How far can you run 300 kcmil wire?

At 20 A on a 120 V single-phase circuit, 300 kcmil copper stays under 3% voltage drop for about 2093 ft one-way. At 240 V the same load reaches roughly 4186 ft, because allowable drop scales with system voltage.

What is the voltage drop of 300 kcmil wire per 100 feet?

About 0.17 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 300 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.

300 kcmil Voltage Drop — Max Run Length by Load

Voltage drop for 300 kcmil copper and aluminium: at 20 A on 120 V single phase the 3% limit is reached at about 2093 ft one-way. Full table by load, voltage and distance.

Quick answer

300 kcmil copper reaches 3% voltage drop at about 2093 ft one-way carrying 20 A on a 120 V single-phase circuit. At 100 ft the same load drops 0.17 V (0.14%), calculated as VD = 2 × K × I × L / cmil with K = 12.9 for copper and 300,000 circular mils.

3% limit @ 20 A, 120 V

2093ft

3% limit @ 20 A, 240 V

4186ft

Drop @ 100 ft, 20 A

0.17V

Ω / 1000 ft (Cu)

0.0346

Maximum one-way run length at 3% drop

Copper 300 kcmil. Aluminium distances are roughly 62% of these figures.

Load	120 V 1Ø	240 V 1Ø	208 V 3Ø	480 V 3Ø
5 A	8,372 ft	16,744 ft	16,757 ft	38,670 ft
10 A	4,186 ft	8,372 ft	8,378 ft	19,335 ft
15 A	2,790 ft	5,581 ft	5,585 ft	12,890 ft
20 A	2,093 ft	4,186 ft	4,189 ft	9,667 ft
25 A	1,674 ft	3,348 ft	3,351 ft	7,734 ft
30 A	1,395 ft	2,790 ft	2,792 ft	6,445 ft
40 A	1,046 ft	2,093 ft	2,094 ft	4,833 ft
50 A	837 ft	1,674 ft	1,675 ft	3,867 ft
60 A	697 ft	1,395 ft	1,396 ft	3,222 ft
80 A	523 ft	1,046 ft	1,047 ft	2,416 ft
100 A	418 ft	837 ft	837 ft	1,933 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%	0.05%
50 ft	0.04%	0.05%	0.07%	0.11%
75 ft	0.05%	0.08%	0.11%	0.16%
100 ft	0.07%	0.11%	0.14%	0.22%
150 ft	0.11%	0.16%	0.22%	0.32%
200 ft	0.14%	0.22%	0.29%	0.43%
300 ft	0.22%	0.32%	0.43%	0.65%

Percent drop, 120 V single phase, copper.

Published standard

Frequently asked

How far can you run 300 kcmil wire?: At 20 A on a 120 V single-phase circuit, 300 kcmil copper stays under 3% voltage drop for about 2093 ft one-way. At 240 V the same load reaches roughly 4186 ft, because allowable drop scales with system voltage.
What is the voltage drop of 300 kcmil wire per 100 feet?: About 0.17 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 300 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.

300 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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