7.1 Feeder Design & Sizing Requirements (NEC Article 215)

7.1 Feeder Design & Sizing Requirements (NEC Article 215)

Quick Answer: Under NEC 215.2(A)(1), feeder conductors must have an allowable ampacity not less than 100% of noncontinuous load plus 125% of continuous load before derating. Neutral conductors (NEC 120.61, formerly 220.61) are sized for the maximum unbalanced load. The NEC Informational Notes recommend limiting voltage drop to 3% on feeders and 5% total across feeders and branch circuits. When ungrounded conductors are increased in size for voltage drop, the equipment grounding conductor must be proportionally upsized under NEC 250.122(B).

A feeder is defined in NEC 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. Feeders serve as the primary arterial distribution network in residential, commercial, and industrial facilities, routing bulk electrical energy from main switchgear or distribution panelboards to subpanels located throughout the structure.

Designing and sizing feeders requires mastering multiple interconnected code articles: Article 215 (Feeder Installation and Minimum Sizing), Article 120 (Feeder Load Calculations and Neutral Sizing, formerly Article 220), Article 240 (Overcurrent Protection and Tap Rules), and Article 250 (Grounding and Bonding). On the Minnesota Journeyworker examination, feeder problems evaluate your ability to handle continuous load factors, neutral derating for harmonics, and multi-step voltage drop computations.

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Feeder Conductor Sizing Mandates (NEC 215.2)

Under NEC 215.2(A)(1), feeder conductors must have an allowable ampacity sufficient to supply the load calculated in accordance with Article 120. The code enforces a two-step sizing procedure parallel to that used for branch circuits:

Step 1: Base Ampacity Before Derating

The allowable ampacity of feeder conductors before applying ambient temperature or conduit fill adjustment factors must not be less than 100% of the noncontinuous load plus 125% of the continuous load:

$\text{Minimum Feeder Ampacity} = \text{Noncontinuous Load} + (1.25 \times \text{Continuous Load})$

This base ampacity is selected directly from NEC Table 310.16 using the column corresponding to the temperature rating of the equipment terminals under NEC 110.14(C) (typically 75°C for circuits rated over 100 amperes or marked for 75°C terminations).

Step 2: Ampacity After Derating

The feeder conductor's derated ampacity—calculated by applying temperature correction factors from Table 310.15(B)(1) and adjustment factors from Table 310.15(C)(1) to the conductor's 90°C rating—must equal or exceed 100% of the total connected load (noncontinuous load + continuous load).

$\text{Derated Ampacity} = \text{Table 310.16 (90°C)} \times \text{Temp Factor} \times \text{Bundling Factor} \ge (I_{\text{noncontinuous}} + I_{\text{continuous}})$

The installer must compare the conductor size required by Step 1 with the conductor size required by Step 2 and install whichever is larger.

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Feeder Overcurrent Protection (NEC 215.3 & 240.4)

Under NEC 215.3, feeders must be protected against overcurrent in accordance with Part I of Article 240. The rating or setting of the feeder overcurrent protective device must not be less than 100% of the noncontinuous load plus 125% of the continuous load:

$\text{Minimum Feeder OCPD Rating} = \text{Noncontinuous Load} + (1.25 \times \text{Continuous Load})$

The Next Higher Standard Rating Rule (NEC 240.4(B))

Where the calculated ampere rating does not correspond to a standard overcurrent device rating in NEC 240.6(A), the installer is permitted to use the next higher standard rating, provided all three of the following conditions are satisfied:

1. The conductors being protected do not supply a multi-outlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
2. The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker.
3. The rating of the overcurrent device does not exceed 800 amperes.

Circuits Exceeding 800 Amperes (NEC 240.4(C))

For feeders rated greater than 800 amperes, the next-higher-standard-rating rule does not apply. The allowable ampacity of the feeder conductors must be equal to or greater than the rating of the overcurrent protective device.

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Feeder Neutral Sizing & Harmonic Considerations (NEC 120.61)

The feeder neutral conductor carries return current from line-to-neutral loads. Under NEC 120.61, the feeder neutral is sized to carry the maximum unbalanced load between the neutral and any one ungrounded conductor, as determined by the Article 120 calculations.

