Electrical Feeders

Learning Objectives

Upon completing this chapter, you will be able to:

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

5.Apply the minimum feeder sizing rules, including the 100% non-continuous plus 125% continuous load requirement.

6.Calculate feeder demand factors for dwellings, commercial kitchens, and farms per Article 220.

7.Size feeders for motor loads, including the 125% rule for the largest motor and the application of Table 430.52.

8.Apply feeder neutral sizing rules, including the reduction permitted for harmonic-producing loads and the prohibition for certain nonlinear loads.

9.Identify the requirements for separately derived systems, including transformer secondary protection and grounding/bonding per Article 250.

10.Navigate the NEC efficiently to locate feeder-related requirements for design, installation, and inspection.

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, a feeder is the conductor set that carries power from the service or a distribution panel to a downstream panelboard, switchboard, or motor control center.

The distinction matters for code application:

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

Feeders run from the service disconnecting means (or the secondary of a transformer, or the output of a generator) to the final overcurrent device protecting a branch circuit.

Branch circuits run from the final overcurrent device to the outlet(s).

A master electrician must correctly identify the boundary points because different ampacity adjustment, grounding, and overcurrent protection rules apply at each stage. For example, the 3-foot clearance requirement for service equipment does not apply to feeder panels, but the working clearance requirements of 110.26 apply to all equipment.

1.2 Sizing Feeders: The General Rule

The foundational sizing rule for feeders is found in 210.19(A)(1) , which is applied by reference to feeders through 215.2(A)(1) . The rule states that feeder conductors must have an ampacity of not less than the larger of:

23.The sum of the non-continuous loads plus 125% of the continuous loads.

24.The sum of the loads after the application of any demand factors permitted by Article 220.

Continuous load is defined as a load where the maximum current is expected to continue for 3 hours or more. For a master, the practical question is always: Is this load continuous? Standard examples include lighting in commercial buildings, HVAC equipment, and electric vehicle supply equipment (which is specifically treated as a continuous load per 625.40).

Example: A commercial panel feeds a 40 A continuous lighting load and a 30 A non-continuous receptacle load. The feeder must be sized for (40 A × 1.25) + 30 A = 80 A. This requires a conductor with an ampacity of at least 80 A before any temperature correction or adjustment factors are applied.

Critical trap: The 125% factor is applied to the load, not to the overcurrent device rating. Many journeymen incorrectly size the conductor to match the breaker. The conductor must be sized to the calculated load; the breaker is then selected to protect the conductor per 240.4.

1.3 Demand Factors and Optional Calculations

Article 220 provides the framework for calculating feeder loads. A master must know both the standard method (Part III) and the optional method (Part IV) for dwellings, and the specific demand factors for other occupancies.

1.3.1 Dwelling Units (Standard Method – 220.40 through 220.61)

The standard method requires calculating the general lighting load at 3 VA per square foot (Table 220.12), plus small-appliance branch circuits at 1,500 VA each (minimum two), and laundry at 1,500 VA. These are then subject to the demand factors of Table 220.42 :

First 3,000 VA at 100%

3,001 to 120,000 VA at 35%

Remainder over 120,000 VA at 25%

The optional method (220.82) for a dwelling unit allows a single calculation based on the total connected load, with a demand factor of 40% applied to the portion of the load exceeding 10 kVA. This method is simpler and almost always yields a smaller service or feeder, but it is only permitted where the total load is served by a single feeder or service.

1.3.2 Commercial Kitchens (Table 220.56)

For commercial electric cooking equipment, dishwasher booster heaters, and water heaters, the feeder demand factor is applied to the total connected load of the kitchen equipment:

First 200 kVA at 100%

Remainder over 200 kVA at 50%

This table is frequently tested. Note that it applies to the feeder serving the kitchen, not to individual branch circuits. The branch circuits themselves must be sized at 100% of the equipment rating (or 125% if continuous).

1.3.3 Farms (220.102 through 220.103)

Farm feeders have specific demand factors based on the number of buildings or loads served. For example, a feeder serving two or more farm buildings may use a demand factor of 100% for the largest load plus 50% for the second largest, with additional loads at 25%. These provisions are rarely used in urban commercial work but appear on the exam and are essential for rural contractors.

1.4 Motor Feeder Sizing

Motor feeders are governed by 430.24 and 430.25. The feeder conductor ampacity must be at least 125% of the full-load current (FLC) of the highest-rated motor in the group, plus the sum of the FLCs of all other motors on the feeder, plus the calculated load of any other loads served.

