How far can you run 18 AWG wire?

At 20 A on a 120 V single-phase circuit, 18 AWG copper stays under 3% voltage drop for about 11 ft one-way. At 240 V the same load reaches roughly 22 ft, because allowable drop scales with system voltage.

What is the voltage drop of 18 AWG wire per 100 feet?

About 31.77 V at 20 A on 120 V single phase, using VD = 2 × K × I × L / cmil with K = 12.9 for copper.

Is 3% voltage drop a code requirement?

It is a recommendation, not a mandatory limit, in most adopted codes: roughly 3% on the branch circuit and 5% total including the feeder. Some local amendments and specifications do make it enforceable, so check what applies to your job.

Does aluminium change the 18 AWG numbers?

Yes. Aluminium uses K = 21.2 instead of 12.9, so drop is about 64% higher for the same size and run, and allowable distance falls by roughly the same proportion.

18 AWG Voltage Drop — Max Run Length by Load

Voltage drop for 18 AWG copper and aluminium: at 20 A on 120 V single phase the 3% limit is reached at about 11 ft one-way. Full table by load, voltage and distance.

Quick answer

18 AWG copper reaches 3% voltage drop at about 11 ft one-way carrying 20 A on a 120 V single-phase circuit. At 100 ft the same load drops 31.77 V (26.48%), calculated as VD = 2 × K × I × L / cmil with K = 12.9 for copper and 1,624 circular mils.

3% limit @ 20 A, 120 V

11ft

3% limit @ 20 A, 240 V

22ft

Drop @ 100 ft, 20 A

31.77V

Ω / 1000 ft (Cu)

6.3861

Maximum one-way run length at 3% drop

Copper 18 AWG. Aluminium distances are roughly 62% of these figures.

Load	120 V 1Ø	240 V 1Ø	208 V 3Ø	480 V 3Ø
5 A	45 ft	90 ft	90 ft	209 ft
10 A	22 ft	45 ft	45 ft	104 ft
15 A	15 ft	30 ft	30 ft	69 ft
20 A	11 ft	22 ft	22 ft	52 ft
25 A	9 ft	18 ft	18 ft	41 ft
30 A	7 ft	15 ft	15 ft	34 ft
40 A	5 ft	11 ft	11 ft	26 ft
50 A	4 ft	9 ft	9 ft	20 ft
60 A	3 ft	7 ft	7 ft	17 ft
80 A	2 ft	5 ft	5 ft	13 ft
100 A	2 ft	4 ft	4 ft	10 ft

Source & method: Derived from VD = 2 × K × I × L / cmil (1.732 × K × I × L / cmil for three phase), solved for L at 3% of nominal voltage. Run your exact circuit.

Voltage drop by distance at common loads

Distance	10 A	15 A	20 A	30 A
25 ft	3.31%	4.96%	6.62%	9.93%
50 ft	6.62%	9.93%	13.24%	19.86%
75 ft	9.93%	14.89%	19.86%	29.79%
100 ft	13.24%	19.86%	26.48%	39.72%
150 ft	19.86%	29.79%	39.72%	59.58%
200 ft	26.48%	39.72%	52.96%	79.43%
300 ft	39.72%	59.58%	79.43%	119.15%

Percent drop, 120 V single phase, copper.

Published standard

Frequently asked

How far can you run 18 AWG wire?: At 20 A on a 120 V single-phase circuit, 18 AWG copper stays under 3% voltage drop for about 11 ft one-way. At 240 V the same load reaches roughly 22 ft, because allowable drop scales with system voltage.
What is the voltage drop of 18 AWG wire per 100 feet?: About 31.77 V at 20 A on 120 V single phase, using VD = 2 × K × I × L / cmil with K = 12.9 for copper.
Is 3% voltage drop a code requirement?: It is a recommendation, not a mandatory limit, in most adopted codes: roughly 3% on the branch circuit and 5% total including the feeder. Some local amendments and specifications do make it enforceable, so check what applies to your job.
Does aluminium change the 18 AWG numbers?: Yes. Aluminium uses K = 21.2 instead of 12.9, so drop is about 64% higher for the same size and run, and allowable distance falls by roughly the same proportion.

18 AWG Voltage Drop — Max Run Length by Load

Maximum one-way run length at 3% drop

Voltage drop by distance at common loads

Frequently asked

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