gas migration.
What is the minimum overhead vertical clearance required for 120/240V open service-drop conductors passing over a residential property driveway?
An overhead feeder conductor passes over a roof with a pitch of 5:12 (greater than 4:12). If the voltage between conductors is 240 volts, what is the minimum vertical clearance required above the roof surface?
Under NEC Table 300.5, what is the minimum burial cover depth required for Schedule 40 PVC conduit containing a 240V subpanel feeder installed under a residential lawn not subject to vehicular traffic?