2.2 Feeders, Feeder Sizing & Calculations

Feeders, Feeder Sizing & Calculations

A feeder consists of 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 (NEC Article 100). Feeders distribute bulk electrical power to subpanels, distribution boards, and motor control centers. Sizing feeders requires mastery of NEC Article 215 (Feeder Installation Rules), Article 220 (Branch-Circuit, Feeder, and Service Calculations), and Article 240 (Overcurrent Protection).

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1. Minimum Feeder Conductor Ampacity (NEC 215.2)

Feeder conductors must have an allowable ampacity sufficient to carry the connected loads without overheating the conductor insulation or exceeding the terminal temperature ratings of the equipment.

The Fundamental Feeder Sizing Equation

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

$\text{Minimum Ampacity} = (125\% \times \text{Continuous Load}) + (100\% \times \text{Non-Continuous Load})$

• Continuous Load: A load where the maximum current is expected to continue for 3 hours or more (NEC Article 100), such as commercial lighting, store display fixtures, or continuous industrial processes.
• Non-Continuous Load: Loads that operate intermittently or for durations under 3 hours, such as residential cooking appliances, water heaters, and convenience receptacles.

Exception for 100%-Rated Assemblies: Where the feeder overcurrent protective device and its complete enclosure are listed for continuous operation at 100% of their rating, the feeder conductors need only be sized for 100% of the continuous load plus 100% of the non-continuous load (NEC 215.2(A)(1)(a) Exception). Note that 100%-rated breakers require specific ventilated enclosures and 90°C conductors sized according to the 75°C ampacity table.

Terminal Temperature Limitations (NEC 110.14(C))

When looking up conductor ampacities in NEC Table 310.16, conductors must be selected based on the temperature ratings of the terminations:

• 100 Amperes or Less (or conductors #14 AWG through #1 AWG): Must be coordinated using the 60°C column unless the equipment terminals are explicitly listed and marked for 75°C.
• Over 100 Amperes (or conductors larger than #1 AWG): Coordinated using the 75°C column.
• 90°C Rated Conductors (e.g., THHN, XHHW-2): May use their higher 90°C ampacity for ambient temperature correction and conduit fill derating, but the final derated ampacity cannot exceed the termination rating (typically 75°C).

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2. Feeder Overcurrent Protection (NEC 215.3 & 240.4)

Feeders must be protected against overcurrent in accordance with NEC Article 240:

1. Rating of Device: The overcurrent device (fuse or circuit breaker) rating must not be less than 125% of the continuous load plus 100% of the non-continuous load (NEC 215.3).
2. Standard Ampere Ratings (NEC 240.6(A)): Standard sizes are 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 amperes, etc.
3. Next Standard Size Up Rule (NEC 240.4(B)): If the calculated ampacity of a feeder conductor does not correspond to a standard overcurrent device rating, the next higher standard rating may be used, provided:
   • The conductors are not part of a multi-outlet branch circuit supplying receptacles;
   • The conductor ampacity is not exceeded by more than the next higher standard rating;
   • The rating of the overcurrent device does not exceed 800 amperes.

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3. Feeder Neutral Sizing & Calculations (NEC 220.61)

The feeder neutral conductor carries the unbalanced return current between the ungrounded phase conductors. Because not all connected loads operate at maximum capacity simultaneously, the NEC permits specific demand reductions when sizing the feeder neutral.

Maximum Unbalanced Load

Under NEC 220.61(A), the feeder neutral must be sized to carry the maximum unbalanced load, defined as the maximum net calculated load between the neutral and any one ungrounded conductor.

Permissible Neutral Demand Reductions

1. Electric Cooking Appliances (NEC 220.61(B)(1)): A 70% demand factor (0.70 multiplier) is applied to the neutral load calculated for household electric ranges, wall ovens, and cooktops determined from NEC Table 220.55.
2. Electric Clothes Dryers (NEC 220.61(B)(2)): A 70% demand factor (0.70 multiplier) is applied to the neutral load calculated for electric dryers determined from NEC Table 220.54.
3. Unbalanced Loads Exceeding 200 Amperes (NEC 220.61(B)): For 3-wire single-phase or 4-wire three-phase systems supplying linear loads, the first 200 amperes of neutral load is calculated at 100%, and the portion exceeding 200 amperes may be multiplied by a 70% demand factor: $\text{Total Neutral Load} = 200\text{ A} + [0.70 \times (\text{Unbalanced Load} - 200\text{ A})]$

Prohibitions on Neutral Reduction (NEC 220.61(C))

Reductions in neutral conductor sizing are strictly prohibited under two critical conditions:

• Non-Linear Loads (NEC 220.61(C)(2)): On 3-phase, 4-wire, wye-connected systems supplying non-linear loads (such as LED lighting drivers, fluorescent electronic ballasts, computers, servers, and variable frequency drives), third-order odd harmonics (triplen harmonics: 3rd, 9th, 15th...) do not cancel out in the neutral. Instead, triplen currents add arithmetically in the neutral conductor, frequently producing neutral currents that exceed the phase currents. Therefore, no neutral reduction is permitted for the portion of the load consisting of non-linear loads.
• Two-Phase Legs of a 3-Phase System (NEC 220.61(C)(1)): Where a 3-wire feeder is derived from two ungrounded conductors and the neutral of a 3-phase, 4-wire wye system (e.g., 208Y/120V system supplying a subpanel with two phases and neutral), the neutral carries vector sum current approximately equal to the phase current even when phases are balanced. No neutral reduction is allowed.

