2.4 Feeders & Outside Branch Circuits

2.4 Feeders & Outside Branch Circuits

Feeders serve as the primary arterial distribution network in an electrical installation, transporting bulk electrical energy from the service equipment or separately derived source to individual branch-circuit panelboards. Because feeders carry substantial currents, errors in conductor sizing, overcurrent protection coordination, or neutral load calculations can lead to catastrophic system failures. Additionally, outside branch circuits and feeders that traverse open air to supply detached buildings must withstand environmental stresses and satisfy rigorous overhead clearance and disconnecting mandates under NEC Articles 215, 225, and 250.

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1. Feeder Definitions & Fundamental Sizing Rules (NEC Article 215)

Definition of a Feeder (Article 100)

A Feeder encompasses all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

  [ Service Equipment ] ──────► [ FEEDER CONDUCTORS ] ──────► [ Subpanel / Panelboard ]
     (Main Breaker)                (NEC Article 215)              (Final OCPDs)
                                                                        │
                                                              [ Branch Circuits ]
                                                               (NEC Article 210)

Feeder Conductor Sizing Formula (NEC 215.2(A)(1))

Feeder conductors must have an ampacity sufficient to supply the load calculated in accordance with Parts III, IV, and V of Article 220. The fundamental rule for sizing ungrounded feeder conductors dictates that the minimum conductor ampacity before derating must be not less than 125% of the continuous load plus 100% of the non-continuous load:

$\text{Minimum Feeder Ampacity} \ge (1.25 \times \text{Continuous Load}) + (1.00 \times \text{Non-Continuous Load})$

• Conductor Ampacity After Derating: If ambient temperature exceeds 30°C (86°F) or more than three current-carrying conductors are routed in the same raceway, the conductor ampacity after applying correction and adjustment factors must still be equal to or greater than the actual load to be served (continuous load + non-continuous load):

$\text{Derated Ampacity} \ge \text{Continuous Load} + \text{Non-Continuous Load}$

• 100%-Rated Overcurrent Devices: Where the feeder overcurrent protective assembly is listed for operation at 100% of its rating, the 125% multiplier for continuous loads is waived (NEC 215.2(A)(1) Exception).

Feeder Overcurrent Protection (NEC 215.3)

Feeders must be protected against overcurrent in accordance with Part I of Article 240. The overcurrent protective device rating must be not less than the non-continuous load plus 125% of the continuous load. Standard ampere ratings established in NEC 240.6 apply (e.g., 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A...).

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2. Feeder Neutral Conductor Sizing (NEC 220.61)

The feeder grounded (neutral) conductor does not necessarily need to be the same size as the ungrounded phase conductors. Under NEC 220.61, the feeder neutral is sized to carry the maximum unbalanced load:

The Maximum Unbalanced Load

The maximum unbalanced load is the maximum net load connected between the neutral and any one ungrounded conductor. For single-phase 120/240V systems, 240V line-to-line loads (such as central air conditioners, electric water heaters, and heat pumps) do not draw current on the neutral conductor and are completely excluded from the neutral calculation.

Neutral Reductions and Derating (NEC 220.61(B))

• Standard Demand Factor: For feeder supplying general loads, an additional demand factor of 70% (0.70) is permitted for that portion of the unbalanced neutral load that exceeds 200 amperes:

$\text{Neutral Load Calculation} = 200\text{ A} + [0.70 \times (\text{Unbalanced Load} - 200\text{ A})]$

• Example: If a commercial unbalanced neutral load calculates to 350 amperes: $\text{Sized Neutral} = 200\text{ A} + [0.70 \times (350\text{ A} - 200\text{ A})] = 200\text{ A} + (0.70 \times 150\text{ A}) = 200\text{ A} + 105\text{ A} = \mathbf{305\text{ Amperes}}$

Prohibited Neutral Reductions (NEC 220.61(C))

The 70% neutral reduction is strictly prohibited for:

1. Any portion of the load consisting of non-linear loads supplied from a 3-phase, 4-wire, wye-connected system.
   • Why this rule exists: Non-linear loads (LED drivers, solid-state computers, variable frequency drives) generate odd triplen harmonic currents (3rd, 9th, 15th harmonics). While fundamental 60 Hz currents cancel in the neutral of a three-phase system, triplen harmonics add arithmetically in the neutral conductor. In high-density data centers or commercial office buildings, the neutral current can actually exceed 130% to 150% of the phase current!
2. Circuits supplying electric discharge lighting (fluorescent, HID).

Minimum Feeder Neutral Size (NEC 215.2(A)(2))

Regardless of how small the calculated unbalanced load is, the feeder grounded conductor must never be sized smaller than the required grounding electrode conductor or equipment bonding jumper specified in NEC Table 250.102(C)(1) based on the size of the ungrounded feeder conductors.