Example: A feeder serves three motors: 10 A, 15 A, and 20 A (all FLC from Tables 430.247–430.250). The feeder must be sized for (20 A × 1.25) + 15 A + 10 A = 50 A.

Critical distinction: The 125% factor applies to the highest-rated motor, not the largest overload relay setting or the motor nameplate current. The FLC is taken from the NEC tables, not the motor nameplate, for conductor sizing. The nameplate is used for overload protection sizing per 430.32.

Motor feeder overcurrent protection is covered by 430.62. The feeder protective device must be sized to permit the starting current of the largest motor plus the full-load current of all other loads, but it cannot exceed the rating determined by Table 430.52 for the largest motor plus the sum of the others. In practice, the feeder breaker is often sized at 150–250% of the largest motor FLC, depending on the motor type and starting characteristics.

Generator feeders follow the same general rules, but special attention is required for the rated current of the generator. Per 445.13, the ampacity of the conductors from the generator terminals to the first overcurrent device must be at least 115% of the generator's nameplate current rating. This is a distinct percentage that differs from the 125% rule for continuous loads and the 125% for the largest motor.

1.5 Neutral (Grounded Conductor) Sizing

The neutral conductor of a feeder must be sized to carry the maximum unbalanced load per 220.61. The neutral is not required to be larger than the ungrounded conductors, and it may be reduced where the load is non-linear or where the connected loads are balanced.

Key provisions:

For a 3-wire, single-phase feeder or a 4-wire, 3-phase feeder, the neutral must carry the maximum unbalanced load.

The neutral of a 3-phase, 4-wire feeder serving only line-to-line loads (e.g., 208 V loads on a 208Y/120 V system) may be sized at 0% (i.e., omitted) if no line-to-neutral loads are present.

For feeders serving harmonic-producing loads (e.g., electronic ballasts, variable frequency drives, data centers), the neutral is considered a current-carrying conductor for ampacity adjustment purposes per 310.15(C)(1) . In some cases, the neutral may need to be larger than the phase conductors.

Exam trap: The 310.15(C)(1) exception for dwelling units allows the neutral to be omitted from the current-carrying conductor count for adjustment purposes, but this does not apply to commercial or industrial feeders where more than 50% of the load is nonlinear.

1.6 Ampacity Adjustments and Corrections

A master must apply the ambient temperature correction factors of Table 310.15(B)(1) and the bundling adjustment factors of Table 310.15(C)(1) . These are applied after the base ampacity is determined from the appropriate column of Table 310.16 (for 75°C terminals) or Table 310.17.

The 75°C terminal rule (110.14(C)): Unless the equipment terminals are specifically listed for 90°C, the conductor ampacity must be based on the 75°C column. This is the single most common cause of undersized feeders in practice. A master must check the termination temperature rating on the panelboard, switchboard, or disconnect.

Adjustment order: The correction and adjustment factors are multiplied together and applied to the base ampacity. The resulting value must be at least the calculated load. The overcurrent device is then selected per 240.4, which permits the next standard size up if the conductor ampacity does not correspond to a standard rating, but only up to 800 A.

1.7 Separately Derived Systems

A separately derived system is a premises wiring system whose power is derived from a battery, solar photovoltaic system, generator, transformer, or converter windings, and that has no direct electrical connection to the supply conductors of any other system. The most common examples are:

A transformer secondary (e.g., 480 V to 208Y/120 V)

A standby generator with a transfer switch that opens the neutral

An uninterruptible power supply (UPS) output

1.7.1 Grounding and Bonding (250.30)

The requirements for grounding a separately derived system are found in 250.30(A) . The system must have:

74.A system bonding jumper connecting the grounded conductor to the equipment grounding conductor at the source or at the first disconnecting means.

75.A grounding electrode conductor connecting the grounded conductor to a grounding electrode (typically the building steel or a ground rod).

76.The grounded conductor must not be bonded to the equipment grounding conductor at more than one point (except as permitted for parallel paths).

Critical trap: For a transformer, the neutral of the secondary must be bonded to the transformer enclosure and to the equipment grounding conductor at the transformer, not at the downstream panelboard. Bonding at both locations creates a parallel neutral path and violates 250.30(A)(1).

1.7.2 Transformer Secondary Protection (240.21(C))

The primary overcurrent device may protect the secondary conductors if the primary device is sized per the transformer's primary current and the secondary conductors meet the length and size requirements of 240.21(C)(1) through (C)(6). The most common allowance is the 25-foot tap rule of 240.21(C)(2), which permits secondary conductors to be tapped without secondary overcurrent protection if:

The ampacity of the secondary conductors is at least one-third of the rating of the primary protective device (multiplied by the turns ratio).

