5.1 Feeder Sizing & Voltage Drop Calculations

5.1 Feeder Sizing & Voltage Drop Calculations

Quick Reference: 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). Sizing feeder conductors requires a two-step engineering verification: (1) calculating thermal ampacity under NEC Article 215 and Article 220 (accounting for $125%$ continuous loads and terminal ratings under NEC 110.14(C)), and (2) verifying that feeder length and load do not cause excessive voltage drop that impairs equipment operation or violates mandatory energy conservation codes.

1. Feeder Conductor Ampacity Sizing (NEC 215.2)

Feeder conductors must be sized to carry the total calculated load without exceeding their temperature ratings. The basic ampacity rules for feeders operating at $1000\text{ V}$ or less are established in NEC 215.2(A).

+-----------------------------------------------------------------------------+
|                   NEC 215.2 FEEDER CONDUCTOR SIZING RULES                   |
+-----------------------------------------------------------------------------+
|  BASELINE RULE (NEC 215.2(A)(1)(a)):                                        |
|  Minimum Ampacity = (1.25 x Continuous Load) + (1.00 x Non-Continuous Load) |
|                                                                             |
|  100% RATED ASSEMBLY EXCEPTION (NEC 215.2(A)(1)(b)):                        |
|  If the overcurrent protective device and switchboard/panelboard assembly   |
|  are listed for 100% continuous operation:                                  |
|  Minimum Ampacity = 1.00 x (Continuous Load + Non-Continuous Load)          |
|                                                                             |
|  TERMINAL TEMPERATURE LIMITATIONS (NEC 110.14(C)):                          |
|  • <= 100A (or 14 AWG to 1 AWG): Size from 60°C column (unless 75°C marked)|
|  • > 100A (or larger than 1 AWG): Size from 75°C column                     |
|  • Derating adjustments (temperature/fill) start from 90°C column (THHN)    |
+-----------------------------------------------------------------------------+

Definitions and Core Requirements

• Continuous Load (Article 100): A load where the maximum current is expected to continue for 3 hours or more (e.g., commercial office lighting, fixed electric space heating, commercial refrigeration, electric vehicle charging stations).
• Non-Continuous Load: Loads that cycle on and off or operate for less than 3 hours consecutively (e.g., general convenience receptacles, waste disposers, residential water heaters, elevators).
• Minimum Feeder Size Floor (NEC 215.2(A)(2)–(3)):
  • Feeder conductors supplying a 2-wire branch circuit from a 30A or larger supply must be at least 10 AWG Copper.
  • Feeders supplying two or more 2-wire branch circuits must be at least 10 AWG Copper.
  • Feeders supplying three or more 2-wire branch circuits must be at least 8 AWG Copper.
  • Feeders supplying a single-family dwelling unit where the feeder serves $100%$ of the dwelling load must have an ampacity of not less than 100A 3-wire (NEC 215.2(A)(3) / 230.79(C)).

2. Minimum Feeder Grounded (Neutral) Conductor Sizing (NEC 215.2(A)(2))

The feeder grounded conductor (neutral) serves two critical functions: carrying line-to-neutral load unbalance and providing an equipment grounding reference for downstream systems.

Neutral Sizing Criteria:

1. Calculated Unbalanced Load (NEC 220.61): The feeder neutral must have an allowable ampacity sufficient to carry the maximum calculated unbalanced load determined in accordance with NEC Article 220 Part III.
2. The Equipment Grounding Conductor Floor (NEC 215.2(A)(2)): Where a feeder supplies branch circuits that require an equipment grounding conductor (EGC) under Article 250, the feeder grounded conductor shall not be smaller than the minimum size EGC required by NEC Table 250.122 for the feeder overcurrent protective device rating.

+-----------------------------------------------------------------------------+
|                  FEEDER NEUTRAL CONDUCTOR SIZING DUAL-CHECK                 |
|                                                                             |
|   [Check 1: Calculated Load]        [Check 2: Table 250.122 Safety Floor]   |
|   Calculate unbalanced load         Lookup feeder OCPD rating in Table      |
|   under NEC 220.61 (100% of         250.122 to determine minimum EGC wire   |
|   120V loads + 70% cooking/dryer)   gauge.                                  |
|                 \                                 /                         |
|                  \                               /                          |
|                   [SELECT LARGER OF THE TWO VALUES]                         |
+-----------------------------------------------------------------------------+

[!IMPORTANT] Plan Review Example — Neutral Floor Check: A 400A feeder supplies a subpanel with a calculated unbalanced neutral load of only $45\text{ A}$ (which would thermally fit on an $8\text{ AWG Cu}$ conductor rated at $50\text{A}$). Under Table 250.122, a 400A overcurrent protective device requires a minimum 3 AWG Copper equipment grounding conductor. Therefore, the plans examiner must require the feeder neutral to be at least 3 AWG Copper, rejecting the 8 AWG design.

