Electrical Feeders

Learning Objectives

Upon completing this chapter, the candidate will be able to:

4.Define a feeder and distinguish it from a branch circuit and a service conductor.

5.Apply the minimum sizing rules for feeders supplying continuous and noncontinuous loads, including the 125% factor.

6.Calculate feeder conductor ampacity and minimum neutral size for 3-phase, 4-wire systems with nonlinear loads.

7.Apply the feeder tap rules (10-foot, 25-foot, and 100-foot) for commercial and industrial installations.

8.Size feeders for motor loads, multiple motors, and motor-plus-other-load combinations per Article 430.

9.Identify the requirements for separately derived systems (transformers, generators) regarding grounding, bonding, and overcurrent protection.

10.Navigate the NEC efficiently to locate feeder-related requirements during the open-book exam.

1.1 Feeder Definition and Scope

A feeder is defined in Article 100 as 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. In practical terms, the feeder begins at the service disconnect (or transformer secondary, or generator output) and ends at the panelboard or disconnect that supplies the branch circuits.

Key Distinctions for the Master:

Service conductors run from the utility point of connection to the service disconnecting means.

Feeders run from the service disconnect to the branch-circuit panelboard.

Branch circuits run from the final overcurrent device to the outlets or utilization equipment.

The master electrician must identify the exact transition point on a one-line diagram. A common field error is mislabeling the conductors from a meter to a main breaker as a feeder—they are service conductors. Conversely, conductors from a main breaker to a sub-panel are feeders.

1.2 Sizing Feeders: The 125% Rule and Continuous Loads

The foundational rule for feeder sizing is found in NEC 215.2(A)(1) . A feeder must have an ampacity of not less than the sum of the noncontinuous loads plus 125% of the continuous loads. The 125% factor accounts for heat buildup over prolonged operation (3 hours or more).

Calculation Formula:

Feeder Ampacity ≥ (Noncontinuous Load) + (1.25 × Continuous Load)

Critical Thresholds:

Continuous Load: A load where the maximum current is expected to continue for 3 hours or more (Article 100).

Standard Sizes: Once the calculated ampacity is determined, the conductor must be selected from the standard sizes in Table 310.4 (formerly 310.15(B)(16) in older codes; the 2026 NEC reorganized conductor tables). The next standard size up must be used unless the next standard overcurrent device rating is permitted by the "next-size-up" rule in 240.4(B) .

Exam Trap:

Many candidates calculate the load correctly but forget to apply the ambient temperature correction factor and the number-of-conductors adjustment factor from Table 310.15(B)(1) and Table 310.15(C)(1) . The master must check the termination temperature rating (usually 75°C for standard equipment) versus the conductor insulation rating (90°C for XHHW-2 or THHN). The ampacity must be based on the lower of the termination rating or the conductor rating after adjustments.

Example:

A feeder supplies a 200A continuous load and a 50A noncontinuous load.

Calculated load = 50A + (1.25 × 200A) = 300A.

You select a conductor with an ampacity of at least 300A at the 75°C termination column. If you use 90°C conductors, you may use the 90°C column for derating purposes only if the final ampacity is still ≥ 300A at the 75°C termination rating.

1.3 Minimum Feeder Neutral Size and Nonlinear Loads

NEC 215.2(A)(2) addresses the grounded (neutral) conductor. The neutral must be sized to carry the maximum unbalanced load, but it cannot be smaller than the required grounding electrode conductor (Table 250.66) in certain configurations.

Nonlinear Loads (Harmonics):

For feeders supplying data centers, LED lighting, or variable frequency drives (VFDs), the neutral carries triplen harmonics (3rd, 9th, 15th). These currents are additive on the neutral. NEC 310.15(E) requires that the neutral be counted as a current-carrying conductor when the load is nonlinear.

Master-Level Rule:

If more than 50% of the load is nonlinear, the neutral must be considered a current-carrying conductor. This triggers a derating factor of 80% (4-6 current-carrying conductors) from Table 310.15(C)(1) . In severe cases, the neutral may need to be full-size or even oversized relative to the phase conductors.

Exam Trap:

For a 3-phase, 4-wire wye system supplying a linear balanced load, the neutral carries little to no current. However, for a nonlinear load, the neutral can carry up to 173% of the phase current. The master must specify a full-size neutral and apply the derating factor.

1.4 Feeder Taps: 10-Foot, 25-Foot, and 100-Foot Rules

Article 240.21(B) permits feeder taps—conductors connected to a feeder that supply a panelboard or equipment without individual overcurrent protection at the tap point. The tap conductor must terminate in an overcurrent device rated for the tap conductor ampacity.

