4.1 Feeder Sizing, Demand Factors & Neutral Calculations

Feeder Sizing, Demand Factors & Neutral Calculations

Feeders transport bulk electrical power from service equipment or separately derived sources to branch-circuit panelboards. Governed by NEC Article 215 (Feeders) and NEC Article 220 (Branch-Circuit, Feeder, and Service Calculations), sizing feeder phase and neutral conductors requires precise application of continuous load multipliers, terminal temperature ratings, demand factors, and harmonic distortion rules. For the Alabama Journeyman Electrician examination, sizing feeders accurately is a core competency tested on the Prov blueprint.

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1. Feeder Definition & Distribution Hierarchy (NEC Article 100)

NEC Article 100 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 protective device (OCPD).

In the electrical distribution hierarchy, service conductors run from the utility service point to the service disconnect, feeders distribute power to subpanels, and branch circuits supply utilization equipment. Because feeders carry the cumulative electrical loads of multiple branch circuits, an undersized feeder creates severe thermal degradation, dangerous voltage drops, and widespread system failure.

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2. Sizing Feeder Phase Conductors (NEC 215.2(A)(1))

Under NEC 215.2(A)(1), feeder phase conductors must have an allowable ampacity not less than:

$I_{\text{feeder}} = (I_{\text{continuous}} \times 1.25) + I_{\text{noncontinuous}}$

A continuous load (NEC Article 100) continues for 3 hours or more. The 125% multiplier prevents sustained thermal buildup on overcurrent protective devices and equipment terminations.

Two-Step Sizing Process

Conductor selection must satisfy two separate requirements:

1. Terminal & Overcurrent Protection (NEC 215.2(A)(1)(a)): The conductor's Table 310.16 ampacity—evaluated at equipment terminal ratings per NEC 110.14(C) (typically 75°C for equipment over 100A)—must equal or exceed 125% continuous plus 100% noncontinuous load before derating.
2. Adjustment & Correction (NEC 215.2(A)(1)(b)): After ambient temperature correction (Table 310.15(B)(1)) and raceway bundling adjustment (Table 310.15(C)(1)), the derated ampacity must equal or exceed 100% continuous plus 100% noncontinuous load.

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3. Feeder Neutral Sizing & Maximum Unbalanced Load (NEC 220.61)

Unlike phase conductors, a feeder neutral conductor is not sized for total connected wattage. Instead, NEC 220.61(A) requires the neutral to carry the maximum unbalanced load—the maximum net load between the neutral and any one ungrounded phase conductor.

• Pure 240V line-to-line loads (such as straight 240V water heaters or condensing units) draw current exclusively between hot legs, placing zero return current on the neutral. These loads are excluded from neutral sizing.
• Balanced 3-phase line-to-line loads also place zero current on the neutral.

Permissible Neutral Demand Reductions (NEC 220.61(B))

For specific high-amperage appliances and heavy feeders, NEC 220.61(B) permits demand reductions:

1. Cooking Appliances: Sized per Table 220.55, a 70% demand factor applies to household electric ranges, wall ovens, and cooktops.
2. Clothes Dryers: Sized per Table 220.54, a 70% demand factor applies to household dryers.
3. Feeder Neutral Loads Over 200 Amperes: For 3-wire 120/240V and 4-wire wye systems, that portion of the unbalanced neutral load exceeding 200A receives a 70% demand factor (the first 200A is taken at 100%):

$\text{Neutral Load} = 200\text{ A} + \left[(\text{Total Unbalanced Load} - 200\text{ A}) \times 0.70\right]$

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4. Non-Linear Harmonic Loads (NEC 220.61(C)(2))

Under NEC 220.61(C)(2), no neutral reduction is permitted for the portion of the load consisting of non-linear loads on a 4-wire, 3-phase, wye system.

Non-linear loads (LED drivers, computers, electronic ballasts, VFDs) draw current in short pulses, generating triplen harmonics (notably 3rd harmonics at 180 Hz). While fundamental 60 Hz currents cancel in a balanced wye neutral, triplen harmonics are in phase and add arithmetically in the neutral conductor:

$I_{\text{neutral}} = \sqrt{I_1^2 + I_2^2 + I_3^2 + 3(I_3^{\text{harmonic}})^2}$

Neutral currents in electronic facilities can exceed phase currents. Under NEC 310.15(E)(3), where non-linear loads predominate, the neutral must be counted as a current-carrying conductor for Table 310.15(C)(1) derating.

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5. Feeder Neutral to Separate Buildings (NEC 250.32)

When a feeder supplies a separate building or structure, NEC 250.32(B)(1) requires an insulated grounded conductor and a separate equipment grounding conductor. The neutral conductor must not be smaller than the minimum supply-side bonding jumper in NEC Table 250.102(C)(1) based on the largest ungrounded feeder conductor.

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6. Sizing Summary & Worked Calculation Example

Worked Calculation: Commercial Feeder

A 120/208V, 3-phase, 4-wire subpanel with 75°C THHN copper conductors supplies:

• Continuous commercial LED lighting: $60\text{ A}$ per phase
• Noncontinuous general loads: $90\text{ A}$ per phase
• Balanced 3-phase non-linear server loads: $80\text{ A}$ per phase

Phase Conductor Sizing: $I_{\text{phase}} = (60\text{ A} \times 1.25) + 90\text{ A} + 80\text{ A} = 75\text{ A} + 90\text{ A} + 80\text{ A} = 245\text{ A}$ Per Table 310.16 (75°C column), 250 kcmil copper (255A) is selected.

Neutral Sizing: Because the LED lighting and server loads are non-linear, NEC 220.61(C)(2) prohibits neutral reduction. The neutral conductor must be sized at 250 kcmil copper to handle the harmonic load, and all 4 conductors must be counted as current-carrying per NEC 310.15(E)(3).