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Voltage Drop Calculator

Voltage drop in volts and percent for copper or aluminum wire, 22 AWG to 1000 kcmil, DC, single- or three-phase, from one-way feet, amps and source volts. How it works

Use the Voltage Drop Calculator

Voltage presets
Voltage drop
3.95 V
Of source 3.29% · Longest run at 3% 91.1 ft (27.8 m)

3.29% of the source (above the 3% limit)

End Voltage: 116.05 V

Resistance of one conductor: 0.1975 ohm (12 AWG, copper, one way).

At 10 A and a 3% limit this wire can run 91.1 ft (27.8 m) one way, so the run entered is too long for it.

Smallest listed wire that keeps the drop at or below 3%: 10 AWG. This is voltage drop only: the current rating of the wire, its insulation and the terminals set a minimum size too.

Drop by wire size for these inputs

WireDrop (V)Drop (%)
22 AWG40.1933.49
20 AWG25.2921.08
18 AWG15.9313.27
16 AWG10.008.33
14 AWG6.285.23
12 AWG (selected)3.953.29
10 AWG2.492.07
8 AWG1.561.30
6 AWG0.980.82
4 AWG0.620.52
3 AWG0.490.41
2 AWG0.390.32
1 AWG0.310.26
1/0 AWG0.240.20
2/0 AWG0.190.16
3/0 AWG0.150.13
4/0 AWG0.120.10
250 kcmil0.100.09
300 kcmil0.090.07
350 kcmil0.070.06
400 kcmil0.060.05
500 kcmil0.050.04
600 kcmil0.040.04
750 kcmil0.030.03
1000 kcmil0.030.02

Resistance only, at the NEC 75 °C constants (K = 12.9 copper, 21.2 aluminum ohm-cmil per foot). Reactance and the real conductor temperature are not included, and the load current is assumed constant. The 3% and 5% figures are NEC informational-note recommendations, not requirements.

About the Voltage Drop Calculator

12 AWG copper carrying 10 A for 100 ft one way on 120 V drops 3.95 V, 3.29% of the source, which is above the 3% the NEC recommends for a branch circuit; 10 AWG drops 2.49 V (2.07%). Voltage drop = 2 × K × L × I ÷ A for DC or single-phase, and √3 in place of the 2 for three-phase, with K = 12.9 for copper or 21.2 for aluminum, L the one-way length in feet and A the wire area in circular mils.

Voltage drop on a 120 V copper circuit, 100 ft one way

Wire10 A15 A20 A30 A
14 AWG6.28 V (5.23%)9.42 V (7.85%)12.55 V (10.46%)18.83 V (15.69%)
12 AWG3.95 V (3.29%)5.93 V (4.94%)7.90 V (6.58%)11.85 V (9.88%)
10 AWG2.49 V (2.07%)3.73 V (3.11%)4.97 V (4.14%)7.46 V (6.21%)
8 AWG1.56 V (1.30%)2.34 V (1.95%)3.13 V (2.60%)4.69 V (3.91%)
6 AWG0.98 V (0.82%)1.47 V (1.23%)1.97 V (1.64%)2.95 V (2.46%)

DC or single-phase, K = 12.9 at 75 °C. At 240 V the volts dropped are the same and the percentage is half. These are voltage-drop figures, not current ratings.

Smallest copper wire that keeps the drop within 3% at 120 V

One-way length10 A15 A20 A30 A
25 ft16 AWG14 AWG14 AWG12 AWG
50 ft14 AWG12 AWG10 AWG8 AWG
75 ft12 AWG10 AWG8 AWG8 AWG
100 ft10 AWG8 AWG8 AWG6 AWG
150 ft8 AWG8 AWG6 AWG4 AWG
200 ft8 AWG6 AWG4 AWG3 AWG

Single-phase, from the sizes the calculator lists (22 AWG to 1000 kcmil), judged on the percentage as shown to two decimals. The current rating of the wire, its insulation and the terminals can call for a larger size; this table does not check them.

Smallest copper wire that keeps a 12 V DC drop within 3%

One-way length5 A10 A20 A30 A
5 ft16 AWG14 AWG10 AWG8 AWG
10 ft14 AWG10 AWG8 AWG6 AWG
15 ft12 AWG8 AWG6 AWG4 AWG
20 ft10 AWG8 AWG4 AWG3 AWG
25 ft10 AWG6 AWG4 AWG2 AWG

A 3% drop at 12 V is 0.36 V. The length is from the battery to the load; the formula counts the return wire. Current ratings are not checked.

Smallest copper wire for 10 A within 3% at 12, 24 and 48 V DC

One-way length12 V24 V48 V
5 ft14 AWG16 AWG20 AWG
10 ft10 AWG14 AWG16 AWG
25 ft6 AWG10 AWG12 AWG
50 ft4 AWG6 AWG10 AWG

The same current loses the same volts at any system voltage, so doubling the voltage halves the percentage and allows a thinner wire. At 1 A and 12 V the smallest size within 3% is 22 AWG at 5 ft one way, 20 AWG at 10 ft and 18 AWG at 20 ft. Current ratings are not checked.

