5.1 Feeder Conductor Sizing, Neutral Calculations & Voltage Drop

5.1 Feeder Conductor Sizing, Neutral Calculations & Voltage Drop

In electrical distribution systems, a feeder consists of all circuit conductors between the service equipment (or the generator/transformer source of a separately derived system) and the final branch-circuit overcurrent device. Sizing feeder conductors requires a rigorous understanding of continuous vs. noncontinuous loads, ambient temperature and raceway fill adjustments, neutral demand factors, and voltage drop optimization.

On the Idaho Journeyman Electrician examination, feeder sizing calculations represent one of the most common and multi-step mathematical categories. Journeymen must be able to accurately navigate NEC Article 215 (Feeders), Article 220 (Branch-Circuit, Feeder, and Service Calculations), Article 250 (Grounding and Bonding), and Article 310 (Conductors for General Wiring).

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1. Feeder Conductor Sizing Fundamentals (NEC 215.2)

The primary rule for feeder conductor sizing is established in NEC 215.2(A)(1): feeder conductors must have an allowable ampacity not less than the sum of 125% of the continuous load plus 100% of the noncontinuous load.

+-----------------------------------------------------------------------------+
|                   FEEDER CONDUCTOR SIZING FORMULA (NEC 215.2)               |
|                                                                             |
|   Minimum Required Feeder Ampacity = (Continuous Load * 1.25) +             |
|                                      (Noncontinuous Load * 1.00)            |
|                                                                             |
|   [CONDITION 1: OCPD & TERMINAL SIZING (110.14(C) & 215.2(A)(1)(a))]         |
|   * Conductor ampacity must equal 125% Continuous + 100% Noncontinuous      |
|   * Evaluated against terminal temperature column (typically 75°C)          |
|                                                                             |
|   [CONDITION 2: DERATING / RACING ADJUSTMENTS (310.15 & 215.2(A)(1)(b))]    |
|   * Conductor ampacity after applying temp correction & fill adjustment     |
|     must equal 100% Continuous + 100% Noncontinuous                         |
+-----------------------------------------------------------------------------+

The Two-Step Sizing Rule

When sizing feeders subjected to ambient temperature correction or conduit fill derating, electricians must evaluate both criteria independently and select the larger conductor:

1. Step 1 (Terminal & OCPD Protection): Conductor selected from Table 310.16 at the terminal rating (typically 75°C per NEC 110.14(C)) must be sized for at least 125% of continuous load + 100% of noncontinuous load.
2. Step 2 (Conductor Ampacity After Derating): The conductor's ampacity from the 90°C column (e.g., THHN/THWN-2), multiplied by the ambient temperature correction factor (Table 310.15(B)(1)) and the conductor fill adjustment factor (Table 310.15(C)(1)), must be at least equal to 100% of continuous load + 100% of noncontinuous load.

+-----------------------------------------------------------------------------+
|                    TWO-STEP FEEDER EVALUATION WORKFLOW                      |
|                                                                             |
|   [GIVEN: 80A Continuous Load + 40A Noncontinuous Load in 40°C Ambient]     |
|                                                                             |
|   Step 1: Terminal Sizing (75°C Column, No Derating)                        |
|   - Minimum Ampacity = (80A * 1.25) + 40A = 100A + 40A = 140A               |
|   - Table 310.16 (75°C Cu) -> 1/0 AWG Cu is rated 150A (Complies >= 140A)   |
|                                                                             |
|   Step 2: Derated Ampacity Check (90°C Column Derated)                      |
|   - Required load to carry = 80A + 40A = 120A                               |
|   - 40°C correction factor for 90°C conductor = 0.91 (Table 310.15(B)(1))   |
|   - Try 1/0 AWG THHN (90°C rating = 170A):                                  |
|     Derated Ampacity = 170A * 0.91 = 154.7A (Complies >= 120A)              |
|                                                                             |
|   Conclusion: 1/0 AWG Copper THHN satisfies both code requirements.         |
+-----------------------------------------------------------------------------+

100% Rated Overcurrent Protective Devices Exception

Under the Exception to NEC 215.2(A)(1), if the feeder assembly—including the overcurrent protective device (OCPD)—is listed for continuous operation at 100 percent of its rating, the conductor ampacity does not need to be scaled up by 125% for continuous loads. However, 100%-rated breakers are virtually never used in residential or standard commercial subpanels; they are primarily found in industrial draw-out switchgear rated 400A and above.

