4.4 Outside Branch Circuits and Feeders (Article 225) & Feeder Sizing (Article 215)

Outside Branch Circuits & Feeders (NEC Articles 215 & 225)

Feeders are the primary arteries of an electrical distribution network, carrying bulk power from the service equipment or separately derived source to downstream subpanels and branch circuit overcurrent devices. When feeders and branch circuits extend outdoors to supply remote buildings, security lighting, or detached workshops, they encounter extreme physical stresses including wind loads, ice accumulation, thermal expansion, vehicle traffic, and environmental deterioration. NEC Article 215 (Feeders) and Article 225 (Outside Branch Circuits and Feeders) establish the engineering standards required to ensure electrical safety and long-term reliability.

---

1. Feeder Conductor Sizing & Overcurrent Protection (NEC Article 215)

A feeder consists of 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.

+-----------------------------------------------------------------------------+
|                   FEEDER SIZING FORMULA (NEC 215.2(A)(1))                   |
|                                                                             |
|   MINIMUM FEEDER AMPACITY:                                                  |
|   Ampacity >= (100% of Non-Continuous Load) + (125% of Continuous Load)    |
|                                                                             |
|   FEEDER OVERCURRENT PROTECTION (NEC 215.3):                                |
|   OCPD Rating >= (100% of Non-Continuous Load) + (125% of Continuous Load)  |
+-----------------------------------------------------------------------------+

Feeder Neutral Sizing (NEC 220.61):

• The feeder grounded conductor (neutral) must be sized to carry the maximum unbalanced load between the neutral and any one ungrounded conductor.
• Demand Reductions Permitted: In dwellings, a 70% demand factor is applied to electric range and electric dryer neutral loads per NEC 220.61(B)(1).
• Harmonic Non-Linear Loads (220.61(C)(2)): No neutral reduction is permitted for that portion of the load consisting of non-linear loads (LED lighting drivers, computers, VFDs), as third-order triplen harmonics add constructively in the neutral conductor.

---

2. Feeder Voltage Drop Calculations (NEC 215.2(A) Inf. Note 2)

While the NEC does not make voltage drop mandatory for general branch circuits and feeders in the base code text, Informational Note 2 to NEC 215.2(A) and Informational Note 4 to NEC 210.19(A) specify critical engineering design standards:

• Maximum Feeder Voltage Drop: Not to exceed 3% under full design load.
• Maximum Combined Feeder + Branch Circuit Voltage Drop: Not to exceed 5% total to the farthest outlet.

+-----------------------------------------------------------------------------+
|                   VOLTAGE DROP CALCULATION FORMULAS                         |
|                                                                             |
|   SINGLE-PHASE CIRCUITS:                     THREE-PHASE CIRCUITS:          |
|            2 x K x I x L                              1.732 x K x I x L     |
|   VD  =  -----------------                   VD  =  --------------------    |
|                  CM                                          CM             |
|                                                                             |
|   Where:                                                                    |
|   - K  = Conductor Resistivity (Copper = 12.9 ohms-cmil/ft; Al = 21.2)      |
|   - I  = Circuit Load Current in Amperes                                    |
|   - L  = One-Way Length of Feeder in Feet                                   |
|   - CM = Conductor Cross-Sectional Area in Circular Mils (Chapter 9 Table 8)|
+-----------------------------------------------------------------------------+

Worked Voltage Drop Example:

A 240V single-phase commercial feeder carries an 80A continuous load a one-way distance of 180 feet using #3 AWG Copper conductors ($CM = 52,620$ from Chapter 9, Table 8).

$VD = \frac{2 \times 12.9 \times 80 \times 180}{52,620} = \frac{371,520}{52,620} = \mathbf{7.06\text{ Volts}}$ $\%VD = \left(\frac{7.06\text{ V}}{240\text{ V}}\right) \times 100 = \mathbf{2.94\%}$

Because $2.94% < 3.0%$, the feeder complies with the 3% maximum recommended voltage drop.

