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SZ-07Sizing · Electrical

Voltage drop calculator

Volts dropped, percent drop and the voltage left at the load for a copper or aluminum run, single- or three-phase — plus the smallest size that meets your target. Drawn to scale. Voltage drop only: ampacity is a separate check.

Voltage drop on one run

Example: a 30 A, 240 V single-phase load 150 ft from the panel on 10 AWG copper. Change any number.

drop on 10 AWG copper Example

11.18 volts dropped, 4.66 percent; 228.82 volts at the load. Smallest size for 3 percent: 8 AWG.

11.18 V dropped228.82 V at the load

Smallest size for 3%: 8 AWG copper (2.93%)

A calculation, not a code requirement.

See it drawn to scale Change the numbers

Run profile and one-line diagramDrawn to scale: distance in ft, drop in %
Voltage along the run for the example, with the one-line diagram, drawn to scaleVoltage along a 150 ft run: 240 volts at the source falling to 228.82 V at the load, a 4.66% drop with 10 AWG copper at 30 amps. The 3% and 5% NEC informational-note levels are marked as recommendations, not limits. The drop reaches the 3% target at 96.59 ft. The smallest size that meets 3% is 8 AWG.AVoltage along the runDropVolts0%2402%235.24%230.46%225.60501001505% totalTarget 3%96.59 ft4.66%BOne-line2 conductors10 AWG Cu240.00 V228.82 VSourceLoad150 ft one wayCConductor areacircles to scaleChosen10 AWG Cu10,383 cmilSmallest for 3%8 AWG Cu16,510 cmil
  • 5% total: NEC informational-note level (a recommendation)
  • Your target drop
Dropped
11.18 Vout and back
At the load
228.82 Vof 240 V
Max run, 3%
96.59 ftwith 10 AWG

2 × 12.9 × 30 A × 150 ft ÷ 10,383 cmil = 11.18 V11.18 ÷ 240 V = 4.66%

Already showing the example.

Read with the result

  • NEC informational notes suggest ≤ 3% on a branch circuit and ≤ 5% feeder + branch combined. They are recommendations, not code requirements.EC&M; IAEI (citing NEC 90.5(C)). Note numbers change between editions, so none is quoted here.
  • Voltage drop only: a size that meets the drop target may still be too small for the current, the insulation or the terminals. Check ampacity separately.EC&M notes the NEC ampacity tables don’t take voltage drop into account; the reverse holds for this formula, which ignores ampacity.
  • Estimate by the K-factor method at 75 °C (dc resistance; no power factor or conductor reactance, the same assumptions EEPower’s examples state). Methods using AC impedance, power factor and conduit type can give a different answer.IAEI Magazine (2017); EEPower (2022).
  • 10 AWG: 10,383 circular mils, computed from the AWG definition. Printed tables round the diameter first and can differ in the fourth figure.NBS Handbook 100 (1966).

Change the numbers

Phase
Conductor

14 AWG to 1000 kcmil. AWG areas from the gauge definition, kcmil at nominal area.

Source to load. The formula counts the way back.

Have kW? Convert to amps.

e.g. 120 or 240 V.

For the smallest-size line, 0.5–10%. 3% matches the branch-circuit note.

Optional: from the service to the panel this run starts at. Checks the 5% total.

Allowed drop7.20 V3% of 240 V

Drop: 4.66% · 11.18 V. Back to the result

How the math works

The K-factor method IAEI describes: dc resistance at 75 °C folded into one constant per metal, the conductor’s area in circular mils, and the one-way length of the run. No code table is reproduced; conductor areas come from the definition of the wire gauge.

  1. Conductor areaCM = (5 × 92(36 − n) ÷ 39)² for n AWG; 1/0 = 0 … 4/0 = −3kcmil sizes at their nominal area: 250 kcmil = 250,000 cmil.
  2. Volts droppedVD = 2 × K × I × L ÷ CM (single-phase)
    VD = √3 × K × I × L ÷ CM (three-phase)    K = 12.9 for copper, 21.2 for aluminum (Ω·cmil/ft at 75 °C). L is one way; the 2 counts the return path.
  3. Percent and load volts% = VD ÷ V × 100
    at the load = V − VD
  4. Smallest sizeCM needed = m × K × I × L ÷ (V × target% ÷ 100)Then the first size at or above it. m = 2 single-phase, √3 three-phase. A calculation against your target, not a code requirement.
  5. Longest runL = (V × target% ÷ 100) × CM ÷ (m × K × I)

Worked examples

  • 12 AWG Cu, 16 A, 100 ft, 120 V2 × 12.9 × 16 × 100 ÷ 6,530 = 6.32 V = 5.27%
  • 4/0 Cu, 3-phase, 150 A, 200 ft, 480 V√3 × 12.9 × 150 × 200 ÷ 211,600 = 3.17 V = 0.66%
  • 1/0 Al, 100 A, 150 ft, 240 V2 × 21.2 × 100 × 150 ÷ 105,535 = 6.03 V = 2.51%
  • Branch + feeder: 12 AWG Cu, 20 A, 50 ft, 120 V, feeder 1.5%2 × 12.9 × 20 × 50 ÷ 6,530 = 3.95 V = 3.29%; with the feeder’s 1.5%: 4.79% (within the 5% note)
  • Smallest Cu for 3%: 30 A, 150 ft, 240 V2 × 12.9 × 30 × 150 ÷ 7.2 = 16,125 cmil → 8 AWG (7.03 V = 2.93%)