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2. Neutral Conductor Sizing & Demand Factors (NEC 220.61)

The feeder grounded (neutral) conductor does not carry current from pure line-to-line 240V, 208V, or 480V balanced loads. Under NEC 220.61(A), the feeder neutral conductor is sized to carry the maximum unbalanced load.

Maximum Unbalanced Load

The maximum unbalanced load is defined as the maximum net calculated load between the neutral conductor and any one ungrounded (hot) conductor. In single-phase 120/240V systems, this is the total connected 120V line-to-neutral load.

+-----------------------------------------------------------------------------+
|                     NEC 220.61 NEUTRAL DEMAND FACTORS                       |
|                                                                             |
|   [ELECTRIC RANGES, OVENS & COOKING APPLIANCES (220.61(B)(1))]              |
|   * Feeder neutral load = 70% of the demand load calculated per Table 220.55|
|                                                                             |
|   [ELECTRIC CLOTHES DRYERS (220.61(B)(2))]                                  |
|   * Feeder neutral load = 70% of the demand load calculated per Table 220.54|
|                                                                             |
|   [GENERAL FEEDER NEUTRAL LOADS OVER 200 AMPERES (220.61(B))]               |
|   * First 200 Amperes of unbalanced neutral load   ---> 100% Demand Factor  |
|   * Portion of unbalanced load exceeding 200A      ---> 70% Demand Factor   |
+-----------------------------------------------------------------------------+

Prohibited Neutral Reductions (NEC 220.61(C))

Electricians must never reduce the neutral conductor size under the following specific conditions:

1. Multiwire Circuits Supplying Nonlinear Loads (220.61(C)(2)): Electric-discharge lighting (fluorescent, metal halide, LED drivers), personal computers, electronic data processing equipment, and variable frequency drives (VFDs). These nonlinear loads produce triplen harmonics (primarily the 3rd harmonic, 180 Hz), which do not cancel out in the neutral but rather add together algebraically, causing the neutral current to potentially exceed the phase currents.
2. 3-Phase, 4-Wire, Wye-Connected Circuits with 2 Phase Conductors and Neutral (220.61(C)(1)): In a 208Y/120V system where only 2 ungrounded conductors and a common neutral are derived (such as in an apartment subpanel), the neutral carries approximately the same current as the phase conductors. No reduction is permitted.

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3. Equipment Grounding Conductor (EGC) in Feeders (NEC 250.122)

Every feeder installed in a raceway or cable assembly must include or provide an equipment grounding conductor (EGC) to establish an effective ground-fault current path back to the electrical source.

Basic Sizing from Table 250.122

The minimum size of copper or aluminum EGC is determined by the rating or setting of the upstream feeder overcurrent protective device (fuse or circuit breaker) guarding the ungrounded conductors.

Mandatory Proportional Upsizing Rule (NEC 250.122(B))

Under NEC 250.122(B), where ungrounded conductors are increased in size from the minimum required for the load (e.g., upsized for voltage drop compensation or excessive length), the equipment grounding conductors must be proportionally increased in circular mil area.