3. Voltage Drop Physics & Code Recommendations

When electrical current flows through a conductor, the conductor's internal resistance ($R$) and inductive reactance ($X_L$) cause an electrical potential loss between the source and the load, known as voltage drop ($VD = I \times Z$). Excessive voltage drop causes:

• Motor overheating, reduced starting torque ($T \propto V^2$), and nuisance thermal overload tripping.
• Fluorescent and LED driver flicker, dimming, and premature ballast/driver failure.
• Overheating of electronic power supplies and data server reboot cycles.
• Significant wasted energy dissipated as heat across the building's raceway system.

The Fundamental 3% / 5% Rule

While the National Electrical Code is primarily a safety standard and not an efficiency manual, it provides clear voltage drop benchmarks in its Informational Notes:

• NEC 215.2(A) Informational Note No. 2: Recommends that feeder conductors be sized to provide a maximum voltage drop of 3% at the farthest outlet supplying heating, power, or lighting loads.
• NEC 210.19(A) Informational Note No. 4: Recommends that branch-circuit conductors be sized for a maximum voltage drop of 3%.
• Total Combined System Limit: The maximum total combined voltage drop on both the feeder and the branch circuit to the farthest outlet should not exceed 5% of the nominal system voltage.

+-----------------------------------------------------------------------------+
|                     VOLTAGE DROP THRESHOLDS BY NOMINAL VOLTAGE              |
+-----------------------+-----------------------+-----------------------------+
| Nominal System Voltage| 3% Maximum Feeder Drop| 5% Maximum Total System Drop|
+-----------------------+-----------------------+-----------------------------+
| 120 V (1-Phase L-N)   | 3.60 Volts            | 6.00 Volts                  |
| 208 V (3-Phase L-L)   | 6.24 Volts            | 10.40 Volts                 |
| 240 V (1-Phase L-L)   | 7.20 Volts            | 12.00 Volts                 |
| 277 V (1-Phase L-N)   | 8.31 Volts            | 13.85 Volts                 |
| 480 V (3-Phase L-L)   | 14.40 Volts           | 24.00 Volts                 |
+-----------------------+-----------------------+-----------------------------+

When Voltage Drop Becomes Mandatory

Although NEC Informational Notes are non-mandatory guidance in general applications, plans examiners must enforce voltage drop under the following statutory conditions:

1. Energy Conservation Codes (IECC / ASHRAE 90.1): International Energy Conservation Code (IECC Section C405) and ASHRAE 90.1 mandate that feeders shall not exceed 2% voltage drop and branch circuits shall not exceed 3% voltage drop at rated design load.
2. Fire Pump Circuits (NEC 695.6(C)): Conductor voltage drop is strictly limited to 15% maximum during motor locked-rotor starting conditions and 5% maximum under $115%$ of normal pump motor full-load running current.
3. Sensitive Electronic Equipment (NEC 647.4(D)): Limits total voltage drop on 120V technical power feeders and branch circuits to 1.5% branch, 2.5% combined total.
4. Solar PV System Interconnections (NEC 705.12): Grid-tied inverters require low voltage drop ($< 2%$) to prevent AC overvoltage trip errors at the utility interface.

4. Mathematical Formulas for Voltage Drop Calculations

For standard plan review calculations where power factor is near unity ($0.90$ to $1.0$) or circuits are under $1000\text{ kcmil}$, Ohm's Law direct-current constant approximations ($K$-factor method) provide precise, code-accepted results.

1. Single-Phase AC Circuits (2-Wire or 3-Wire)

In a single-phase circuit, current flows out on the ungrounded phase conductor and returns through the neutral or second phase conductor. The total circuit length is $2 \times L$ (out and back):

$VD_{1\phi} = \frac{2 \times K \times I \times L}{CM}$

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

$\%VD = \left( \frac{VD}{V_{\text{nominal}}} \right) \times 100$

2. Three-Phase AC Circuits (3-Wire or 4-Wire Balanced)

In a balanced three-phase system, current returns through the other two phase conductors displaced by $120^\circ$. The effective length multiplier is $\sqrt{3} \approx 1.732$:

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

$\text{Required } CM = \frac{\sqrt{3} \times K \times I \times L}{VD_{\text{allowable}}} = \frac{1.732 \times K \times I \times L}{VD_{\text{allowable}}}$

$\%VD = \left( \frac{VD}{V_{\text{line-to-line}}} \right) \times 100$

Where:

• $VD$ = Conductor voltage drop in Volts.
• $K$ = Conductor resistivity constant (direct current resistance of a circular mil-foot of conductor at operating temperature):
  • Copper ($75^\circ\text{C}$): $K = 12.9\ \Omega\cdot\text{cmil/ft}$
  • Aluminum ($75^\circ\text{C}$): $K = 21.2\ \Omega\cdot\text{cmil/ft}$
  • (Note: At $25^\circ\text{C}$, $K_{\text{Cu}} = 11.0$ and $K_{\text{Al}} = 18.0$, but plans examiners must always use the $75^\circ\text{C}$ values for fully loaded circuits in accordance with NEC Chapter 9 Table 8).
• $I$ = Actual or design load current in Amperes.
• $L$ = One-way length of the feeder run from source to subpanel in feet.
• $CM$ = Conductor cross-sectional area in circular mils (from NEC Chapter 9, Table 8).

