3.4 Feeder Sizing, Overcurrent Protection & Voltage Drop

3.4 Feeder Sizing, Overcurrent Protection & Voltage Drop

Feeders comprise all circuit conductors between the service equipment (or the generator/transformer source of a separately derived system) and the final branch-circuit overcurrent device. Master electrical calculations on the Wisconsin Journeyman exam require precise application of NEC Article 215 (Feeders), NEC Article 220 (Calculations), NEC Article 240 (Overcurrent Protection), and the fundamental physics of voltage drop.

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1. Feeder Sizing Fundamentals (NEC 215.2)

Under NEC 215.2(A)(1), feeder conductors must have an allowable ampacity not less than the minimum required to supply the calculated load. The general formula for determining minimum feeder conductor ampacity is:

$\text{Minimum Feeder Ampacity} = 1.00 \times (\text{Non-continuous Loads}) + 1.25 \times (\text{Continuous Loads})$

A continuous load is defined in NEC Article 100 as a load where the maximum current is expected to continue for 3 hours or more (e.g., commercial retail lighting, office fluorescent/LED banks, space heating, and water heating). Non-continuous loads operate intermittently.

Worked Example: Commercial Feeder Sizing

An electrical feeder supplies a subpanel in a commercial office building with the following balanced 208Y/120V loads:

• Non-continuous receptacle and office equipment load: 45 amperes
• Continuous general office lighting load: 80 amperes

1. Calculate the minimum conductor ampacity: $\text{Minimum Ampacity} = 45\text{ A} + (1.25 \times 80\text{ A}) = 45\text{ A} + 100\text{ A} = 145\text{ Amperes}$

2. Conductor Selection from NEC Table 310.16 (75°C Copper THHN):

   • 1 AWG Copper = 130 Amperes (Insufficient: $130\text{ A} < 145\text{ A}$)
   • 1/0 AWG Copper = 150 Amperes (Compliant: $150\text{ A} \ge 145\text{ A}$)

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2. Feeder Neutral Sizing & Reductions (NEC 220.61)

The feeder neutral conductor must be sized to carry the maximum unbalanced load determined between the neutral and any one ungrounded conductor under NEC 220.61(A).

Permitted Neutral Reductions (NEC 220.61(B))

To reflect practical diversity in load usage, the code permits specific demand factors when calculating feeder neutral conductors:

1. Cooking Appliances & Dryers: A demand factor of 70% (0.70) is permitted for the neutral load of electric ranges, wall ovens, counter-mounted cooking units, and electric clothes dryers.
2. Neutral Loads Over 200 Amperes: For feeders supplying 3-wire single-phase or 4-wire three-phase systems, that portion of the unbalanced neutral load in excess of 200 amperes is permitted to be reduced by a 70% demand factor:
   • First 200 Amperes of neutral load: Calculated at 100% (200 A).
   • Portion over 200 Amperes: Calculated at 70% (0.70).

Worked Example: Feeder Neutral Derating Over 200A

A multi-family residential building feeder has a calculated maximum linear unbalanced neutral load of 360 amperes.

1. First 200 amperes @ 100%: $200\text{ A} \times 1.00 = 200\text{ Amperes}$
2. Portion exceeding 200 amperes: $360\text{ A} - 200\text{ A} = 160\text{ Amperes}$
3. Apply 70% demand factor to excess: $160\text{ A} \times 0.70 = 112\text{ Amperes}$
4. Total minimum neutral ampacity: $\text{Neutral Ampacity} = 200\text{ A} + 112\text{ A} = 312\text{ Amperes}$

Prohibited Neutral Reductions (NEC 220.61(C))

Reductions to the feeder neutral conductor are strictly prohibited under two circumstances:

• Nonlinear Loads: No neutral reduction is allowed for circuits supplying electronic data processing equipment, LED electronic drivers, computers, or fluorescent lighting with electronic ballasts. These loads produce 3rd, 9th, and 15th harmonic currents (triplen harmonics) that do not cancel in the neutral but add arithmetically, causing neutral current to exceed phase current.
• 3-Wire from 4-Wire, 3-Phase Systems: Where a 3-wire circuit is derived from a 4-wire, 3-phase wye-connected system (consisting of two phase conductors and the neutral), the common neutral carries approximately the same current as the phase conductors and cannot be reduced.

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3. Overcurrent Protection & The Next Higher Standard Rating (NEC 240)

Standard Overcurrent Device Ratings (NEC Table 240.6(A))

Journeyman candidates must memorize the standard ampere ratings for fuses and fixed-trip inverse-time circuit breakers:

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

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

Where the allowable ampacity of a conductor does not correspond to a standard ampere rating in Table 240.6(A), the next higher standard rating OCPD is permitted to protect the conductor, provided ALL THREE of the following conditions are met:

1. The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
2. The ampacity of the conductors does not correspond to a standard rating in Table 240.6(A).
3. The next higher standard rating selected does not exceed 800 amperes.