+-----------------------------------------------------------------------------+
|                 PROPORTIONAL EGC UPSIZING FORMULA (250.122(B))              |
|                                                                             |
|   New EGC Circular Mils = Original EGC Circular Mils * (New Phase kcmil /   |
|                                                         Original Phase kcmil)|
|                                                                             |
|   [WORKED EXAMPLE]                                                          |
|   * Feeder OCPD: 100A Breaker. Minimum Phase Conductor: 3 AWG Cu (52,620 CM)|
|   * Table 250.122 Original EGC: 8 AWG Cu (16,510 Circular Mils)             |
|   * Phase conductors upsized for voltage drop to: 1/0 AWG Cu (105,600 CM)   |
|                                                                             |
|   Calculation:                                                              |
|   Multiplier = 105,600 CM / 52,620 CM = 2.0068                             |
|   New EGC CM = 16,510 CM * 2.0068 = 33,132 Circular Mils                    |
|                                                                             |
|   Reference Table 8, Chapter 9:                                             |
|   - 6 AWG Copper = 26,240 CM (Too small)                                    |
|   - 4 AWG Copper = 41,740 CM (Complies >= 33,132 CM)                        |
|   Selected EGC: 4 AWG Copper                                                |
+-----------------------------------------------------------------------------+

[!WARNING] Common Exam Trap on 250.122(B): PSI exam questions frequently ask for the required EGC size after stating that phase conductors were upsized two trade sizes to compensate for voltage drop on a long run. Answering with the standard Table 250.122 size (e.g., 8 AWG for a 100A breaker) is incorrect. You must calculate the circular mil ratio using Chapter 9, Table 8 and upsize the EGC accordingly.

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4. Feeder Conductor Identification & High-Leg Delta Systems

Feeder conductors must be properly identified and marked according to NEC 215.12 and NEC 110.15.

+-----------------------------------------------------------------------------+
|                     FEEDER IDENTIFICATION PROTOCOLS                         |
|                                                                             |
|   [GROUNDED (NEUTRAL) CONDUCTOR (NEC 200.6 & 215.12(A))]                    |
|   * 6 AWG and smaller: Continuous white or gray outer finish, or three      |
|     continuous white stripes along full length.                             |
|   * 4 AWG and larger: Continuous white/gray finish OR distinctive white     |
|     marking tape encasing conductor at all terminations and pull points.    |
|                                                                             |
|   [EQUIPMENT GROUNDING CONDUCTOR (NEC 250.119 & 215.12(B))]                 |
|   * Bare conductor, continuous green finish, or green with yellow stripes.  |
|                                                                             |
|   [UNGROUNDED (HOT) PHASE CONDUCTORS (NEC 215.12(C))]                       |
|   * Where premises contain multiple voltage systems (e.g., 120/208V and     |
|     277/480V), each ungrounded conductor must be identified by phase and    |
|     system voltage at all termination, connection, and splice points.       |
|   * Common 120/208V System: Black (Phase A), Red (Phase B), Blue (Phase C)  |
|   * Common 277/480V System: Brown (Phase A), Orange (Phase B), Yellow (C)   |
+-----------------------------------------------------------------------------+

High-Leg Delta Marking (NEC 110.15 & 215.12(C)(2))

On a 4-wire, delta-connected secondary where the midpoint of one phase winding is grounded to supply 120V loads (known as a 120/240V 3-phase, 4-wire high-leg delta), the phase busbar or conductor with the higher voltage to ground (208V to neutral nominal) must be durably and permanently marked with an orange finish (or other effective means such as orange tagging) at every point where a connection is made.

• In switchboards and panelboards, the high leg must occupy the B-phase (center) position (NEC 408.3(E)(1)).

+-----------------------------------------------------------------------------+
|                120/240V 3-PHASE 4-WIRE HIGH-LEG DELTA VOLTAGES              |
|                                                                             |
|                Phase A to Phase B   ================> 240V                  |
|                Phase B to Phase C   ================> 240V                  |
|                Phase A to Phase C   ================> 240V                  |
|                                                                             |
|                Phase A to Neutral (Center Tap) =====> 120V                  |
|                Phase C to Neutral (Center Tap) =====> 120V                  |
|                PHASE B (HIGH-LEG / ORANGE) TO NEUTRAL => 208V (120V * 1.732)|
+-----------------------------------------------------------------------------+

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5. Voltage Drop Sizing & Recommendations

While the National Electrical Code is primarily a safety standard and not a design manual, NEC Informational Note 215.2(A)(1) and 210.19(A) Informational Note No. 2 provide clear engineering recommendations to ensure electrical equipment operates within manufacturer voltage tolerances.