5. NEC Chapter 9, Table 8 Conductor Properties Reference

Plans examiners must verify conductor cross-sectional areas using the standard values published in NEC Chapter 9, Table 8:

6. Conductor Upsizing & Equipment Grounding Conductor Adjustment (NEC 250.122(B))

A critical, frequently violated requirement in electrical engineering is the mandatory proportional adjustment of the Equipment Grounding Conductor (EGC) when phase conductors are increased in size.

The Proportional Sizing Mandate (NEC 250.122(B))

Under NEC 250.122(B), where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation (e.g., upsized for voltage drop, ambient temperature, conduit fill, or future spare capacity), wire-type equipment grounding conductors shall be increased in size proportionally according to the circular mil area of the ungrounded conductors.

$\text{Proportional Multiplier} = \frac{\text{Actual (Upsized) Phase Conductor } CM}{\text{Minimum Required Phase Conductor } CM}$

$\text{Minimum Required EGC } CM = \text{Standard Table 250.122 EGC } CM \times \text{Proportional Multiplier}$

[!CAUTION] Plan Review Pitfall — Failure to Upsize EGC: If a designer upsizes a 100A feeder's phase conductors from $3\text{ AWG Cu}$ to $1/0\text{ AWG Cu}$ to combat a 350-foot run, the designer cannot leave an $8\text{ AWG Cu}$ EGC in the conduit. An 8 AWG ground wire on a 350-foot run has such high total loop impedance that during a phase-to-ground short circuit, fault current is severely choked, delaying or preventing breaker magnetic trip and melting the undersized EGC!

7. Step-by-Step Worked Feeder Engineering Examples

Example 1: 480V 3-Phase Industrial Feeder Sizing & Voltage Drop

• Project Parameters: A new distribution switchboard supplies a remote motor control center (MCC) located $400\text{ feet}$ away. The system is $480\text{Y}/277\text{V}$, 3-phase, 4-wire. The feeder load is $160\text{ A}$ continuous and $40\text{ A}$ non-continuous. Feeder voltage drop must not exceed $2.5%$.
• Step 1: Calculate Minimum Required Ampacity (NEC 215.2(A)(1)) $\text{Minimum Ampacity} = (1.25 \times 160\text{ A}) + (1.00 \times 40\text{ A}) = 200\text{ A} + 40\text{ A} = \mathbf{240\text{ A}}$
• Step 2: Select Overcurrent Protective Device & Base Conductor (Table 310.16 at 75°C)
  • Next standard OCPD rating above $240\text{ A}$ (NEC 240.6) = $250\text{ A}$ breaker.
  • Base conductor for $240\text{ A}$ from Table 310.16 at 75°C = $250\text{ kcmil Copper}$ ($255\text{ A}$ rating).
  • Base EGC from Table 250.122 for a 250A breaker = $4\text{ AWG Copper}$ ($41,740\text{ CM}$).
• Step 3: Calculate Voltage Drop on Base Conductor ($250\text{ kcmil Cu} = 250,000\text{ CM}$)
  • Total operating load current = $160\text{ A} + 40\text{ A} = 200\text{ A}$.
  • Maximum allowable voltage drop = $480\text{ V} \times 0.025 = 12.0\text{ V}$. $VD = \frac{1.732 \times 12.9 \times 200\text{ A} \times 400\text{ ft}}{250,000\text{ CM}} = \frac{1,787,424}{250,000} = 7.15\text{ Volts}$ $\%VD = \left( \frac{7.15\text{ V}}{480\text{ V}} \right) \times 100 = \mathbf{1.49\%}$
  • Result: $1.49% < 2.5%$. The $250\text{ kcmil Cu}$ conductor complies without needing further upsizing.

