How far can I run 12 gauge wire on a 20 amp circuit?

About 45 feet one way at 120 V with a full 20 A load for a 3% drop, using K = 12.9 and 12 AWG at 6,530 circular mils (NEC Chapter 9 Table 8). At 240 V the same wire reaches about 91 feet.

Is a 3% voltage drop required by the NEC?

No. The 3% branch-circuit and 5% total figures appear in informational notes to 210.19 and 215.2(A)(2), which are recommendations. Some jurisdictions and energy codes make them mandatory, and sensitive equipment may need less drop.

Is the distance one way or round trip?

One way: the length of cable from the panel to the load. The formula already doubles it (the 2 in VD = 2·K·I·L ÷ CM) to account for current flowing out and back.

What K value should I use for aluminum?

21.2 ohm-circular mils per foot at 75°C, compared with 12.9 for copper. Aluminum runs therefore reach roughly 61% of the copper distance for the same size and load.

Should I calculate at the breaker rating or the actual load?

Use the actual expected load. Sizing for the full breaker rating is conservative and simple, which is how this chart is built; a calculator lets you enter the real current.

Voltage Drop Chart: Max Run Length by Wire Size & Amps

Maximum one-way distance in feet for a 3% voltage drop at 120 V, copper, by wire size and load current
Copper size	15 A ft	20 A ft	30 A ft	40 A ft	50 A ft	60 A ft	100 A ft
14 AWG	38	—	—	—	—	—	—
12 AWG	60	45	—	—	—	—	—
10 AWG	96	72	48	—	—	—	—
8 AWG	153	115	76	57	46	—	—
6 AWG	244	183	122	91	73	61	—
4 AWG	388	291	194	145	116	97	—
3 AWG	489	367	244	183	146	122	73
2 AWG	617	462	308	231	185	154	92
1 AWG	778	583	389	291	233	194	116
1/0 AWG	982	736	491	368	294	245	147
2/0 AWG	1,238	928	619	464	371	309	185
3/0 AWG	1,560	1,170	780	585	468	390	234
4/0 AWG	1,968	1,476	984	738	590	492	295

Maximum one-way distance in feet for a 3% voltage drop at 240 V, copper, by wire size and load current
Copper size	15 A ft	20 A ft	30 A ft	40 A ft	50 A ft	60 A ft	100 A ft
14 AWG	76	—	—	—	—	—	—
12 AWG	121	91	—	—	—	—	—
10 AWG	193	144	96	—	—	—	—
8 AWG	307	230	153	115	92	—	—
6 AWG	488	366	244	183	146	122	—
4 AWG	776	582	388	291	232	194	—
3 AWG	978	734	489	367	293	244	146
2 AWG	1,234	925	617	462	370	308	185
1 AWG	1,557	1,167	778	583	467	389	233
1/0 AWG	1,964	1,473	982	736	589	491	294
2/0 AWG	2,476	1,857	1,238	928	742	619	371
3/0 AWG	3,121	2,341	1,560	1,170	936	780	468
4/0 AWG	3,936	2,952	1,968	1,476	1,181	984	590

Conductor area in circular mils and DC resistance at 75°C in ohms per 1,000 feet (stranded, uncoated copper and aluminum)
Size	Area cmil	Copper Ω/kFT	Aluminum Ω/kFT	Implied K, copper Ω·cmil/ft
14 AWG	4,110	3.14	5.17	12.9
12 AWG	6,530	1.98	3.25	12.9
10 AWG	10,380	1.24	2.04	12.9
8 AWG	16,510	0.778	1.28	12.8
6 AWG	26,240	0.491	0.808	12.9
4 AWG	41,740	0.308	0.508	12.9
3 AWG	52,620	0.245	0.403	12.9
2 AWG	66,360	0.194	0.319	12.9
1 AWG	83,690	0.154	0.253	12.9
1/0 AWG	105,600	0.122	0.201	12.9
2/0 AWG	133,100	0.0967	0.159	12.9
3/0 AWG	167,800	0.0766	0.126	12.9
4/0 AWG	211,600	0.0608	0.1	12.9
250 kcmil	250,000	0.0515	0.0847	12.9
300 kcmil	300,000	0.0429	0.0707	12.9
350 kcmil	350,000	0.0367	0.0605	12.8
400 kcmil	400,000	0.0321	0.0529	12.8
500 kcmil	500,000	0.0258	0.0424	12.9

How to read the voltage drop chart

Pick the row for the copper wire size and the column for the load in amps. The number is the longest one-way run, in feet, that keeps the voltage drop at or below 3% on a single-phase, two-wire circuit. If your run is longer, move down to a larger wire until the distance in that column covers it.

A dash means the wire is too small for that current in the first place: the load exceeds the 75°C ampacity in NEC Table 310.16 or the 240.4(D) breaker limit. Ampacity always comes first; voltage drop only ever pushes the size up. The wire ampacity chart has the full table.

The formula behind it

VD = 2 × K × I × L ÷ CM

VD is the voltage drop in volts, K is the resistivity constant (12.9 ohm-cmil/ft for copper, 21.2 for aluminum, both at 75°C), I is the current in amps, L is the one-way length in feet and CM is the conductor area in circular mils from NEC Chapter 9 Table 8. Rearranged for distance, L = VD × CM ÷ (2 × K × I), which is what generates every cell above.

Worked example: a 16 A load on 12 AWG copper 60 feet from the panel at 120 V gives 2 × 12.9 × 16 × 60 ÷ 6,530 = 3.79 V, or 3.2%. That is just over the 3% target. Moving up to 10 AWG (10,380 cmil) gives 2.39 V, or 2.0%.

Why 240 volts goes twice as far

The drop in volts for a given wire and current is the same at 120 V and 240 V. But 3% of 240 V is twice as many volts as 3% of 120 V, so the allowable distance doubles. That is one reason well pumps, heat pumps and other long-run loads are usually 240 V.

Common voltage drop mistakes

Entering the round-trip length; the formula already accounts for both conductors.
Using the copper K for aluminum conductors.
Ignoring motor starting current, which can briefly cause several times the running drop.
Treating the chart as an ampacity table. Check Table 310.16 first.

The K-factor method uses DC resistance. For large conductors on AC circuits, especially in steel conduit, NEC Chapter 9 Table 9 impedance gives a slightly more exact result. Conductor data is from NFPA 70, 2023 edition; local amendments may apply.

Reviewed October 11, 2026 by the Tradesman Calculator editorial team · Figures are from NEC 2023; local amendments may apply · Report an error

Voltage drop chart

Maximum run at 120 volts (3% drop)

Maximum run at 240 volts (3% drop)

Conductor area and resistance