4.3 Outside Branch Circuits & Feeders

Outside Branch Circuits & Feeders

Electrical distribution frequently extends beyond the exterior walls of a primary building. Whether supplying detached garages, storage warehouses, exterior site lighting poles, agricultural outbuildings, or industrial processing structures, outside branch circuits and feeders operate in harsh environmental conditions subject to physical damage, lightning surges, vehicular contact, and extreme temperature fluctuations. NEC Article 225 governs the wiring methods, physical clearances, and disconnecting means for outside circuits, while NEC Article 250 (specifically 250.32) establishes the mandatory grounding and bonding architecture for supplying separate structures. Mastering these dual articles is critical for Kentucky Journeyman Electrician candidates.

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1. Overhead Conductor Clearances from Ground (NEC 225.18)

When branch circuits or feeders are installed as open overhead spans or multiconductor aerial cables supported by poles or building attachments, they present severe physical contact hazards to pedestrians, maintenance workers, and vehicular traffic. NEC 225.18 establishes mandatory minimum vertical clearances above finished grade based on voltage level and terrain usage:

The Vertical Clearance Hierarchy (NEC 225.18)

1. 10 Feet (3.0 meters):
   Required above finished grade, sidewalks, or from any platform or projection from which conductors might be reached, where the supply voltage does not exceed 150 volts to ground and the area is accessible to pedestrians only.
2. 12 Feet (3.7 meters):
   Required over residential property and driveways, and those commercial areas not subject to truck traffic, where the voltage does not exceed 300 volts to ground (typical 120/240V single-phase or 208Y/120V 3-phase residential/light commercial installations).
3. 15 Feet (4.5 meters):
   Required for those residential and commercial areas not subject to truck traffic where the voltage exceeds 300 volts to ground (e.g., 480Y/277-volt commercial systems supplying parking lot or site lighting).
4. 18 Feet (5.5 meters):
   Required 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 agricultural land, grazing land, orchards, and construction sites), regardless of voltage up to 1000V.
5. 24.5 Feet (7.5 meters):
   Required over track rails of railroads.

Exam Tip: Notice that residential driveways require 12 feet for voltages up to 300V to ground. However, if a driveway serves a commercial business where delivery vans or trucks enter, the clearance immediately jumps from 12 feet to 18 feet under NEC 225.18(4).

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2. Clearances from Buildings & Roofs (NEC 225.19)

Overhead spans passing over or near buildings and structures must maintain strict separation to protect painters, roofers, firefighters, and occupants.

Vertical Clearance Above Roofs (NEC 225.19(A))

Overhead spans 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 not less than 3 feet (900 mm) in all directions from the edge of the roof. Do not confuse this with services: in the 2023 NEC, overhead service conductors need 8 feet 6 inches above a roof (NEC 230.24(A)).

Exceptions to the 8-Foot Roof Clearance Rule:

1. Steep Pitch Roofs (NEC 225.19(A) Exception No. 2):
   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 (a 4/12 pitch or steeper), the vertical clearance is permitted to be reduced to 3 feet (900 mm).
   Rationale: People cannot readily walk or stand on steep-pitch roofs, dramatically reducing accidental contact risk.
2. Roof Overhang Mast Exception (NEC 225.19(A) Exception No. 3):
   Where the voltage between conductors does not exceed 300 volts, the vertical clearance is permitted to be reduced to not less than 18 inches (450 mm) if:
   • The conductors pass over not more than 4 feet (1.2 meters) of the roof overhang/eave, AND
   • The conductors terminate at a through-the-roof raceway mast or approved overhead support located within 4 feet of the roof edge.

                         Overhead Feeder Span
                         ───────────────────┐
                                            │ 18 in. min.
                                   ┌────────┴────────┐
                                   │ Mast Head       │
                                   │                 │
             ┌─────────────────────┤ (Within 4 ft    │
             │  Roof Overhang /    │  of roof edge)  │
             │  Eave (Max 4 ft)    │                 │
    ─────────┴─────────────────────┘                 │ (Through-roof raceway)
    Building Exterior Wall                           │

Clearances from Building Openings (NEC 225.19(D))

Overhead conductors must never be installed where someone leaning out of a window or climbing a fire escape could contact an energized wire:

• Windows, Doors, Fire Escapes: Conductors must maintain a clearance of not less than 3 feet (900 mm) from windows that are designed to be opened, doors, porches, balconies, ladders, stairs, and fire escapes.
• Conductors Above Windows: Conductors installed above the top level of a window are permitted to be closer than 3 feet, because occupants cannot lean upwards out of a window opening to touch conductors running across the wall above the lintel.

Summary Table: Article 225 Outside Clearances

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3. Disconnecting Means for Separate Buildings (NEC 225.31 – 225.39)

When a feeder or branch circuit leaves one building to supply a separate building or detached structure, firefighters, maintenance personnel, and emergency responders must be able to shut off power quickly without running back to the main building.

General Requirement (NEC 225.31)

Under NEC 225.31, a disconnecting means must be provided for all ungrounded conductors that supply or pass through a building or structure.

Disconnect Location (NEC 225.32)

The disconnecting means shall be installed at a readily accessible location either:

1. Outside the building or structure, OR
2. Inside nearest the point of entrance of the supply conductors.

Exception No. 1: For installations under single management, where documented safe switching procedures are established and maintained and the installation is monitored by qualified individuals, the disconnect is permitted to be located elsewhere on the premises.