Values the calculator uses

K, copper
12.9 Ω·cmil/ft at 75 °CIAEI Magazine (2017), from the NEC’s Chapter 9 Table 8 dc resistance (IAEI’s example: 1 AWG, 0.154 Ω per 1,000 ft × 83,690 cmil = 12.9). Our check from NBS Handbook 100: (10.371 + 55 × 0.0409) × 1.02 = 12.873.
K, aluminum
21.2 Ω·cmil/ft at 75 °CIAEI Magazine (2017), from the same table. Our check from NBS Handbook 109 Table 1, EC-H19 aluminum (61% IACS), with the 2% stranding allowance NBS Handbook 100 uses for copper: 17.002 × (1 + 0.00403 × 55) × 1.02 = 21.186. EC-H19 is one conductor alloy; aluminum building wire may use others, so treat the third figure as approximate.
Conductor areas
From the AWG definition: No. 0000 = 0.4600 in., No. 36 = 0.0050 in.NBS Handbook 100, Copper Wire Tables (1966), §2.1–2.2. Computed areas can differ from printed tables in the fourth significant figure (tables round the diameter first); not checked against NEC Table 8 for every size.
3% and 5%
Branch circuit ≤ 3%; feeder + branch ≤ 5% — recommendationsNEC informational notes, as quoted by EC&M (and paraphrased by EEPower). Informational notes are not requirements (NEC 90.5(C), cited by IAEI).

Conductor resistance the calculator uses

Ohms per 1,000 ft of one conductor at 75 °C: K ÷ circular mils × 1,000. Our arithmetic, not the NEC’s Chapter 9 Table 8 or 9, which are not reproduced here. IAEI reports that for any Table 8 conductor “the k-factor will roughly be 12.9 or very close to it for copper, and 21.2 or very close to it for aluminum”, so Table 8’s dc values should agree closely. We did not check Table 9’s AC values.

SizeArea (cmil)Copper, ohms per 1,000 ftAluminum, ohms per 1,000 ft
14 AWG4,1073.14125.1623
12 AWG6,5301.97553.2466
10 AWG10,3831.24242.0418
8 AWG16,5100.78141.2841
6 AWG26,2510.49140.8076
4 AWG41,7410.30900.5079
3 AWG52,6350.24510.4028
2 AWG66,3710.19440.3194
1 AWG83,6930.15410.2533
1/0 AWG105,5350.12220.2009
2/0 AWG133,0770.09690.1593
3/0 AWG167,8060.07690.1263
4/0 AWG211,6000.06100.1002
250 kcmil250,0000.05160.0848
300 kcmil300,0000.04300.0707
350 kcmil350,0000.03690.0606
400 kcmil400,0000.03230.0530
500 kcmil500,0000.02580.0424
600 kcmil600,0000.02150.0353
750 kcmil750,0000.01720.0283
1000 kcmil1,000,0000.01290.0212

Ω/kft = ohms per 1,000 ft. AWG areas from the gauge definition (NBS Handbook 100), kcmil sizes at their nominal area; printed tables round the diameter first and can differ in the fourth figure.

Before you use the number

  • An estimate by the K-factor method. Methods that use AC impedance, the load’s power factor and the raceway type can give a different answer for the same run. K assumes 75 °C conductors, so the result leans high for lightly loaded runs.
  • 3% and 5% are recommendations. They come from NEC informational notes, which are not enforceable requirements. Some articles do set mandatory limits — Article 647, sensitive electronic equipment, at 1.5% and 2.5% — and equipment instructions may require a minimum voltage.
  • Voltage drop is not ampacity. A size that meets the drop target may still be too small for the current, the insulation rating or the terminals. This calculator does not check ampacity.
  • One-way length. Measure from the source to the load along the run; the formula accounts for the way back.
  • The authority having jurisdiction decides. Read the code edition your jurisdiction enforces; this is general information, not code advice.

The formula, copper against aluminum and what the NEC says: Voltage drop explained: the K formula, 3% and 5%.

Sources

Opened and checked again October 28, 2026 (first read October 4, 2026; NBS Handbook 109 first read October 28, 2026). The worked examples and the K checks are our own arithmetic.

  1. Voltage Drop Formulas (March/April 2017)IAEI Magazine (International Association of Electrical Inspectors)
  2. Voltage Drop Calculations (September/October 2019)IAEI Magazine (International Association of Electrical Inspectors)
  3. Voltage Drop Limitations on Branch Circuits (2015)EC&M
  4. Watch Out for that Voltage Drop (2015)EC&M
  5. National Electrical Code Basics: Computing Voltage Drop in Branch Circuits and Feeders, Part 1 (2022)EEPower
  6. Copper Wire Tables, Handbook 100 (1966)National Bureau of Standards (now NIST)
  7. Aluminum Wire Tables, Handbook 109 (1972)National Bureau of Standards (now NIST), with The Aluminum Association
  8. THHN/THWN-2 spec sheet (Rev 02/2026)Cerrowire
  9. NFPA 70 (NEC) Code DevelopmentNational Fire Protection Association

  • 4.05Guide

    Voltage Drop Explained: The K Formula, 3% and 5%

    How to work voltage drop with the K-factor formula, copper vs aluminum, single vs three phase, and what the NEC's 3% and 5% notes really say.

    √3 × 12.9 × 150 × 200 ÷ 211,600 = 3.17 V

    Length
    6 min
    Updated

  • 4.04Guide

    kW to Amps: Single-Phase vs Three-Phase Math

    How to turn kW or kVA into amps on single-phase and three-phase supplies, where power factor and line-to-line voltage come in, and the nameplate check.

    Length
    6 min
    Updated

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