Permitted Reductions (NEC 120.61(B))

To avoid running excessively oversized copper for neutral conductors, the NEC permits demand factor reductions for specific large loads:

• Electric Ranges and Cooking Appliances: The feeder neutral demand for household electric ranges, wall ovens, and cooking units is calculated at 70% of the load determined under Table 120.55.
• Electric Clothes Dryers: The feeder neutral demand for household dryers is calculated at 70% of the load determined under Table 120.54.
• Loads in Excess of 200 Amperes: For that portion of the total unbalanced load that exceeds 200 amperes, a demand factor of 70% may be applied (provided the system does not supply non-linear harmonic loads).

Non-Linear Loads & Harmonic Derating (NEC 120.61(C) & 310.15(E))

In commercial facilities supplied by 208Y/120V or 480Y/277V 3-phase, 4-wire wye systems, electronic equipment (LED electronic drivers, computer power supplies, variable frequency drives) draws current in short, high-magnitude pulses rather than smooth sinusoids. These pulses create triplen harmonics (predominantly the 3rd harmonic at 180 Hz and 9th harmonic at 540 Hz).

Because triplen harmonics are zero-sequence components, they do not cancel in the neutral conductor—they add arithmetically in the neutral. Under severe harmonic conditions, neutral current can exceed the phase conductor current.

Therefore, the NEC enforces two critical restrictions:

1. No Neutral Reduction (NEC 120.61(C)): There shall be no reduction in neutral capacity for that portion of the load consisting of non-linear loads.
2. Current-Carrying Conductor Status (NEC 310.15(E)): On a 3-phase, 4-wire wye system where a major portion of the load consists of non-linear loads, the neutral conductor carries substantial harmonic currents and must be counted as a current-carrying conductor when applying conduit fill derating factors under Table 310.15(C)(1)!

Minimum Feeder Neutral Size (NEC 215.2(A)(2))

Regardless of how small the calculated unbalanced neutral load may be, the feeder grounded conductor may not be smaller than the equipment grounding conductor required by 250.122, based on the rating of the feeder OCPD. (Where grounded conductors are run in parallel, the 250.122(F) parallel rule does not apply to this minimum.) Do not confuse this with the service grounded conductor, whose minimum comes from Table 250.102(C)(1).

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Voltage Drop Engineering & Formulas

While the National Electrical Code does not strictly enforce voltage drop as a mandatory requirement in Article 215 (voltage drop is an Informational Note, not mandatory text), Minnesota Journeyworker examinations frequently feature voltage drop calculations to verify applied engineering competence.

The NEC Informational Note Recommendations

Informational Notes in 210.19 and 215.2 suggest:

• Maximum voltage drop on a feeder conductor alone: 3%.
• Maximum voltage drop on a branch circuit alone: 3%.
• Maximum total combined voltage drop from the service to the farthest outlet: 5%.

Basic Voltage Drop Equations (The Ohm's Law Method)

Voltage drop is caused by conductor resistance ($V_d = I \times R$). The resistance of a conductor is proportional to its length and resistivity, and inversely proportional to its cross-sectional area in circular mils (CM):

$R = \frac{K \times D}{CM}$

Where:

• $K$ = Specific resistivity of the conductor material in ohms-circular mil per foot at 75°C:
  • Copper: $K \approx 12.9\ \Omega\cdot\text{cmil/ft}$
  • Aluminum: $K \approx 21.2\ \Omega\cdot\text{cmil/ft}$
• $D$ = One-way length of the feeder in feet.
• $I$ = Load current in amperes.
• $CM$ = Conductor cross-sectional area in circular mils (from NEC Chapter 9, Table 8).