[!WARNING] The 800-Ampere Ceiling (NEC 240.4(C)): If a conductor's ampacity exceeds 800 amperes and does not correspond to a standard rating, you cannot round up. The overcurrent device rating must be rounded DOWN to a standard rating equal to or less than the conductor ampacity!

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4. Feeder Tap Rules (NEC 240.21(B))

As a foundational rule under NEC 240.21, overcurrent protection must be provided at the point where conductors receive their supply. Feeder taps represent specific exceptions permitting smaller tap conductors to be connected to a larger feeder without overcurrent protection at the tap point:

[!CAUTION] No Taps of Taps: You can never tap a tap conductor. An electrical tap must originate directly from a feeder protected by an overcurrent device.

5. Voltage Drop Calculations & Formulas

While the National Electrical Code does not make voltage drop limits strictly mandatory in Article 210/215 text, Informational Note No. 2 to NEC 210.19(A) and Informational Note No. 2 to NEC 215.2(A)(1) establish design recommendations for operational efficiency and equipment longevity:

• Branch Circuit Voltage Drop: Maximum 3%.
• Feeder Voltage Drop: Maximum 3%.
• Total Combined Voltage Drop: Maximum 5% from the service disconnect to the farthest outlet.

The Ohmic Formulas for Voltage Drop

$\text{Single-Phase Voltage Drop: } VD = \frac{2 \times K \times I \times L}{CM}$

$\text{Three-Phase Voltage Drop: } VD = \frac{\sqrt{3} \times K \times I \times L}{CM} = \frac{1.732 \times K \times I \times L}{CM}$

To solve for the required conductor cross-sectional area in Circular Mils (CM) to satisfy a target voltage drop limit:

$CM_{\text{Single-Phase}} = \frac{2 \times K \times I \times L}{VD_{\text{allowed}}}$

$CM_{\text{Three-Phase}} = \frac{1.732 \times K \times I \times L}{VD_{\text{allowed}}}$

Formula Variables Defined:

• $K$ (Resistivity Constant): Specific resistance of conductor material in ohms-circular mil per foot at 75°C:
  • Copper ($K$) = 12.9 $\Omega\cdot\text{cmil/ft}$
  • Aluminum ($K$) = 21.2 $\Omega\cdot\text{cmil/ft}$
• $I$: Actual circuit design current in amperes.
• $L$: One-way length of circuit from source to load in feet (do NOT double $L$; the factor of 2 in the single-phase equation already accounts for the return conductor).
• $CM$: Conductor area in circular mils (lookup in NEC Chapter 9, Table 8).
• $VD$: Allowable voltage drop in volts ($V_{\text{nominal}} \times %VD$).

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6. Step-by-Step Worked Voltage Drop Calculation

Exam Problem:

A 240-volt, single-phase subpanel feeder carries a continuous load of 40 amperes over a one-way distance of 150 feet using copper conductors. Calculate the maximum permitted voltage drop in volts for a 3% limit, determine the minimum conductor size in circular mils, and select the appropriate conductor from NEC Chapter 9, Table 8.

Step 1: Calculate Maximum Permissible Voltage Drop ($VD$)

$VD_{\text{allowed}} = 240\text{ Volts} \times 0.03 = 7.2\text{ Volts}$

Step 2: Calculate Required Circular Mils ($CM$)

Using $K = 12.9$, $I = 40\text{ A}$, $L = 150\text{ ft}$, and $VD = 7.2\text{ V}$:

$CM = \frac{2 \times K \times I \times L}{VD_{\text{allowed}}}$

$CM = \frac{2 \times 12.9 \times 40 \times 150}{7.2}$

$CM = \frac{154,800}{7.2} = 21,500\text{ Circular Mils}$

Step 3: Conductor Selection from NEC Chapter 9, Table 8

Examine the Conductor Properties table for uncoated copper:

• 8 AWG Copper: $16,510\text{ CM}$ ($16,510 < 21,500$ — violates 3% limit)
• 6 AWG Copper: $26,240\text{ CM}$ ($26,240 > 21,500$ — compliant!)

Answer: Select 6 AWG Copper conductors.

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Common Exam Traps & Pitfalls

1. Doubling the Distance in Voltage Drop: The single-phase formula ($2 \times K \times I \times L$) already includes the factor of 2. Doubling the distance manually produces a 4x conductor over-sizing error.
2. Single-Phase vs. Three-Phase Constant: Using 2 for a three-phase calculation instead of $1.732$ ($\sqrt{3}$) will yield incorrect circular mil values on exam calculation items.
3. 800A Round-Up Boundary: Remembering that NEC 240.4(B) allows rounding up to the next standard breaker ONLY up to 800A. At 801A or above, you must round DOWN per NEC 240.4(C).
4. Feeder Tap Rules: Attempting to use the 10-foot tap rule with conductors rated at less than 10% of the feeder breaker, or the 25-foot tap rule with conductors rated at less than 33.3% of the feeder breaker.