Voltage Drop Calculator

A voltage drop calculator tells you how many volts a wire loses between the panel and the load, and whether that loss stays under 3%. For a single-phase copper circuit the answer is VD = 2 × 12.9 × one-way feet × amps ÷ circular mils. A 20 A load 100 ft away on 12 AWG copper loses 7.90 V (6.58% of 120 V), more than twice the recommended limit, so the run needs 8 AWG.

Enter your circuit voltage, the load current, the one-way run length, and the wire gauge above. The calculator returns the volts lost, the percent drop, the voltage that actually reaches the load, and a pass/fail check against 3%.

The voltage drop formula

Every foot of wire has a small resistance. Current flowing through that resistance turns some voltage into heat before it reaches the load (Ohm’s law: V = I × R). Longer wire, more current, and thinner wire all mean more drop.

For single-phase AC and DC circuits:

VD = (2 × K × L × I) ÷ CM

• VD = voltage drop in volts
• 2 = the current travels out and back, so the wire length is doubled
• K = resistance constant of the conductor: 12.9 ohm-cmil/ft for copper, 21.2 for aluminum (values at a 75 °C conductor temperature)
• L = one-way length in feet
• I = load current in amps
• CM = conductor area in circular mils (6,530 for 12 AWG)

Percent drop = VD ÷ source voltage × 100. Voltage at the load = source voltage − VD.

You can also flip the formula to find the longest run a wire can handle at a target drop: max one-way length = (allowed % ÷ 100 × V × CM) ÷ (2 × K × I). That is how the distance table below was built.

For three-phase circuits the multiplier changes from 2 to √3 (1.732). This calculator handles single-phase and DC only.

Worked examples

These numbers come straight from the voltage drop calculator above, so you can enter them yourself and get the same results.

Example 1: 120 V, 20 A, 100 ft, 12 AWG copper (a detached-garage outlet circuit)

• Voltage drop: (2 × 12.9 × 100 × 20) ÷ 6,530 = 7.90 V
• Percent drop: 7.90 ÷ 120 × 100 = 6.58%
• Voltage at the load: 120 − 7.90 = 112.10 V
• Result: over 3%. Moving to 10 AWG gives 4.97 V (4.14%), still over. 8 AWG gives 3.13 V (2.60%), which passes.

Example 2: 240 V, 30 A, 150 ft, 10 AWG copper (a well pump or shop heater)

• Voltage drop: (2 × 12.9 × 150 × 30) ÷ 10,380 = 11.18 V
• Percent drop: 11.18 ÷ 240 × 100 = 4.66%
• Voltage at the load: 228.82 V
• Result: over 3%. 8 AWG drops 7.03 V, which is 2.93%, just under the limit.

Example 3: 120 V, 15 A, 50 ft, 14 AWG copper (a lighting circuit)

• Voltage drop: (2 × 12.9 × 50 × 15) ÷ 4,110 = 4.71 V
• Percent drop: 3.92%, over the limit
• Switching to 12 AWG: (2 × 12.9 × 50 × 15) ÷ 6,530 = 2.96 V, or 2.47%

Each step up of two AWG sizes cuts the drop by roughly 37%, because the copper area grows by about 59%.

Wire resistance by AWG size

The table shows each common size’s area and its approximate resistance per 1,000 ft, derived from K ÷ circular mils. These track the DC resistance values in NEC Chapter 9, Table 8 (75 °C) closely; use that table for exact solid versus stranded figures.

AWG	Area (circular mils)	Copper (Ω per 1,000 ft)	Aluminum (Ω per 1,000 ft)
14	4,110	3.14	—
12	6,530	1.98	3.25
10	10,380	1.24	2.04
8	16,510	0.78	1.28
6	26,240	0.49	0.81
4	41,740	0.31	0.51
3	52,620	0.25	0.40
2	66,360	0.19	0.32
1	83,690	0.15	0.25
1/0	105,600	0.12	0.20

To use resistance directly: VD = 2 × L × I × (Ω per 1,000 ft ÷ 1,000). For 12 AWG copper at 100 ft and 20 A, that is 2 × 100 × 20 × 0.00198 = 7.92 V, matching the circular-mil method within rounding.

