How far can you run 250 kcmil wire?

At 20 A on a 120 V single-phase circuit, 250 kcmil copper stays under 3% voltage drop for about 1744 ft one-way. At 240 V the same load reaches roughly 3488 ft, because allowable drop scales with system voltage.

What is the voltage drop of 250 kcmil wire per 100 feet?

About 0.21 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 250 kcmil 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.

250 kcmil Voltage Drop — Max Run Length by Load

Voltage drop for 250 kcmil copper and aluminium: at 20 A on 120 V single phase the 3% limit is reached at about 1744 ft one-way. Full table by load, voltage and distance.

Quick answer

250 kcmil copper reaches 3% voltage drop at about 1744 ft one-way carrying 20 A on a 120 V single-phase circuit. At 100 ft the same load drops 0.21 V (0.17%), calculated as VD = 2 × K × I × L / cmil with K = 12.9 for copper and 250,000 circular mils.

3% limit @ 20 A, 120 V

1744ft

3% limit @ 20 A, 240 V

3488ft

Drop @ 100 ft, 20 A

0.21V

Ω / 1000 ft (Cu)

0.0415

Maximum one-way run length at 3% drop

Copper 250 kcmil. Aluminium distances are roughly 62% of these figures.

Load	120 V 1Ø	240 V 1Ø	208 V 3Ø	480 V 3Ø
5 A	6,976 ft	13,953 ft	13,964 ft	32,225 ft
10 A	3,488 ft	6,976 ft	6,982 ft	16,112 ft
15 A	2,325 ft	4,651 ft	4,654 ft	10,741 ft
20 A	1,744 ft	3,488 ft	3,491 ft	8,056 ft
25 A	1,395 ft	2,790 ft	2,792 ft	6,445 ft
30 A	1,162 ft	2,325 ft	2,327 ft	5,370 ft
40 A	872 ft	1,744 ft	1,745 ft	4,028 ft
50 A	697 ft	1,395 ft	1,396 ft	3,222 ft
60 A	581 ft	1,162 ft	1,163 ft	2,685 ft
80 A	436 ft	872 ft	872 ft	2,014 ft
100 A	348 ft	697 ft	698 ft	1,611 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	0.02%	0.03%	0.04%	0.06%
50 ft	0.04%	0.06%	0.09%	0.13%
75 ft	0.06%	0.1%	0.13%	0.19%
100 ft	0.09%	0.13%	0.17%	0.26%
150 ft	0.13%	0.19%	0.26%	0.39%
200 ft	0.17%	0.26%	0.34%	0.52%
300 ft	0.26%	0.39%	0.52%	0.77%

Percent drop, 120 V single phase, copper.

Published standard

Frequently asked

How far can you run 250 kcmil wire?: At 20 A on a 120 V single-phase circuit, 250 kcmil copper stays under 3% voltage drop for about 1744 ft one-way. At 240 V the same load reaches roughly 3488 ft, because allowable drop scales with system voltage.
What is the voltage drop of 250 kcmil wire per 100 feet?: About 0.21 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 250 kcmil 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.

250 kcmil 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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