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3. High-Leg Identification & Bus Arrangement (NEC 110.15 & 408.3(E))

In older commercial and light industrial installations, utilities frequently provide a 3-phase, 4-wire delta-connected system (often called a "red-leg", "wild-leg", or "high-leg" delta). This system is created using three single-phase transformers connected in a delta configuration, with the center-tap of one transformer winding connected to ground to provide 120V for lighting.

                                    Phase B ("High Leg" / 208V to Ground)
                                                 ▲
                                                / \
                                               /   \
                                              /     \
                                   240V      /       \     240V
                                            /         \
                                           /           \
                                          /             \
                                         /   Center-Tap  \
                                        ▼───────●─────────▼
             Phase A (120V to Ground)        Neutral        Phase C (120V to Ground)
                                         |◄───120V───►|◄───120V───►|
                                         |◄──────────240V─────────►|

Voltage Relationships in a 120/240V High-Leg Delta

• Phase A to Phase B: 240 Volts
• Phase B to Phase C: 240 Volts
• Phase C to Phase A: 240 Volts
• Phase A to Neutral (Grounded Conductor): 120 Volts
• Phase C to Neutral (Grounded Conductor): 120 Volts
• Phase B to Neutral (Grounded Conductor):

$V_{B-\text{Neutral}} = 120\text{ V} \times \sqrt{3} = 120\text{ V} \times 1.732 \approx \mathbf{208\text{ Volts}}$

Mandatory Code Requirements for High-Leg Delta Systems

1. High-Leg Identification (NEC 110.15): On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, the conductor having the higher phase voltage to ground must be durably identified by an outer finish that is orange in color, or by tagging or other effective means, at each point where a connection is made if the grounded conductor is also present.
2. Panelboard Bus Arrangement (NEC 408.3(E)(1)): In switchboards and panelboards, the high-leg conductor must connect to the "B" phase (center bus). This ensures standard left-to-right (A-B-C) or top-to-bottom phase arrangement.
   • Exception: Meter socket enclosures provided by utilities often mandate the high leg on the right-hand (C-phase) position due to utility metering standards.
3. Catastrophic Field Hazard: Under no circumstances can a 120V single-phase load be connected between Phase B and the neutral. Doing so subjects the 120V appliance to 208 volts, resulting in immediate catastrophic burnout, insulation melting, and potential fire.

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4. Outside Branch Circuits & Feeders (NEC Article 225)

When electrical conductors run outdoors between separate buildings or structures (such as from a primary dwelling to a detached workshop, garage, or barn), they are governed by NEC Article 225.

Overhead Conductor Clearances Above Finished Grade (NEC 225.18)

Overhead spans of open conductors and multi-conductor cables not exceeding 600 volts must comply with the following vertical clearances above finished grade:

 ┌─────────────────────────────────────────────────────────────────────────────────┐
 │ 10 FEET (3.0 m)  ── Above finished grade, sidewalks, platforms accessible only  │
 │                     to pedestrians; circuits <= 150V to ground                  │
 ├─────────────────────────────────────────────────────────────────────────────────┤
 │ 12 FEET (3.7 m)  ── Over residential property and driveways; commercial areas   │
 │                     not subject to truck traffic; circuits <= 300V to ground     │
 ├─────────────────────────────────────────────────────────────────────────────────┤
 │ 15 FEET (4.5 m)  ── Over residential driveways/property where circuit voltage    │
 │                     exceeds 300V to ground                                      │
 ├─────────────────────────────────────────────────────────────────────────────────┤
 │ 18 FEET (5.5 m)  ── Over public streets, roads, alleys, commercial parking      │
 │                     lots with truck traffic, and vehicle-traversed lands        │
 ├─────────────────────────────────────────────────────────────────────────────────┤
 │ 24.5 FEET (7.5 m)── Over tracks of railroads                                    │
 └─────────────────────────────────────────────────────────────────────────────────┘

Exam Trap: Pay close attention to the voltage-to-ground specification. For a residential driveway with standard 120/240V single-phase power, the voltage to ground is 120 volts (which is less than 300V), so the mandatory clearance is 12 feet. If the installation were a 480V 3-phase commercial system crossing an area not subject to truck traffic, the voltage to ground is 277V (still under 300V, so 12 ft applies); if it were a 480V delta ungrounded system, the voltage to ground exceeds 300V, bumping the requirement to 15 feet.

Clearances from Building Openings (NEC 225.19(D))

Overhead outside conductors must maintain a clearance of not less than 3 feet (900 mm) from:

• Windows that are designed to be opened
• Doors
• Porches, balconies, and fire escapes
• Stairs and ladders

Critical Exception: Conductors run above the top level of a window are permitted to be less than 3 feet from the window, because persons leaning out of the window cannot reach or contact conductors positioned above the top window frame.