Emergency Disconnects for Dwellings Supplied by Feeders (NEC 225.41, new in 2023)

The 2023 NEC added a counterpart to the service rule in 230.85. A one- or two-family dwelling supplied by a feeder, such as a house fed from a meter pedestal or from another building, must have an emergency disconnecting means at a readily accessible outdoor location so first responders can shut off power without entering the house.

Maximum Number of Disconnects: The "Six-Handle Rule" (NEC 225.33)

Under NEC 225.33(A), the building disconnect may consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard or panelboard. There must not be more than six operations of the hand to completely disconnect all ungrounded conductors supplying the separate building.

Grouping and Identification (NEC 225.34 & 225.37)

• Grouping (225.34): Where two to six disconnects are installed, they must be grouped together at one location.
• Identification of Multiple Supplies (225.37): Where a separate building is supplied by more than one feeder or branch circuit (or by an on-site solar/generator system in addition to the feeder), a permanent plaque or directory must be installed at each feeder disconnect denoting all other feeders, branch circuits, and power supply sources feeding that structure.

Minimum Ampere Ratings for Building Disconnects (NEC 225.39)

The disconnecting means must have a rating not less than the calculated load to be served, and must satisfy these statutory minimum ratings:

• One-Circuit Installation (225.39(A)): Not less than 15 amperes.
• Two-Circuit Installation (225.39(B)): Not less than 30 amperes.
• One-Family Dwelling (225.39(C)): Not less than 100 amperes, 3-wire for a feeder supplying a single-family dwelling.
• All Other Installations (225.39(D)): Not less than 60 amperes for all other buildings or structures supplied by more than two 2-wire branch circuits.

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4. Grounding & Bonding at Separate Buildings (NEC 250.32)

Grounding at a separate building or detached structure is one of the most critical life-safety requirements in the NEC. Improper wiring creates objectionable circulating neutral currents over the earth and metal water piping, energizes building structural steel, and exposes personnel to fatal touch potentials.

The Mandatory 4-Wire / 5-Wire Supply (NEC 250.32(B)(1))

Under current NEC editions, any feeder supplying a detached building or structure must include an Equipment Grounding Conductor (EGC) run with the phase conductors and neutral conductor (4-wire feeder for 120/240V single-phase; 5-wire feeder for 3-phase wye systems).

Main Service Panel (Building 1)               Subpanel Disconnect (Building 2)
┌─────────────────────────────────┐           ┌─────────────────────────────────┐
│ Phase A ────────────────────────┼───────────┼─► Main Lugs / Phase Bus         │
│ Phase B ────────────────────────┼───────────┼─► Main Lugs / Phase Bus         │
│                                 │           │                                 │
│ [Neutral Bar] ──────────────────┼───────────┼─► [ISOLATED Neutral Bar]        │
│        │ (Main Bonding Jumper)  │           │   (Floating - NO green bonding  │
│ [Ground Bar]                    │           │    screw or strap installed!)   │
│        │                        │           │                                 │
│        └──────── EGC ───────────┼───────────┼─► [Equipment Ground Bar]        │
└─────────────────────────────────┘           │          │                      │
                                              │          │ (Bonded to enclosure)│
                                              └──────────┼──────────────────────┘
                                                         │
                                                         ▼ (Grounding Electrode Conductor)
                                              ┌─────────────────────────────────┐
                                              │ Grounding Electrode System      │
                                              │ (Two ground rods / CEE / etc.)  │
                                              └─────────────────────────────────┘

The Golden Rule: Isolate the Neutral at the Subpanel!

At the separate building disconnect or subpanel:

1. The Grounded (Neutral) Conductor MUST BE ISOLATED from the enclosure!
   The green main bonding jumper screw or bonding strap supplied with the panelboard must NOT be installed. The neutral bar must remain completely isolated from the metallic chassis.
2. The Equipment Grounding Conductor connects to the Equipment Ground Bar:
   The EGC originating from Building 1 bonds directly to the subpanel metal enclosure and equipment grounding terminal bar.
3. Why Isolation is Essential:
   If the neutral is bonded to the enclosure at the second building, neutral return current will divide and flow in parallel across both the neutral wire and the equipment grounding conductor (and through the earth and metal water lines). This continuous objectionable current creates shock hazards, overheats metal raceways, and trips GFCI and AFCI protection devices.

Grounding Electrode System Required at Separate Building (NEC 250.32(A))

A separate building or structure supplied by a feeder must have a grounding electrode system installed in accordance with Part III of Article 250 (such as two driven ground rods spaced at least 6 feet apart per 250.53(A)(3), a concrete-encased electrode / Ufer ground per 250.52(A)(3), or a ground ring). A grounding electrode conductor (GEC) sized per Table 250.66 must connect the grounding electrode system directly to the building disconnect enclosure and equipment ground bar.

The Single Branch Circuit Exception (NEC 250.32(A) Exception)

A grounding electrode system is not required at a separate building or structure where the installation is supplied by only one branch circuit (including a multiwire branch circuit) that includes an equipment grounding conductor for grounding the equipment within the structure (for example, a single 120V 20A circuit or a single 120/240V multiwire circuit feeding a residential garden shed with two light fixtures and a single receptacle outlet).