Feeders

Feeders

Learning Objectives

Upon completing this chapter, you should be able to:

1.1 The Feeder Defined: Scope and Boundaries

The NEC defines a feeder 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. This is a critical boundary definition. In practice, the feeder begins at the load side of the service disconnect or at the terminals of a transformer secondary, and it ends at the overcurrent device protecting the branch circuits.

A master electrician must be able to instantly classify conductors on a one-line diagram. A conductor supplying a panelboard that, in turn, supplies luminaires and receptacles is a feeder. The conductors from that panelboard to the individual luminaires are branch circuits. The conductors from the utility transformer to the service disconnect are service conductors, not feeders. This distinction drives which set of rules applies.

Key Point: The feeder is the workhorse of a distribution system. It is where the majority of system coordination and voltage-drop decisions are made.

1.2 Sizing and Ampacity: The Core Calculation

1.2.1 Minimum Size (Article 215.2)

The minimum feeder conductor size, before any adjustment or correction factors, must have an ampacity of not less than the sum of two components:

This is the foundational rule. The 125% factor for continuous loads is a heat-management requirement, ensuring the conductor does not operate at its thermal limit for extended periods (3 hours or more). The calculation is based on the load, not the overcurrent device.

Example: A 3-phase, 4-wire feeder supplies a 120/208V panelboard. The calculated load is 150A continuous and 50A noncontinuous.

1.2.2 The 800A Exception (215.2(A)(1) Exception No. 1)

For feeders supplying loads that are either all noncontinuous or where the overcurrent device is rated 800A or less, the 125% factor may be applied to the overcurrent device rating instead of the load. This is a practical allowance. If the OCPD is 800A or less, the feeder is sized to the OCPD, which inherently provides the 125% factor for continuous loads.

Trap: This exception does not apply to feeders supplying a mix of continuous and noncontinuous loads where the OCPD exceeds 800A. In those cases, you must calculate the load precisely.

1.2.3 Voltage Drop (215.2(A)(1) Informational Note, 210.19(A) Informational Note)

The NEC does not mandate a specific voltage drop percentage for feeders as a code requirement, but the informational notes recommend a maximum of 3% for feeders and a total of 5% for feeders plus branch circuits. While informational notes are not enforceable code, a master electrician must treat them as a design standard. For long runs, the calculated ampacity may be insufficient. You must increase the conductor size to limit voltage drop. This is a supervision point: always verify the actual length of the feeder run on the plans.

Formula for 3-phase: VD = (1.732 × K × I × L) / Cmils, where K is the conductor resistivity (approximately 12.9 for copper, 21.2 for aluminum), I is the current, L is the one-way length in feet, and Cmils is the circular mil area of the conductor.

1.3 The Neutral Conductor: Nonlinear Loads and Harmonics

1.3.1 Basic Sizing (215.2(A)(2))

The neutral conductor must be sized to carry the maximum unbalanced load. For a 3-phase, 4-wire wye system supplying line-to-neutral loads, this is the maximum calculated load on any single phase.

1.3.2 The 70% Rule for Nonlinear Loads (220.61(C)(1))

This is a critical master-level distinction. In a 3-phase, 4-wire wye system where the major portion of the load consists of nonlinear loads (e.g., electronic ballasts, VFDs, computers, LED drivers), the neutral conductor must be considered a current-carrying conductor. Triplen harmonics (3rd, 9th, 15th) are zero-sequence currents that do not cancel on the neutral; they add arithmetically.

The code permits the neutral to be sized at 70% of the phase conductor ampacity for this scenario, but only if the neutral is not counted as a current-carrying conductor for ampacity adjustment purposes. This is a complex interaction.

Supervision Point: On a job site, if you see a feeder with a neutral that is visibly smaller than the phase conductors, you must verify that the load is not predominantly nonlinear. If it is, the neutral may overheat. The 70% rule is a minimum, not a recommendation.

1.3.3 Neutral as a Current-Carrying Conductor (310.15(E))

For ampacity adjustment (the "bundle" derating), the neutral is counted as a current-carrying conductor when:

Trap: In a 3-phase, 4-wire wye feeder supplying only line-to-line loads (e.g., a 3-phase motor), the neutral is not a current-carrying conductor. It is only a grounding/bonding conductor or an equipment grounding conductor. Do not count it for derating.