How to use this tool

  1. Pick the material, the wire size (22 AWG to 1000 kcmil, or 0.5 to 500 mm²) and the circuit: DC or single-phase, or three-phase line to line.
  2. Enter the one-way length from the source to the load in feet, the load current in amps and the source voltage, or tap a preset: 12, 24, 48, 120 or 240 V.
  3. Pick the drop limit (1, 2, 3 or 5%). The drop, the longest run the chosen wire carries within the limit, the smallest listed wire within it and the drop on every size update as you type.
  4. A link can fill the form: voltage, amps, length, gauge (0 to 0000 also read as 1/0 to 4/0, or a metric size such as 2.5mm2), material, circuit and maxdrop, as in ?voltage=24&amps=10&length=25&gauge=10.

Frequently asked questions

What is the voltage drop on 12 gauge wire at 100 feet?
On copper at 120 V, 3.95 V (3.29%) at 10 A and 7.90 V (6.58%) at 20 A, which leaves 112.10 V at the load. 10 AWG at 20 A drops 4.97 V (4.14%). The 100 ft is one way; the formula already counts the return conductor.
Is the length one way or round trip?
One way, from the source to the load. The formula doubles it for the go and return conductors. Entering the round-trip 200 ft for a 100 ft run of 12 AWG at 10 A and 120 V gives 7.90 V instead of 3.95 V.
What voltage drop does the NEC allow?
Informational notes to NEC 210.19 (branch circuits) and 215.2 (feeders) recommend sizing conductors for no more than 3% drop on the branch circuit or the feeder, and 5% for feeder and branch together. Informational notes are not enforceable requirements (NEC 90.5(C)), though a local amendment or a project specification can make a limit binding.
How do you calculate three-phase voltage drop?
Use √3 (1.732) in place of the 2: VD = 1.732 × K × L × I ÷ A, line to line. A 480 V three-phase feeder of 1/0 copper carrying 100 A for 200 ft drops 4.23 V (0.88%); the same run in aluminum drops 6.95 V (1.45%). In AC conductors of 2/0 and larger, skin effect makes the resistance higher than the DC value used here, so the real drop is somewhat larger; the page shows a note for those sizes instead of correcting K.
What size wire do I need for 12 V at 20 amps?
For a drop within 3% on copper: 10 AWG up to 5 ft one way, 8 AWG at 10 ft, 6 AWG at 15 ft and 4 AWG at 20 to 25 ft. At 15 ft, 10 AWG loses 0.75 V, 6.21% of 12 V. The wire’s current rating still has to be checked separately.
Why does aluminum wire drop more voltage than copper?
Aluminum has the higher resistance: K is 21.2 against 12.9 for copper, so the same size and run drops about 1.64 times as much. 12 AWG at 10 A for 100 ft on 120 V drops 6.49 V (5.41%) in aluminum and 3.95 V (3.29%) in copper.

Long runs, low voltages and three-phase feeders

Electricians check branch circuits and feeders; solar, RV and boat installers check 12 V and 24 V DC runs, where one lost volt matters. A 15 ft run at 12 V carrying 20 A loses 0.75 V on 10 AWG copper, 6.21% of the battery voltage; the page names 6 AWG as the smallest size within 3%. Presets set 12, 24, 48, 120 or 240 V, a table lists the drop on every size from 22 AWG to 1000 kcmil, and the limit can be 1, 2, 3 or 5%. The result also gives the longest run within the limit: 12 AWG copper at 10 A and 120 V reaches 91.1 ft (27.8 m) at 3%.

The size it names is for voltage drop alone; current rating, insulation and terminals can call for a larger one, and the page does not check them.

The voltage drop formula and the NEC constants

VD = 2 × K × L × I ÷ A for DC or single-phase

VD = √3 × K × L × I ÷ A for three-phase, line to line

K is 12.9 for copper or 21.2 for aluminum, in ohm·cmil per foot at 75 °C. L is the one-way length in feet (the 2 counts both conductors), I the amps and A the conductor area in circular mils from NEC Chapter 9 Table 8, 6,530 for 12 AWG and 10,380 for 10 AWG.

Example, the defaults: 12 AWG copper, 100 ft one way, 10 A at 120 V. 2 × 12.9 × 100 × 10 ÷ 6530 = 3.95 V, 3.29% of 120 V and leaves 116.05 V at the load; 10 AWG drops 2.49 V (2.07%).

Metric sizes (0.5 to 500 mm², IEC 60228) use 1 mm² = 1,973.5 circular mils: 2.5 mm² copper on the same run drops 5.23 V (4.36%); 4 mm² stays within 3%.

The 3% branch and 5% total limits come from informational notes to NEC 210.19 and 215.2: recommendations, not requirements.

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