Minimum Feeder Grounded Conductor Size (NEC 215.2(A)(2))

The feeder grounded (neutral) conductor can never be smaller than the minimum Equipment Grounding Conductor (EGC) required by NEC Table 250.122, based on the rating of the feeder overcurrent protective device.

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4. Common Neutral Conductors (NEC 215.4)

A common neutral conductor is permitted for multiple feeders under strict conditions:

• Up to two or three sets of 3-wire feeders; or
• Two sets of 4-wire or 5-wire feeders.
• Raceway Enclosure Rule: All conductors of the common neutral feeder system must be installed in the same raceway or enclosure (NEC 215.4(B)). Running conductors in separate conduits causes severe inductive heating of metal conduit walls due to unbalanced magnetic fields.

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5. Feeder Voltage Drop Calculations

Excessive voltage drop causes motor overheating, reduced lighting output, premature ballast/driver failure, and erratic operation of microprocessor-controlled electronics.

Code Status of Voltage Drop

Voltage drop limits in the NEC are found in Informational Notes (non-mandatory guidelines unless adopted as local code amendments):

• NEC 215.2(A)(1) Informational Note No. 2: Feeder conductors sized to prevent a voltage drop exceeding 3% at the farthest outlet supplying heating, power, or lighting loads.
• Overall System Limit: Sizing conductors for an overall maximum voltage drop not exceeding 5% combined on both the feeder and the branch circuit to the farthest outlet provides reasonable efficiency of operation.

Mathematical Formulas for Voltage Drop

Voltage drop is calculated using Ohm's law ($V = I \times R$) accounting for total loop resistance. On Connecticut licensing exams, direct circular mil equations are used:

Single-Phase Voltage Drop:

$VD = \frac{2 \times K \times I \times L}{CM}$

Three-Phase Voltage Drop:

$VD = \frac{\sqrt{3} \times K \times I \times L}{CM} = \frac{1.732 \times K \times I \times L}{CM}$

Sizing for a Specific Allowable Voltage Drop (Circular Mils):

$CM_{\text{single-phase}} = \frac{2 \times K \times I \times L}{VD_{\text{allowable}}}$ $CM_{\text{three-phase}} = \frac{1.732 \times K \times I \times L}{VD_{\text{allowable}}}$

Where:

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

Conductor Areas and Properties (NEC Chapter 9, Table 8)

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6. Comprehensive Worked Voltage Drop Calculations

Worked Example 1: Single-Phase Residential Subpanel Feeder

Scenario: A 240V single-phase subpanel feeder supplies an 80-ampere non-continuous load located 150 feet from the main service panelboard using copper THHN conductors. Sizing must not exceed a 3% feeder voltage drop.

1. Calculate Maximum Allowable Voltage Drop: $VD_{\text{allowable}} = 240\text{ V} \times 0.03 = 7.2\text{ Volts}$

2. Calculate Required Circular Mil Area: $CM = \frac{2 \times K \times I \times L}{VD_{\text{allowable}}} = \frac{2 \times 12.9 \times 80\text{ A} \times 150\text{ ft}}{7.2\text{ V}}$ $CM = \frac{309,600}{7.2} = 43,000\text{ Circular Mils}$

3. Select Conductor from NEC Chapter 9, Table 8:

   • #4 AWG copper has 41,740 cmil (less than 43,000 cmil; voltage drop would exceed 3%).
   • #3 AWG copper has 52,620 cmil (adequate).

4. Verify Conductor Ampacity in NEC Table 310.16:

   • #3 AWG THHN copper at 75°C has an ampacity of 85 amperes, which satisfies the 80-ampere continuous/non-continuous load requirement.
   • Conclusion: Install #3 AWG Copper.

Worked Example 2: Three-Phase Commercial Feeder

Scenario: A 208Y/120V 3-phase, 4-wire feeder carries a balanced load of 100 amperes of continuous commercial kitchen equipment over a distance of 200 feet using copper conductors. Voltage drop cannot exceed 3%.

1. Determine Sizing Current (NEC 215.2): $I_{\text{design}} = 100\text{ A} \times 1.25 = 125\text{ Amperes}$ From Table 310.16, 125A requires a minimum #1 AWG Copper (75°C rating = 130A).
2. Calculate Maximum Allowable Voltage Drop: $VD_{\text{allowable}} = 208\text{ V} \times 0.03 = 6.24\text{ Volts}$
3. Calculate Required Circular Mils at 100A Operating Current: $CM = \frac{1.732 \times 12.9 \times 100\text{ A} \times 200\text{ ft}}{6.24\text{ V}} = \frac{446,856}{6.24} = 71,611.5\text{ cmil}$
4. Select Conductor:
   • #1 AWG has 83,690 cmil, which exceeds 71,611 cmil.
   • Conclusion: #1 AWG Copper satisfies both ampacity and voltage drop requirements.