+-----------------------------------------------------------------------------+
|                     NEC VOLTAGE DROP RECOMMENDATIONS                        |
|                                                                             |
|   [FEEDER CONDUCTORS ALONE]                                                 |
|   * Maximum Recommended Voltage Drop: 3% of Nominal System Voltage          |
|                                                                             |
|   [BRANCH CIRCUIT CONDUCTORS ALONE]                                         |
|   * Maximum Recommended Voltage Drop: 3% of Nominal System Voltage          |
|                                                                             |
|   [COMBINED TOTAL: FEEDER + BRANCH CIRCUIT TO FARTHEST OUTLET]              |
|   * Maximum Total Recommended Voltage Drop: 5% of Nominal System Voltage    |
+-----------------------------------------------------------------------------+

Voltage Drop Formulas

Single-Phase Voltage Drop: V_D = (2 * K * I * L) / CM
Three-Phase Voltage Drop:  V_D = (1.732 * K * I * L) / CM

Required Conductor Area (Single-Phase): CM = (2 * K * I * L) / V_D(allowable)
Required Conductor Area (Three-Phase):  CM = (1.732 * K * I * L) / V_D(allowable)

Where:

• K = Direct current resistivity constant (K = 12.9 for Copper, K = 21.2 for Aluminum at 75°C).
• I = Load current in amperes.
• L = Length of one-way conductor run in feet.
• CM = Circular mil area of conductor (from Chapter 9, Table 8).
• V_D = Voltage drop in volts.

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6. Worked Feeder Calculation Examples

Example 1: Residential Subpanel Feeder (120/240V Single-Phase)

Problem: A feeder supplies a subpanel in a detached workshop located 150 feet from the main dwelling service panel. The calculated noncontinuous load is 45A at 240V and the continuous load is 28A at 240V. All conductors are Copper THHN in PVC conduit. Terminal ratings are 75°C. Max allowable voltage drop is 3%.

Step 1: Calculate Minimum Required Ampacity (NEC 215.2(A)(1))

Minimum Ampacity = (28 A * 1.25) + (45 A * 1.00) = 35 A + 45 A = 80 A

• From Table 310.16 (75°C Cu), a 4 AWG Cu conductor is rated 85A (satisfies the 80A requirement).
• Standard OCPD = 90A or 80A breaker (NEC 240.6).

Step 2: Voltage Drop Verification at 73A Full Load (28 A + 45 A)

• Allowable V_D = 240 V * 0.03 = 7.2 V.
• Circular Mils of 4 AWG Cu (Table 8) = 41,740 CM.

V_D = (2 * 12.9 * 73 * 150) / 41,740 = 282,510 / 41,740 = 6.77 V
Percentage Drop = (6.77 / 240) * 100% = 2.82% (Complies <= 3%)

• Result: Conductor remains 4 AWG Copper with an 8 AWG Copper EGC (Table 250.122 for 80A/90A OCPD).

Example 2: Commercial Feeder with Neutral Harmonics (208Y/120V 3-Phase)

Problem: A commercial feeder supplies a 208Y/120V subpanel dedicated entirely to 120V LED architectural luminaires and office computer workstations totaling 280 amperes of calculated balanced linear and nonlinear load. What is the minimum required neutral conductor size?

Analysis:

1. Under NEC 220.61(C)(2), because the load consists of nonlinear solid-state LED power supplies and data processing equipment, no 70% reduction over 200A is permitted.
2. The neutral conductor must carry the full 280A continuous harmonic current.
3. Continuous load sizing (215.2): 280 A * 1.25 = 350 A.
4. From Table 310.16 (75°C Copper), 500 kcmil Copper (rated 380A) is required for both the phase and neutral conductors.

[!NOTE] Idaho Field Practice Tip: When installing commercial feeders supplying EV fast charging or massive LED high-bay arrays, state electrical inspectors routinely check for proper 100% neutral sizing and verify that harmonic derating was not improperly applied.