Feeders

Feeders

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 Definitions and Scope: Feeder vs. Branch Circuit

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.

Key Distinctions for the Master:

A common field error is misidentifying a conductor that runs from a panelboard to a sub-panel. That is a feeder, not a branch circuit. This distinction is critical because the rules for ampacity, overcurrent protection, and grounding differ.

1.2 Sizing Feeders: The Core Calculation (NEC 215.2)

The minimum feeder conductor size must have an ampacity sufficient for the load, computed per Article 220. The fundamental rule is found in NEC 215.2(A)(1) :

The ampacity of the feeder conductors shall not be less than the sum of the non-continuous loads plus 125% of the continuous loads.

Continuous Load is defined as a load where the maximum current is expected to continue for 3 hours or more.

Example Calculation:

A commercial kitchen has a non-continuous load of 60 A and a continuous load of 80 A.

Master's Note: This 125% factor is applied to the load, not the conductor. After determining the minimum ampacity, you must then apply ambient temperature correction and conductor bundling adjustment factors from Table 310.16 and its associated correction factors. The final conductor size must satisfy both the load requirement and the adjusted ampacity requirement.

1.3 The Neutral (Grounded) Conductor Sizing

The feeder neutral must be sized to carry the maximum unbalanced load. However, the rules are nuanced.

NEC 215.2(A)(2) requires the neutral to have an ampacity not less than the maximum unbalanced load. For a 3-phase, 4-wire system, this is the maximum net calculated load between the neutral and any one ungrounded conductor.

Reduced Neutral:

The neutral is permitted to be smaller than the ungrounded conductors, provided:

The Nonlinear Load Trap:

For feeders serving nonlinear loads (e.g., electronic ballasts, variable frequency drives, computers), the neutral can carry significant triplen harmonics (3rd, 9th, 15th...). These harmonics are additive on the neutral. NEC 310.15(E) requires the neutral to be counted as a current-carrying conductor for ampacity adjustment purposes when serving nonlinear loads. In severe cases, the neutral may need to be full-size or even larger than the phase conductors.

Master's Note: For a feeder supplying a panelboard that feeds data centers or offices with many computers, do not automatically reduce the neutral. Treat it as a current-carrying conductor, which triggers a 50% adjustment factor for four or more current-carrying conductors in a cable or raceway.

1.4 Feeder Overcurrent Protection (NEC 215.3)

Feeder conductors must be protected against overcurrent per their ampacity.

The "Next Size Up" Rule:

Where the calculated ampacity does not correspond to a standard rating of a fuse or circuit breaker, you may use the next higher standard rating, but only if:

Standard Ratings (NEC 240.6): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000.

Example:

A calculated load requires a conductor ampacity of 145 A. You select a 1/0 AWG conductor (150 A at 75°C). The next standard OCPD above 145 A is 150 A. This is permitted because the conductor ampacity (150 A) is equal to the OCPD rating.

Example with a Trap:

A calculated load is 155 A. You select a 2/0 AWG conductor (175 A at 75°C). The next standard OCPD above 155 A is 175 A. This is permitted. However, if you selected a 1/0 AWG (150 A), you cannot protect it with a 175 A breaker because 150 A is less than the calculated load of 155 A.

Tap Rules:

Feeders can be tapped (NEC 240.21(B)) to supply smaller panelboards, but the tap conductors must terminate in a single OCPD and meet specific length and ampacity requirements (e.g., 3 ft, 10 ft, 25 ft rules). A master must know these exceptions for industrial applications where running a full-size feeder to every small load is impractical.

1.5 Feeder Installation Requirements

NEC 215.12 covers the minimum installation requirements:

Voltage Drop (Informational but Critical):

NEC 215.2(A)(4) Informational Note No. 2 recommends that feeder conductors be sized to limit voltage drop to 3% for the feeder, and the total voltage drop for the feeder and branch circuit to 5%. While this is not a mandatory "shall" requirement, it is a design requirement that a master must address. The exam will test your ability to calculate voltage drop for long runs.

Voltage Drop Formula (Single-Phase):

VD = (2 × K × I × L) / CM

Where K = 12.9 for copper, 21.2 for aluminum; I = current; L = one-way length in feet; CM = circular mils.

Voltage Drop Formula (Three-Phase):

VD = (1.732 × K × I × L) / CM

Master's Note: For a 480 V, 3-phase motor feeder running 400 feet, the voltage drop often dictates a conductor size larger than the ampacity requirement. Always check voltage drop on long runs.

1.6 Feeders for Specific Loads

1.6.1 Motor Feeders (NEC 430.24 and 430.25)

A feeder supplying a group of motors must have an ampacity of at least:

Example:

A feeder supplies three motors: 10 HP (14 A FLC), 15 HP (21 A FLC), and 25 HP (34 A FLC) at 460 V, 3-phase.

The feeder OCPD is sized per NEC 430.62, which requires the maximum rating of the feeder protective device to be based on the largest branch-circuit protective device plus the sum of the FLCs of the other motors. This is a coordination issue—the feeder OCPD must allow the largest motor to start without opening, but must protect the feeder.

1.6.2 Transformer Feeders (Separately Derived Systems)

When a feeder supplies the primary side of a transformer, the transformer is a separately derived system. The secondary conductors are a new feeder.

Primary Protection (NEC 450.3): The primary OCPD must protect the transformer against overcurrent. If the primary OCPD is rated at 125% of the primary current (or the next standard size), the secondary conductors may be protected by the primary device under specific conditions (NEC 240.21(C)).

Secondary Conductor Protection: The secondary conductors must be protected by an OCPD on the secondary side, unless they comply with the "secondary conductor tap" rules in NEC 240.21(C) . These rules allow the secondary conductors to be tapped without a secondary OCPD if they terminate in a single OCPD and meet length and ampacity requirements.

Master's Note: For a 75 kVA, 480 V to 208Y/120 V transformer:

1.6.3 Generator Feeders

Generators are also separately derived systems (unless they are non-separately derived, which is rare). The feeder from the generator output terminals to the transfer switch or distribution panel must be sized per the generator's rated output current.

Critical Rule: The generator's rated output is the continuous rating. The feeder must be sized for 125% of the continuous load, but the generator's rated output already includes the continuous rating. Therefore, the feeder ampacity must be at least 100% of the generator's rated output current, but the OCPD must be sized to protect the generator windings.

1.7 Code Navigation: Where to Find It

1.8 Inspection and Supervision Points

As a master electrician, you are responsible for the work being code-compliant. On-site, verify the following:

1.9 Common Exam Traps

1.10 Summary

Feeder sizing and protection is the heart of commercial and industrial electrical design. A master electrician must be able to:

Mastery of Article 215, combined with a working knowledge of Articles 220, 240, 310, 430, and 450, will allow you to design safe, code-compliant feeder systems and pass the Wyoming Master Electrician exam.