---

3. Outside Overhead Conductors & Clearances (NEC Article 225)

Overhead conductors running between buildings, poles, or structures must maintain mandatory physical clearances to prevent contact with vehicles, pedestrians, and machinery:

+-----------------------------------------------------------------------------+
|              OVERHEAD CLEARANCES ABOVE FINISHED GRADE (NEC 225.18)          |
|                                                                             |
|   [24.5 FEET] --- Over Track Rails of Railroads                             |
|                                                                             |
|   [18.0 FEET] --- Over Public Streets, Roads, Alleys, Parking Areas Subject  |
|                   to Truck Traffic, & Commercial Driveways                  |
|                                                                             |
|   [15.0 FEET] --- Over Residential Driveways & Commercial Non-Truck Areas    |
|                   (Circuits > 300V to Ground)                               |
|                                                                             |
|   [12.0 FEET] --- Over Residential Property, Driveways & Commercial         |
|                   Non-Truck Areas (Circuits <= 300V to Ground)              |
|                                                                             |
|   [10.0 FEET] --- Above Finished Grade, Sidewalks, or Pedestrian-Only       |
|                   Platforms (Circuits <= 150V to Ground)                    |
+-----------------------------------------------------------------------------+

Clearances from Buildings (NEC 225.19):

• Roofs (225.19(A)): Overhead conductors must maintain a vertical clearance of not less than 8.0 feet (2.44 m) above the roof surface. For steep-pitch roofs (slope not less than 4 inches in 12 inches), clearance may be reduced to 3.0 feet (914 mm).
• Roof Overhang Exception: Where a feeder passes over not more than 4 feet of roof overhang and terminates in a through-the-roof raceway/mast, clearance may be reduced to 18 inches (450 mm).
• Windows, Doors, & Fire Escapes (225.19(D)): Conductors shall have a clearance of at least 3.0 feet (900 mm) from windows that are designed to open, doors, porches, balconies, or fire escapes.

Overhead Spans & Conductor Minimum Sizes (NEC 225.6):

• Spans Up to 50 Feet: Minimum #10 AWG Copper or #8 AWG Aluminum.
• Spans Over 50 Feet: Minimum #8 AWG Copper or #6 AWG Aluminum.
• Festoon Lighting Spans (225.6(B)): Overhead conductors for festoon lighting must be at least #12 AWG and must be supported by a messenger wire with listed strain insulators if the span exceeds 40 feet.

Support on Vegetation Prohibited (NEC 225.26):

"Vegetation such as trees shall not be used for support of overhead conductor spans."

Electricians must never attach overhead spans, messenger wires, or outside feeder cables to live trees. Trees sway in storms, grow continuously, and shed limbs, creating extreme shock, fire, and wire-drop hazards.

---

4. Disconnecting Means for Separate Buildings (NEC 225.31–225.39 & 250.32)

When a feeder or branch circuit supplies a detached building or structure (such as a detached garage, commercial warehouse, or agricultural barn), NEC 225 mandates a local disconnecting means:

+-----------------------------------------------------------------------------+
|               DETACHED STRUCTURE DISCONNECT & GROUNDING (NEC 225 & 250.32)  |
|                                                                             |
|   [MAIN BUILDING PANEL]                      [DETACHED BUILDING SUBPANEL]   |
|   Feeder Hot 1 ----------------------------> [ Main Disconnect / Breaker ]  |
|   Feeder Hot 2 ----------------------------> [ Max 6 Disconnects (225.33)]  |
|   Feeder Neutral --------------------------> [ ISOLATED NEUTRAL BUSBAR ]    |
|   Equipment Ground (EGC) ------------------> [ EQUIPMENT GROUND BUSBAR ]    |
|                                                             |               |
|                                                [ Grounding Electrode System]|
|                                                (Ground Rods / Concrete Ufer)|
+-----------------------------------------------------------------------------+

Detached Building Disconnect Rules:

1. Location (NEC 225.32): Installed at a readily accessible location either outside the building/structure or inside nearest the point of entrance of the supply conductors.
2. Maximum Number of Disconnects (NEC 225.33 - "Rule of Six"): The disconnecting means shall consist of not more than six switches or six circuit breakers mounted in a single enclosure or group of separate enclosures.
3. Grouping (NEC 225.34): The two to six disconnects must be grouped together.
4. Minimum Disconnect Ratings (NEC 225.39):
   • Single-circuit installation: 15 Amperes minimum.
   • Two-circuit installation: 30 Amperes minimum.
   • Single-family dwelling: 100 Amperes 3-wire minimum.
   • All other installations (commercial/workshops): 60 Amperes minimum.
5. Grounding & Neutral Isolation (NEC 250.32):
   • An Equipment Grounding Conductor (EGC) must be run with the feeder.
   • A Grounding Electrode System (ground rods, ground ring, or concrete-encased electrode) must be established at the detached building.
   • The Neutral and EGC must remain ISOLATED at the subpanel. The main bonding jumper is NOT installed; bonding neutral to ground at a subpanel creates dangerous parallel ground return paths.