acalculator

What is the voltage drop?

Pick the wire metal and AWG size, type the supply voltage, the load current and the one-way length. The voltage drop calculator shows the volts lost in the wires, the percent drop and the voltage left at the load.

Your numbers

Units
Conductor
Circuit
Temperature and parallel runs
Voltage drop
7.73 V

Over 100 ft of 12 AWG wire at 20 A, the voltage drop is 7.73 V (6.44% of 120 V), leaving 112.27 V at the load.

Voltage drop, percent
6.44%
Voltage at the load
112.27 V
Resistance, Ω per 1,000 ft
1.9316
Resistance, Ω per km
6.3371
Diameter, inches
0.0808
Area, mm²
3.309

Voltage drop: 7.73 V. Over 100 ft of 12 AWG wire at 20 A, the voltage drop is 7.73 V (6.44% of 120 V), leaving 112.27 V at the load.

How to calculate

Computes the voltage drop, the percent drop and the voltage at the load for a copper or aluminium circuit, from the AWG wire size, the one-way length, the current, the supply voltage and the conductor temperature.

Example with the default inputs (Conductor Copper, Wire size (AWG) 12 AWG, Circuit DC or single-phase, Supply voltage 120 V, Load current 20 A, One-way length 100 ft, Conductors in parallel 1, Conductor temperature 75 °C): Over 100 ft of 12 AWG wire at 20 A, the voltage drop is 7.73 V (6.44% of 120 V), leaving 112.27 V at the load.

Method: d = 0.005 in × 92^((36 − n) ÷ 39); R = ρ₂₀ × (1 + α × (T − 20 °C)) ÷ (π d² ÷ 4); drop = k × L × I × R ÷ conductors in parallel, with k = 2 (DC or single-phase) or √3 (three-phase).

  • Solid round wire at the AWG diameter; stranded wire of the same gauge has a little more resistance.
  • Copper is annealed copper of 100% IACS, 1/58 Ω·mm²/m at 20 °C with α = 0.00393; aluminium is 61% IACS, 0.028264 Ω·mm²/m with α = 0.00403.
  • AC circuits count resistance only: no inductive reactance, skin effect or power factor, which add a little drop in large wires.
  • The load draws the typed current whatever the voltage at its end.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Conductor Copper, Wire size (AWG) 12 AWG, Circuit DC or single-phase, Supply voltage 120 V, Load current 20 A, One-way length 100 ft, Conductors in parallel 1, Conductor temperature 20 °C gives Resistance, Ω per 1,000 ft 1.588, Diameter, inches 0.0808.Source: NBS Handbook 100, Copper Wire Tables (1966), Table 1: 12 AWG, 80.8 mils, 1.588 Ω per 1,000 ft at 20 °C
  2. Conductor Copper, Wire size (AWG) 10 AWG, Circuit DC or single-phase, Supply voltage 120 V, Load current 20 A, One-way length 100 ft, Conductors in parallel 1, Conductor temperature 20 °C gives Resistance, Ω per 1,000 ft 0.9989.Source: NBS Handbook 100, Copper Wire Tables (1966), Table 1: 10 AWG, 0.9989 Ω per 1,000 ft at 20 °C (rounded table value)
  3. Conductor Copper, Wire size (AWG) 12 AWG, Circuit DC or single-phase, Supply voltage 120 V, Load current 20 A, One-way length 100 ft, Conductors in parallel 1, Conductor temperature 75 °C gives Resistance, Ω per 1,000 ft 1.931555, Voltage drop 7.73 V, Voltage drop, percent 6.438518%, Voltage at the load 112.27 V.Source: ASTM B258 diameter formula
  4. Conductor Aluminium, Wire size (AWG) 4/0 AWG, Circuit Three-phase, Supply voltage 480 V, Load current 200 A, One-way length 250 ft, Conductors in parallel 1, Conductor temperature 75 °C gives Diameter, inches 0.46, Voltage drop 8.5 V, Voltage drop, percent 1.770972%.Source: ASTM B258 (4/0 AWG = 0.4600 in)
  5. Conductor Copper, Wire size (AWG) 6 AWG, Circuit DC or single-phase, Supply voltage 230 V, Load current 32 A, One-way length 98.43 ft, Conductors in parallel 1, Conductor temperature 75 °C gives Resistance, Ω per km 1.57634, Voltage drop 3.03 V, Voltage drop, percent 1.315901%.Source: ASTM B258

How it works

  1. Diameter of AWG size n (ASTM B258): d = 0.005 in × 92^((36 − n) ÷ 39), for every size from 20 AWG to 4/0. The sizes 1/0, 2/0, 3/0 and 4/0 are n = 0, −1, −2 and −3.
  2. Cross-section: A = π × d² ÷ 4.
  3. Resistance per metre at conductor temperature T: R = ρ₂₀ × (1 + α × (T − 20 °C)) ÷ A.
    • Copper: ρ₂₀ = 1/58 Ω·mm²/m = 1.7241 × 10⁻⁸ Ω·m, α = 0.00393 per °C.
    • Aluminium: ρ₂₀ = 0.028264 Ω·mm²/m, α = 0.00403 per °C.
  4. Voltage drop = k × L × I × R ÷ N, where L is the one-way length, I the current, N the conductors in parallel per leg, and k = 2 for DC or single-phase AC or √3 for three-phase AC (with the line-to-line voltage).
  5. Percent drop = drop ÷ supply voltage × 100; voltage at the load = supply − drop.