The conductors terminate in a single overcurrent device.

The total length does not exceed 25 feet.

The secondary conductors are protected from physical damage.

For longer runs, the transformer secondary protection must be provided at the secondary side, typically sized per the secondary full-load current at 125% for continuous loads.

1.8 Overcurrent Protection Coordination

For a master, the concept of selective coordination is critical, especially in healthcare, industrial, and emergency systems. Per 240.12 and 700.28 (for emergency systems), overcurrent devices must be selectively coordinated so that a fault on a branch circuit clears the branch device without opening the feeder or service device.

Practical approach: When a feeder breaker is set to trip instantaneously at a value lower than the available fault current at the downstream panel, a fault downstream will trip both breakers. The master must verify that the feeder breaker's instantaneous trip setting is above the maximum fault current available at the load side of the branch breaker.

Exam trap: The 2026 NEC continues to require fully selective coordination for emergency, legally required standby, and critical operations power systems (700.28, 701.27, 708.54). This is not optional and cannot be achieved by simply sizing the feeder breaker larger; the trip curves must be evaluated.

1.9 Code Navigation: Where to Find It

Topic	Article/Section
Feeder definition	Article 100
Feeder sizing (general)	215.2, 210.19(A)(1)
Feeder overcurrent protection	215.3, 240.4
Dwelling load calculations	220.40–220.61, 220.82
Commercial kitchen demand	Table 220.56
Farm demand factors	220.102–220.103
Motor feeder sizing	430.24, 430.25
Motor feeder protection	430.62
Generator conductor sizing	445.13
Neutral sizing	220.61, 310.15(C)(1)
Ampacity tables	310.16, 310.17
Temperature correction	Table 310.15(B)(1)
Adjustment factors	Table 310.15(C)(1)
Terminal temperature limits	110.14(C)
Separately derived systems	250.30, 250.20
Transformer secondary protection	240.21(C)
Selective coordination	240.12, 700.28, 701.27
Working clearances	110.26

1.10 Inspection and Supervision Points

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

96.Conductor size and insulation: Confirm the conductor ampacity is based on the 75°C column (unless terminals are rated 90°C) and that all adjustment/correction factors have been applied.

97.Termination torque: Verify that all lug connections are torqued to the manufacturer's specification. Loose terminations are the leading cause of feeder failures.

98.Grounding and bonding: For a separately derived system, confirm that the system bonding jumper is at the source only, and that the grounding electrode conductor is continuous and properly sized per Table 250.66.

99.Overcurrent device rating: Ensure the feeder breaker is sized to protect the conductor per 240.4, and that it is not larger than the calculated load permits.

100.Spare capacity: For a panelboard feeder, verify that the calculated load does not exceed 80% of the panelboard rating (for continuous loads) and that the feeder has adequate capacity for future expansion, as required by 408.30 for lighting and appliance branch-circuit panelboards.

101.Physical protection: Check that feeders are supported per 334.30 (for NM cable) or 376.30 (for busway), and that conductors entering enclosures are protected from abrasion per 300.4.

1.11 Common Exam Traps

104.The 125% continuous load factor is applied to the load, not the breaker. A 100 A breaker can protect a conductor sized for 80 A of continuous load if the conductor is rated 100 A at 75°C.

105.Motor FLC vs. nameplate. Use the tables (430.247–430.250) for conductor sizing, not the motor nameplate.

106.The neutral counts as a current-carrying conductor for 4-wire, 3-phase feeders serving nonlinear loads. This reduces the allowable ampacity of all conductors in the raceway.

107.The 25-foot tap rule requires the secondary conductors to terminate in a single overcurrent device. Multiple breakers in a panelboard do not qualify.

108.Generator conductors are sized at 115% of the generator nameplate current, not 125%.

109.A separately derived system must be bonded at the source. Bonding at the first downstream panelboard is a violation unless the bonding jumper is located there per the design.

110.The optional dwelling calculation (220.82) cannot be used for a feeder serving only a subpanel in a dwelling; it applies to the entire dwelling load.

Summary

Feeder design is the backbone of commercial and industrial electrical work. The master electrician must integrate load calculations, conductor ampacity, overcurrent protection, and grounding/bonding requirements into a coherent, code-compliant design. Mastery of Article 215, 220, 430, and 250 is essential, not only to pass the Texas Master exam but to supervise installations that are safe, reliable, and legally defensible. Always verify the latest code cycle for amendments, and remember that the NEC is a minimum standard — good engineering practice often exceeds it.

Electrical Feeders