Single-Phase Feeder Voltage Drop Formula

Because current must travel out on the hot conductor and return on the neutral or second phase conductor, total circuit distance is $2 \times D$:

$V_{d\text{ (1-Phase)}} = \frac{2 \times K \times I \times D}{CM}$

Three-Phase Feeder Voltage Drop Formula

In a balanced three-phase system, current vectors are separated by 120°, replacing the multiplier 2 with $\sqrt{3} \approx 1.732$:

$V_{d\text{ (3-Phase)}} = \frac{\sqrt{3} \times K \times I \times D}{CM} = \frac{1.732 \times K \times I \times D}{CM}$

Sizing Conductors for a Target Voltage Drop

To find the minimum conductor circular mil area required to stay within an allowable voltage drop ($V_d$):

$\mathbf{CM_{\text{Single-Phase}} = \frac{2 \times K \times I \times D}{V_d}} \qquad \mathbf{CM_{\text{Three-Phase}} = \frac{\sqrt{3} \times K \times I \times D}{V_d}}$

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Proportional Upsizing of Equipment Grounding Conductors (NEC 250.122(B))

One of the most heavily tested rules on electrical licensing examinations involves the interaction between voltage drop and equipment grounding conductors (EGC).

Under NEC 250.122(B):

Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation (such as increasing wire gauge to prevent excessive voltage drop), equipment grounding conductors must be increased in size proportionally according to the circular mil area of the ungrounded conductors.

Proportional Upsizing Calculation Formula

$\frac{CM_{\text{New EGC}}}{CM_{\text{Table 250.122 EGC}}} = \frac{CM_{\text{New Phase Conductor}}}{CM_{\text{Minimum Phase Conductor}}}$

$CM_{\text{New EGC}} = CM_{\text{Table 250.122 EGC}} \times \left( \frac{CM_{\text{New Phase}}}{CM_{\text{Min Phase}}} \right)$

If you double the circular mil area of the phase conductors to eliminate voltage drop over a 400-foot run, you must double the circular mil area of the equipment grounding conductor! This ensures that fault-circuit impedance remains low enough to quickly trip the upstream circuit breaker during a ground fault.

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Feeders Supplying Separate Buildings or Structures

A feeder that leaves one building to supply another, such as a detached garage or barn, must also meet Article 225 (outside branch circuits and feeders, number of supplies, and the building disconnecting means) and 250.32 (a grounding electrode system at the building and an equipment grounding conductor run with the feeder, with the neutral isolated from the equipment grounding bus). Section 7.2 of this guide covers those rules in detail.

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Practical Exam Scenarios & Trap Avoidance

Scenario 1: Feeder Sizing with Continuous and Noncontinuous Loads

A 120/240V commercial distribution panelboard feeds an 80-ampere continuous electric heating load and a 50-ampere noncontinuous convenience load. What is the minimum conductor ampacity (75°C terminals) and minimum standard circuit breaker size?

• Step 1: Calculate Minimum Ampacity: Apply 125% to continuous load: $\text{Minimum Ampacity} = 50\text{ A} + (1.25 \times 80\text{ A}) = 50\text{ A} + 100\text{ A} = \mathbf{150\text{ Amperes}}$
• Step 2: Select Conductor Size: From Table 310.16 (75°C column), a 1/0 AWG Copper conductor has an allowable ampacity of 150 amperes.
• Step 3: Select Standard OCPD: Under NEC 240.6(A), 150 amperes is a standard rating. A 150A circuit breaker is required.

Scenario 2: Proportional EGC Sizing for Voltage Drop

A 100-ampere single-phase feeder has a minimum calculated load requiring 3 AWG THHN copper (52,620 CM from Chapter 9, Table 8). From Table 250.122, the minimum EGC for a 100A breaker is 8 AWG copper (16,510 CM). To prevent voltage drop on a 350-foot run, the phase conductors are upsized to 1/0 AWG copper (105,600 CM). What size EGC must be installed?

• Step 1: Calculate Upsizing Ratio: $\text{Ratio} = \frac{105,600\text{ CM}}{52,620\text{ CM}} \approx 2.0068$
• Step 2: Calculate New EGC Area: $CM_{\text{New EGC}} = 16,510\text{ CM} \times 2.0068 = \mathbf{33,133\text{ CM}}$
• Step 3: Select Conductor from Chapter 9 Table 8:
  • 6 AWG copper = 26,240 CM (too small).
  • 4 AWG copper = 41,740 CM (exceeds 33,133 CM).
• Conclusion: The equipment grounding conductor must be upsized to 4 AWG copper.