Maximum one-way distance at 3% drop (copper)

This table shows how far you can run each wire size before the drop passes 3%, with the full rated current flowing at the end of the run. Distances are one-way, in feet, rounded down.

Load	Wire	Max distance at 120 V	Max distance at 240 V
15 A	14 AWG	38 ft	76 ft
15 A	12 AWG	60 ft	121 ft
15 A	10 AWG	96 ft	193 ft
20 A	12 AWG	45 ft	91 ft
20 A	10 AWG	72 ft	144 ft
20 A	8 AWG	115 ft	230 ft
30 A	10 AWG	48 ft	96 ft
30 A	8 AWG	76 ft	153 ft
30 A	6 AWG	122 ft	244 ft
40 A	8 AWG	57 ft	115 ft
40 A	6 AWG	91 ft	183 ft
50 A	6 AWG	73 ft	146 ft
50 A	4 AWG	116 ft	232 ft

These are voltage-drop limits only. A wire must also be large enough for the current (its ampacity) and match the breaker, which the breaker size calculator helps with. Upsizing for voltage drop never lets you use a bigger breaker than the load and device ratings allow.

Percent drop at 120 V, 20 A

The same load at different distances, so you can see how fast drop grows with length:

Wire	25 ft	50 ft	75 ft	100 ft	150 ft
12 AWG	1.65%	3.29%	4.94%	6.58%	9.88%
10 AWG	1.04%	2.07%	3.11%	4.14%	6.21%
8 AWG	0.65%	1.30%	1.95%	2.60%	3.91%
6 AWG	0.41%	0.82%	1.23%	1.64%	2.46%

Doubling the length doubles the drop, so a gauge that is fine at 25 ft can fail at 100 ft.

What the NEC says about voltage drop

The National Electrical Code covers voltage drop mostly in informational notes:

• Branch circuits (informational note to 210.19(A)): size conductors so the drop at the farthest outlet is no more than 3%.
• Feeders plus branch circuits (informational note to 215.2(A)): total drop no more than 5%. For example, a 2% feeder leaves room for a 3% branch.

Informational notes are recommendations and are not enforceable code text on their own. The 3% and 5% figures are still the standard design targets, because excessive drop makes motors run hot, LED drivers flicker, and heaters put out less heat.

A few situations do carry mandatory limits, such as sensitive electronic equipment (Article 647) and certain fire pump installations. Local amendments and engineering specs may also set firm limits. Check with your local building department.

How to measure the run

1. Measure the wire path, not the straight-line distance. Follow the route the cable will take: up walls, across the attic, down into a trench. Add a few feet at each end for the panel and the box.
2. Use one-way length. The formula doubles it for you.
3. Use the actual load current. For a motor or appliance, read the nameplate amps. For a general outlet circuit, using the breaker rating gives a conservative worst case. Convert watts with the watts to amps calculator if needed.
4. Use the voltage at the source. Usually 120 V or 240 V for homes.

Common mistakes

• Entering the round-trip length. That doubles the answer. Enter one-way feet only.
• Forgetting the aluminum penalty. This calculator assumes copper. Aluminum drops about 1.64 times as much at the same size.
• Sizing for drop but not for ampacity. A long 14 AWG run might pass the drop check at a small load and still be undersized for a 20 A breaker.
• Checking only the branch. Detached buildings fed by a long feeder also lose voltage on the feeder. Add both to compare against 5%.
• Ignoring motor starting current. Motors briefly draw several times their running current, so a run near 3% at running load can dip hard on startup.

How to fix too much voltage drop

• Use a larger wire (lower AWG number). Two sizes up cuts the drop by about 37%.
• Shorten the run by moving the panel, adding a subpanel, or rerouting.
• Use 240 V instead of 120 V where the equipment allows. The current halves and the percent drop falls to a quarter.
• Split the load across two circuits.

To jump straight to the smallest gauge that meets your target, use the wire size calculator. Then check that the conductors fit your raceway with the conduit fill calculator.

Circuit sizing is code-governed and safety-critical. This voltage drop calculator gives an engineering estimate; confirm conductor size, ampacity, and overcurrent protection with a licensed electrician and the code your jurisdiction has adopted.