The page also shows R per 1,000 ft and per km, the diameter in inches, and the cross-section in mm², for one conductor at T. Temperatures in °F are converted exactly: °C = (°F − 32) ÷ 1.8.

No answer. When the drop would be the whole supply voltage or more, the calculator shows: "The voltage drop would be the whole supply voltage or more: use a larger wire, a shorter run or less current."

Rounding. Volts show with at most 2 decimals, the percent drop with 2, resistances and the diameter with 4, the cross-section with 3, rounded half up, with trailing zeros left off.

Assumptions

  • Solid round wire at the AWG diameter; stranded wire has a little more resistance.
  • AC circuits count resistance only: no reactance, skin effect or power factor.
  • The load draws the typed current whatever the voltage at its end.
  • Supply up to 1,000 kV, current up to 100,000 A, length up to 100 km, 1 to 20 conductors in parallel, conductor temperature from −50 °C to 150 °C.

Worked examples by hand

12 AWG copper at 20 °C. d = 0.005 × 92^(24/39) = 0.08081 in = 2.0525 mm; A = 3.3088 mm². R = 0.017241 ÷ 3.3088 = 0.0052108 Ω/m = 1.5883 Ω per 1,000 ft (the wire table: 1.588).

10 AWG copper at 20 °C. d = 0.1019 in, A = 5.2612 mm², R = 0.9989 Ω per 1,000 ft.

A 20 A load 100 ft from a 120 V panel, 12 AWG copper at 75 °C. R = 1.5883 × (1 + 0.00393 × 55) = 1.9316 Ω per 1,000 ft. Drop = 2 × 100 × 20 × 1.9316 ÷ 1,000 = 7.73 V, which is 6.44%, leaving 112.27 V.

A three-phase 480 V, 200 A feeder, 4/0 aluminium, 250 ft, 75 °C. d = 0.46 in, A = 107.22 mm², R = 0.028264 × (1 + 0.00403 × 55) ÷ 107.22 = 0.00032204 Ω/m = 0.098157 Ω per 1,000 ft. Drop = √3 × 250 × 200 × 0.098157 ÷ 1,000 = 8.50 V = 1.77%.

A 32 A load 30 m from a 230 V supply, 6 AWG copper at 75 °C. A = 13.30 mm², R = 1.5763 Ω/km. Drop = 2 × 30 × 32 × 1.5763 ÷ 1,000 = 3.03 V = 1.32%.

Other questions people ask

How do I calculate voltage drop?

Multiply the wire’s resistance per foot by the length the current travels and by the current: VD = 2 × L × I × R for DC or single-phase AC, where L is the one-way length (the current goes out and back), and VD = √3 × L × I × R for three-phase. A 20 A load 100 ft away on 12 AWG copper at 75 °C (1.9316 Ω per 1,000 ft) loses 2 × 100 × 20 × 0.0019316 = 7.73 V.

What is an acceptable voltage drop?

The US National Electrical Code has a note, not a rule, that suggests at most 3% on a branch circuit and 5% in total from the service to the farthest outlet. On a 120 V circuit, 3% is 3.6 V. If the calculator shows more, try the next larger wire size.

How is the resistance of a wire found from its AWG size?

The AWG diameter is d = 0.005 in × 92^((36 − n) ÷ 39), where n is the gauge (1/0 is 0, 4/0 is −3). The cross-section is π × d² ÷ 4, and the resistance per length is the metal’s resistivity divided by that area. 12 AWG copper is 0.0808 in across and has 1.588 Ω per 1,000 ft at 20 °C, as the classic copper wire tables list.

Why does temperature matter?

Metals conduct worse when hot. Copper’s resistance rises by 0.393% for each °C above 20 °C, aluminium’s by 0.403%. Building wire rated 75 °C, used at that temperature, has about 22% more resistance than at 20 °C, so the calculator starts at 75 °C.

Does aluminium wire have more voltage drop than copper?

Yes, for the same size. Aluminium conducts about 61% as well as copper, so its resistance is about 1.64 times as high. That is why an aluminium conductor is usually one or two sizes larger than a copper one for the same load.

Why does this answer differ a little from the NEC tables?

The calculator uses the AWG formula for solid round wire and the resistivity of the metal, not the NEC Chapter 9 tables. Stranded conductors and coated wire in the tables have slightly more resistance, and AC tables add reactance. For solid 12 AWG copper at 75 °C the formula gives 1.93 Ω per 1,000 ft, the same as the table’s solid value.