What is the formula for voltage drop?

For single phase, VD = 2 x K x I x L / cmil. For three phase, VD = 1.732 x K x I x L / cmil. K is 12.9 for copper and 21.2 for aluminium, I is the load current in amps, L is the one-way run length in feet, and cmil is the conductor cross-sectional area in circular mils.

How much voltage drop is acceptable?

The widely used guideline is 3% maximum on a branch circuit and 5% total for feeder plus branch circuit combined. These are recommendations rather than hard requirements in the NEC, but inspectors and engineers expect designs to meet them.

Does voltage drop use one-way or round-trip length?

Enter the one-way length. The factor of 2 in the single-phase formula already accounts for current flowing out and back, so doubling the length yourself would double the calculated drop.

What size wire do I need for a 100 ft run at 20 amps?

At 120 V single phase, 20 A over 100 ft one-way, 12 AWG copper drops roughly 4.4% which exceeds the 3% guideline. 10 AWG copper drops about 2.8% and is the usual choice for that run.

Why does aluminium need a larger conductor?

Aluminium has about 61% the conductivity of copper, so its voltage drop constant K is 21.2 against 12.9 for copper. For the same current, length and percent drop, aluminium typically needs to be one to two sizes larger.

Voltage Drop Calculator

Percent drop, end-of-run voltage, the smallest conductor that stays under 3%, and how far your current conductor can actually go.

Quick answer

Voltage drop for a single-phase circuit is VD = 2 × K × I × L / cmil, where K is 12.9 for copper and 21.2 for aluminium, I is amps, L is the one-way run in feet and cmil is the conductor area in circular mils. Three-phase replaces the 2 with 1.732. Keep branch circuits at or under 3% drop, and feeder plus branch at or under 5% combined.

Circuit inputs

Load current (A)One-way length (ft)System voltage (V)PhaseConductor sizeMaterial

VD = 2 x K x I x L / cmil

Result

Voltage drop

7.90V

Percent drop

6.58%

Voltage at load

112.1V

Max length at 3%

45ft

Over 5% — this run needs a larger conductor.

Ampacity check: 12 AWG copper is estimated at 24 A at 75 °C, against your 20 A load.

Smallest conductor that satisfies both 3% drop and ampacity: 8 AWG.

Published standard

Maximum one-way run length at 3% drop

Size	10 A	15 A	20 A	30 A	40 A	50 A	60 A	80 A	100 A
14	57	38	—	—	—	—	—	—	—
12	91	60	45	—	—	—	—	—	—
10	144	96	72	48	—	—	—	—	—
8	230	153	115	76	57	—	—	—	—
6	366	244	183	122	91	73	—	—	—
4	582	388	291	194	145	116	97	72	—
3	734	489	367	244	183	146	122	91	—
2	926	617	463	308	231	185	154	115	92
1	1167	778	583	389	291	233	194	145	116
1/0	1472	981	736	490	368	294	245	184	147
2/0	1856	1237	928	618	464	371	309	232	185
3/0	2341	1560	1170	780	585	468	390	292	234
4/0	2952	1968	1476	984	738	590	492	369	295

Copper conductors, 120 V single phase, at the 3% branch-circuit guideline. Values in feet.

Source & method: Length solved from the same formula: L = (V × 3% × cmil) / (multiplier × K × I). Cells marked — exceed the estimated ampacity of that conductor at 75 °C, so the run length is irrelevant. Ampacity estimates are described on the methodology page.

Frequently asked

What is the formula for voltage drop?: For single phase, VD = 2 x K x I x L / cmil. For three phase, VD = 1.732 x K x I x L / cmil. K is 12.9 for copper and 21.2 for aluminium, I is the load current in amps, L is the one-way run length in feet, and cmil is the conductor cross-sectional area in circular mils.
How much voltage drop is acceptable?: The widely used guideline is 3% maximum on a branch circuit and 5% total for feeder plus branch circuit combined. These are recommendations rather than hard requirements in the NEC, but inspectors and engineers expect designs to meet them.
Does voltage drop use one-way or round-trip length?: Enter the one-way length. The factor of 2 in the single-phase formula already accounts for current flowing out and back, so doubling the length yourself would double the calculated drop.
What size wire do I need for a 100 ft run at 20 amps?: At 120 V single phase, 20 A over 100 ft one-way, 12 AWG copper drops roughly 4.4% which exceeds the 3% guideline. 10 AWG copper drops about 2.8% and is the usual choice for that run.
Why does aluminium need a larger conductor?: Aluminium has about 61% the conductivity of copper, so its voltage drop constant K is 21.2 against 12.9 for copper. For the same current, length and percent drop, aluminium typically needs to be one to two sizes larger.

Gear for long runs

Tools for verifying voltage and pulling long runs.

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Result

Maximum one-way run length at 3% drop

Frequently asked

Gear for long runs

Gear for long runs

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