Example 2: 120/240V Single-Phase Remote Feeder with Conductor & EGC Upsizing

• Project Parameters: A 100A, 120/240V single-phase subpanel in a detached commercial storage building is located $300\text{ feet}$ from the main service. The design load is $70\text{ A}$ at $240\text{V}$. Voltage drop on the feeder must not exceed $3.0%$ ($7.20\text{ V}$). Copper THHN conductors in PVC.
• Step 1: Determine Minimum Base Conductors (NEC Table 310.16 & Table 250.122)
  • Minimum conductor for 100A OCPD = $3\text{ AWG Copper}$ ($100\text{ A}$ rating, $52,620\text{ CM}$ from Table 8).
  • Minimum standard EGC for 100A OCPD from Table 250.122 = $8\text{ AWG Copper}$ ($16,510\text{ CM}$ from Table 8).
• Step 2: Check Voltage Drop on Minimum Base 3 AWG Conductor $VD = \frac{2 \times 12.9 \times 70\text{ A} \times 300\text{ ft}}{52,620\text{ CM}} = \frac{541,800}{52,620} = 10.30\text{ Volts} \quad \left( \%VD = \frac{10.30}{240} \times 100 = 4.29\% \right)$
  • Result: $4.29% > 3.0%$. The 3 AWG conductor fails the voltage drop limit and must be upsized.
• Step 3: Calculate Required Circular Mils for 3% Drop ($VD_{\text{max}} = 7.20\text{ V}$) $\text{Required } CM = \frac{2 \times 12.9 \times 70\text{ A} \times 300\text{ ft}}{7.20\text{ V}} = \frac{541,800}{7.20} = \mathbf{75,250\text{ CM}}$
  • Refer to Table 8: Next standard size larger than $75,250\text{ CM}$ is $1/0\text{ AWG Copper}$ ($105,600\text{ CM}$).
• Step 4: Proportionately Upsize the Equipment Grounding Conductor (NEC 250.122(B)) $\text{Proportional Multiplier} = \frac{\text{Upsized Phase } CM}{\text{Original Base Phase } CM} = \frac{105,600\text{ CM (1/0 AWG)}}{52,620\text{ CM (3 AWG)}} = 2.0068$ $\text{Required EGC } CM = 16,510\text{ CM (8 AWG base)} \times 2.0068 = \mathbf{33,132\text{ CM}}$
  • Refer to Table 8: Next standard conductor size $\ge 33,132\text{ CM}$ is $4\text{ AWG Copper}$ ($41,740\text{ CM}$).
• Final Approved Feeder Schedule: (2) 1/0 AWG Cu Phase, (1) 1/0 AWG Cu Neutral, (1) 4 AWG Cu EGC in 1-1/2" Schedule 40 PVC conduit.

8. Plans Examiner Verification Checklist & Review Traps

+-----------------------------------------------------------------------------+
|                    E3 PLANS EXAMINER FEEDER REVIEW CHECKLIST                            |
+---+-------------------------------------------------------------------------------------+
| [ ] | Continuous Load Multiplier (215.2): Verify 125% factor applied to all continuous  |
|     | loads (lighting, heating, EVSE) in feeder ampacity calculations.                  |
+---+-------------------------------------------------------------------------------------+
| [ ] | Temperature Terminal Rating (110.14(C)): Verify ampacity selection matches 75°C   |
|     | rating for circuits >100A; confirm deratings start from 90°C column.              |
+---+-------------------------------------------------------------------------------------+
| [ ] | Neutral Sizing Dual-Check (215.2(A)(2)): Confirm neutral satisfies 220.61 AND is  |
|     | not smaller than the Table 250.122 EGC floor for the feeder breaker.              |
+---+-------------------------------------------------------------------------------------+
| [ ] | 3% Feeder / 5% Total Voltage Drop: Confirm calculations on all feeders exceeding  |
|     | 100 ft length, particularly for 208V, 240V, and 120V circuits.                    |
+---+-------------------------------------------------------------------------------------+
| [ ] | EGC Proportional Upsizing (250.122(B)): Whenever phase conductors are upsized for |
|     | voltage drop, verify that the wire-type EGC is enlarged by the exact CM ratio.    |
+---+-------------------------------------------------------------------------------------+
| [ ] | Energy Code Mandates (IECC/ASHRAE): Check for compliance with 2% feeder / 3%      |
|     | branch circuit mandatory limits where local jurisdiction has adopted energy codes.|
+---+-------------------------------------------------------------------------------------+

[!WARNING] Common Plan Review Trap — Sizing Feeders by Breaker Rating Alone: Designers frequently size feeder conductors based solely on the upstream circuit breaker nameplate rather than performing the actual Article 220 continuous plus non-continuous load calculation. For instance, a 200A feeder breaker carrying 170A of continuous lighting load requires a conductor ampacity of $170\text{ A} \times 1.25 = 212.5\text{ A}$. Installing a $3/0\text{ AWG Cu}$ conductor ($200\text{A}$ at 75°C) is a code violation under NEC 215.2(A)(1), even though the breaker is 200A; the installation requires at least $4/0\text{ AWG Cu}$ ($230\text{A}$) and a $225\text{A}$ or $250\text{A}$ breaker.