1.4 Motor and Mixed Load Feeders (Article 430)

Feeder sizing for motors is a specialized calculation that departs from the general load-based approach.

1.4.1 Feeder Ampacity for Multiple Motors (430.24)

The feeder supplying two or more motors must have an ampacity of not less than:

Important: Use the FLC from the tables in Article 430 (e.g., Table 430.250 for 3-phase AC motors), not the nameplate rating. The nameplate is for overload protection; the tables are for conductor sizing and overcurrent protection.

1.4.2 Feeder Ampacity for Mixed Loads (430.25 and 430.26)

When a feeder supplies a motor load plus other loads (lighting, heating, etc.), the calculation is:

1.4.3 Feeder Overcurrent Protection (430.62)

The feeder overcurrent device (the breaker or fuse protecting the feeder) must be sized to carry the motor load. The maximum size is:

This is a coordination rule. The feeder OCPD must allow the largest motor to start without opening, but it must not exceed the sum of the largest motor's branch-circuit protection and the running current of all other motors.

Trap: The feeder OCPD is not sized at 125% of the largest motor. That is the branch-circuit rule. The feeder rule uses the maximum branch-circuit protective device rating for the largest motor.

1.5 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. The most common SDSs are transformers and generators. The feeder originating from an SDS has specific grounding and bonding requirements.

1.5.1 Grounding and Bonding (250.30)

At the SDS (the transformer secondary or the generator output), you must:

Critical Master Point: The neutral-to-case bond at the SDS is made at the source (the transformer or generator) or at the first disconnecting means of the SDS, but not at both. If you bond at both locations, you create a parallel path for neutral current on the equipment grounding conductors, which is a code violation and a safety hazard.

1.5.2 The 5-Foot Rule (250.30(A)(1) Exception)

The bonding jumper at the SDS can be located at the source or at the first disconnecting means. If it is at the first disconnecting means, the grounded conductor must be run with the phase conductors to that point. The exception allows the GEC to be connected to the grounded conductor at the source, but the bonding jumper must be sized per Table 250.102(C)(1).

1.5.3 Transformer Feeder Protection (240.21(C))

The primary overcurrent protection for a transformer can protect the secondary feeder, but only under specific conditions. The most common allowance is the "primary plus secondary" rule:

If the primary OCPD exceeds 250% of the primary FLC, you must provide secondary protection. This is a complex area that requires careful table work (Table 450.3(B)).

Supervision Point: On a transformer installation, verify the location of the system bonding jumper. It should be clearly identified with a green screw or a clearly marked bonding jumper. Check that the grounded conductor is not bonded to the enclosure at the downstream panelboard.

1.6 Commercial and Industrial Installations

1.6.1 Demand Factors (Article 220)

For commercial feeders, you must apply the demand factors from Part III of Article 220. This includes:

A master must be able to calculate a feeder for a restaurant, a school, or an office building, applying the appropriate demand factors to avoid oversizing.

1.6.2 Kitchen Equipment (220.56)

For commercial kitchens, the feeder can be sized based on the maximum load that can be operated simultaneously, rather than the sum of all equipment. This requires a load management plan or a clear understanding of the kitchen's operation.

1.6.3 Continuous vs. Noncontinuous (Article 100)

The definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. This is a judgment call. A master must classify loads correctly. For example, a parking lot lighting circuit that runs all night is continuous. A convenience receptacle circuit is typically noncontinuous.

1.7 Code Navigation: Where to Find It

1.8 Inspection and Supervision Points

As a master, you are responsible for the final sign-off. On a feeder installation, you must verify:

1.9 Common Exam Traps

1.10 Summary

The feeder is the backbone of any electrical distribution system. Master-level knowledge requires moving beyond simple ampacity calculations to a full understanding of load characterization (continuous vs. noncontinuous), harmonic effects on the neutral, motor load calculations, and the unique grounding and bonding rules for separately derived systems. The ability to navigate the NEC quickly and accurately—knowing that Article 215 is the primary article for feeders, but that 430, 450, and 250 all have critical feeder-related rules—is the difference between a journeyman and a master. Always verify your calculations with the actual code text during the exam, and apply the same rigor to every field inspection.