10-Foot Tap Rule [240.21(B)(1)]:

Tap conductors must have an ampacity not less than the combined computed loads of the circuits supplied.

The tap must not exceed 10 feet in length.

The tap must be enclosed in a raceway or cable.

The tap must not extend beyond the panelboard or switchboard it supplies.

The overcurrent device at the termination must be rated for the tap conductor ampacity.

25-Foot Tap Rule [240.21(B)(2)]:

Applies to industrial installations only.

Tap conductors must have an ampacity of not less than one-third of the rating of the overcurrent device protecting the feeder.

The tap must not exceed 25 feet in length.

The tap must be enclosed in a raceway.

The tap must terminate in a single circuit breaker or set of fuses that limits the load to the tap conductor ampacity.

100-Foot Tap Rule [240.21(B)(4)]:

Applies to high-bay manufacturing buildings (over 35 feet high) with limited access.

Tap conductors must have an ampacity of not less than one-third of the feeder overcurrent device rating.

The tap must not exceed 100 feet in length.

The tap must be supported and enclosed in a raceway.

The tap must terminate in an overcurrent device.

Inspection Point:

The master must verify the physical length of the tap. A 25-foot tap that is 26 feet long is a code violation, regardless of load. Use a tape measure on site; do not rely on the drawing.

1.5 Motor Feeder Sizing (Article 430)

Feeder sizing for motors is distinct from general lighting feeders. The feeder must supply multiple motors and other loads.

NEC 430.24 states that conductors supplying several motors, or a motor(s) and other load(s), must have an ampacity of not less than the sum of:

71.The largest motor's full-load current (FLC) × 1.25.

72.The sum of the FLC of all other motors.

73.The sum of the non-motor loads, calculated per Article 220.

Important Distinction:

Use the FLC from Tables 430.247 through 430.250, not the nameplate full-load amperes (FLA). The tables provide conservative values for standard motor types and voltages.

Motor Feeder Overcurrent Protection [430.62]:

The feeder overcurrent device must be rated the sum of:

78.The largest branch-circuit protective device rating (from 430.52).

79.The sum of the FLC of the other motors.

Exam Trap:

Do not add 125% to the largest motor's protective device rating. The 125% applies to the conductor sizing (430.24). For the overcurrent device, you use the actual branch-circuit protection rating (which may already be 250% of FLC for an inverse-time breaker) plus the FLC of the other motors.

Example:

Feeder supplies Motor A (FLC 20A, branch breaker 50A) and Motor B (FLC 15A, branch breaker 30A).

Conductor ampacity = (20A × 1.25) + 15A = 40A.

Feeder overcurrent device = 50A + 15A = 65A → next standard size up is 70A.

1.6 Separately Derived Systems (Transformers and Generators)

A separately derived system (SDS) is a source of power with no direct electrical connection to the supply conductors, other than through the bonding and grounding path. Examples include transformers and standby generators with a transfer switch.

Grounding and Bonding [250.30]:

The master must ensure the SDS has a system bonding jumper at the source (transformer secondary or generator) that connects the grounded conductor, the equipment grounding conductor, and the grounding electrode conductor.

Key Rules:

The grounded conductor (neutral) must be bonded to the equipment grounding conductor at only one point—the source of the SDS.

A grounding electrode must be connected to the SDS source. The electrode must be as near as practicable to the source, and it can be a building steel, concrete-encased electrode, or ground rod.

The size of the grounding electrode conductor is per Table 250.66, based on the largest ungrounded conductor of the SDS.

Feeder from an SDS:

The conductors from the transformer secondary to the first disconnecting means are considered feeders. The overcurrent protection must be on the secondary side (or a primary protection device sized per 240.21(C) for transformer secondary protection).

Inspection Point:

Verify that the neutral is not bonded at the downstream panelboard. A double bond creates a parallel neutral path, which violates 250.6 and can cause dangerous circulating currents.

1.7 Commercial and Industrial Feeder Calculations

For commercial installations, the master must apply Article 220, Part III for feeder calculations. The standard method (220.40) requires calculating the load per 220.10 and applying demand factors from Table 220.42 (for lighting) and Table 220.44 (for receptacle loads).

Key Demand Factors:

General lighting: 125% of the first 8,000 VA, then 100% of the remainder (Table 220.42).

Receptacles: The first 10 kVA at 100%, the remainder at 50% (Table 220.44).

Kitchen Equipment [220.56]:

For commercial kitchens, the feeder can be sized using the nameplate ratings, but a demand factor of 75% applies when there are four or more pieces of equipment fastened in place.