Disconnecting Means for Detached Buildings (NEC 225.31 & 225.32)

Where a feeder or branch circuit supplies a detached building or structure:

1. Disconnect Required (225.31): A disconnecting means must be provided to disconnect all ungrounded conductors supplying the structure.
2. Location (225.32): The disconnect must be readily accessible and located either outside the building/structure or inside nearest the point of entrance of the conductors.
3. Maximum Number of Disconnects (225.33 - "Rule of Six"): The disconnecting means for each supply shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, a group of separate enclosures, or in or on a switchboard/panelboard.
4. Grouping (225.34): The disconnects must be grouped together.

Grounding at a Separate Building (NEC 250.32)

For a new feeder to a detached building or structure:

• Run an equipment grounding conductor (EGC) with the feeder conductors.
• Establish a grounding electrode system at the detached structure and bond it as required; the electrode does not replace the feeder EGC.
• Keep the feeder grounded (neutral) conductor isolated from equipment enclosures and the equipment-grounding bus on the load side of the service or separately derived system bond.

NEC 250.32(B) contains a narrow existing-installation exception that can permit the grounded conductor to perform the bonding function only when all listed conditions are met, including absence of an EGC and of continuous metallic paths bonded to the grounding systems. Do not apply that exception to new work.

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5. Comprehensive Worked Feeder Sizing Problem

Problem: A commercial subpanel supplies the following continuous and non-continuous balanced 3-phase, 208Y/120V loads:

• Continuous commercial lighting load: 48 amperes
• Non-continuous office receptacle and appliance load: 62 amperes

Conductors are copper with THHN insulation routed in electrical metallic tubing (EMT) through an ambient temperature of 30°C. All termination lugs are rated for 75°C. Calculate:

1. Minimum feeder conductor ampacity.
2. Conductor size selection from NEC Table 310.16.
3. Standard feeder overcurrent protective device rating.

Step 1: Calculate Minimum Feeder Conductor Ampacity (NEC 215.2(A)(1))

$\text{Min Ampacity} = (1.25 \times \text{Continuous Load}) + (1.00 \times \text{Non-Continuous Load})$ $\text{Min Ampacity} = (1.25 \times 48\text{ A}) + (1.00 \times 62\text{ A}) = 60\text{ A} + 62\text{ A} = \mathbf{122\text{ Amperes}}$

Step 2: Select Feeder Conductor Size (NEC Table 310.16)

Because termination lugs are rated at 75°C, conductor ampacity must be verified using the 75°C column of Table 310.16:

• 1 AWG Copper: 130 Amperes (75°C column).
• (Note: 2 AWG Copper is rated for 115A, which is insufficient to carry 122A).
• Therefore, select 1 AWG THHN Copper (rated for 130A at 75°C).

Step 3: Determine Feeder Overcurrent Protective Device (NEC 215.3 & 240.4)

1. The OCPD must be rated at not less than 122 amperes ($48 \times 1.25 + 62$).
2. The conductor ampacity is 130 amperes.
3. Under NEC 240.4(B) (the "Next Standard Size Up" rule), where conductor ampacity does not correspond to a standard breaker rating, the next standard size higher is permitted, provided the conductor does not feed multi-outlet branch circuits and the rating does not exceed 800A.
4. Standard breaker sizes per NEC 240.6 include 100A, 110A, 125A, 150A...
5. A 125-ampere circuit breaker satisfies both requirements: it is greater than the 122A continuous calculation and does not exceed the 130A conductor rating.

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6. Common Massachusetts Exam Traps: Feeders & Outside Circuits

• Trap 1: The High-Leg Orange Wire Rule. Questions frequently ask: "In a 120/240V 3-phase 4-wire delta panelboard, what color identification is required on the phase conductor exhibiting 208V to ground?" The only correct answer is Orange (or other effective tagging/marking). Connecting this wire to Phase A instead of Phase B inside the panel is a direct violation of NEC 408.3(E)(1).
• Trap 2: Floating the Subpanel Neutral. Exam questions often present a scenario where an apprentice bonds the neutral bar to the enclosure in a subpanel or detached garage panelboard. This creates a parallel path for neutral current over the equipment grounding conductor, energizing metal conduits and creating an immediate shock hazard. For a new feeder, the neutral bar must remain isolated from the enclosure and equipment-grounding bar; evaluate only a clearly stated existing-installation exception under 250.32(B).
• Trap 3: Clearance Over Driveways vs. Alleys. Remember the difference: residential driveways require 12 feet, whereas commercial alleys, roads, and truck parking lots require 18 feet.