Exam Trap:

Do not apply the residential demand factors to a commercial occupancy. The tables are occupancy-specific.

1.8 Overcurrent Protection Coordination

Selective Coordination [240.12 and 700.28]:

For emergency systems (Article 700), legally required standby systems (701), and critical operations power systems (708), the overcurrent devices must be selectively coordinated. This means the feeder overcurrent device must open before the branch-circuit device, isolating the fault without de-energizing the healthy loads.

Master-Level Application:

The master must review the time-current curves (TCCs) of the fuses or breakers. For fuses, this is straightforward (e.g., a 200A fuse upstream of a 100A fuse). For circuit breakers, the instantaneous trip settings must be evaluated.

Inspection Point:

On site, the master verifies that the ampere ratings are properly nested (e.g., 400A main, 200A feeder, 100A branch). If a fault occurs on the 100A branch, the 200A feeder should not trip. This is a critical life-safety check for hospitals and high-rise buildings.

1.9 Code Navigation: Where to Find It

Use this quick-reference table to locate feeder requirements during the exam.

Concept	NEC Reference
Feeder definition	Article 100
Feeder conductor sizing (minimum ampacity)	215.2(A)(1)
Feeder neutral sizing	215.2(A)(2)
Feeder overcurrent protection	215.3
Feeder taps (10, 25, 100-foot)	240.21(B)
Conductor ampacity tables	Table 310.4 (and 310.15)
Ambient temp correction factors	Table 310.15(B)(1)
Adjustment factors (bundle)	Table 310.15(C)(1)
Motor feeder conductors	430.24
Motor feeder overcurrent protection	430.62
Motor FLC tables	Tables 430.247–430.250
Transformer secondary protection	240.21(C)
Separately derived system grounding	250.30
Grounding electrode conductor sizing	Table 250.66
Commercial feeder calculations	220.40, 220.42, 220.44
Kitchen equipment demand	220.56
Selective coordination (emergency)	700.28, 701.27, 708.54

1.10 Inspection and Supervision Points

When the master signs off on a feeder installation, the following must be verified on site:

125.Conductor Identification: The grounded conductor (neutral) must be white or gray and continuous. The equipment grounding conductor must be green or bare. Phase conductors must be identified per the system voltage (e.g., 277/480V requires specific color coding per company policy, but NEC only requires the neutral and ground to be identified).

126.Termination Torque: All feeder terminations must be torqued to the manufacturer's specification. A loose termination on a 400A feeder is a leading cause of electrical fires.

127.Raceway Fill: Verify that the conduit fill does not exceed the limits of Chapter 9, Table 1 and the notes. A feeder pulling into a conduit that is over 40% fill is a violation.

128.Bonding at the Service: For a service feeder, the neutral must be bonded to the service equipment enclosure via the main bonding jumper. For a feeder to a sub-panel, the neutral must be isolated (floating) from the enclosure.

129.Voltage Drop: While not mandatory for general feeders (it is a fine print note, not a code rule), the master should recommend a maximum of 3% voltage drop for the feeder and 5% total. For motor circuits, excessive voltage drop reduces starting torque.

1.11 Common Exam Traps

Trap 1: Using the 90°C column without checking terminations. The 90°C ampacity is only for derating. The final ampacity cannot exceed the 75°C column if the equipment is rated 75°C.

Trap 2: Forgetting the 125% factor on the largest motor only. For multiple motors, you apply 125% to the largest FLC, not to every motor.

Trap 3: Confusing FLC (from tables) with FLA (from nameplate). Always use the tables for feeder sizing.

Trap 4: Applying the 25-foot tap rule to a commercial office. It is for industrial installations only.

Trap 5: Bonding the neutral at the sub-panel. The neutral-ground bond belongs at the service or the SDS source only.

Trap 6: Ignoring the neutral as a current-carrying conductor. If the load is nonlinear (VFDs, electronic ballasts, computers), the neutral counts, triggering a derating factor.

Trap 7: Sizing the feeder to the sum of the branch breakers. The feeder is sized to the calculated load, not the sum of the overcurrent devices. Demand factors apply.

Summary

The master electrician's role in feeder design is to ensure the safe, code-compliant delivery of power from the source to the final branch circuits. This requires a deep understanding of continuous load factors, conductor ampacity adjustments, motor calculations, and the unique rules for separately derived systems. The ability to navigate the NEC quickly and accurately—especially Articles 215, 220, 240, 250, and 430—is the difference between a journeyman and a master. Always verify the calculated load against the conductor ampacity after all adjustment and correction factors have been applied, and never sign off on an installation without physically verifying the neutral-ground bonding points.

Electrical Feeders