How to use the wire voltage drop calculator
- Enter the current the load draws, the one-way length of the run and the length unit. The return path is added for you.
- Choose the wire size (AWG) and enter the supply voltage at the source.
- Open Advanced options to change the resistivity of the metal and the drop you want to stay under.
- Read the voltage drop, the share of the supply, the voltage left at the load and the power lost as heat, then compare the nearby wire sizes in the table.
Formula
diameter (mm) = 0.127 x 92^((36 - n) / 39) area = pi x diameter^2 / 4
round-trip resistance = 2 x resistivity x length / area
voltage drop = current x resistance power lost = current^2 x resistance
The number n is the gauge, with 1/0, 2/0, 3/0 and 4/0 counted as 0, -1, -2 and -3. Resistivity is in ohm square millimeters per meter and length is converted to meters. The longest run for a target drop is the target share of the supply times the area, divided by 2 x resistivity x current.
15 amps over 50 feet of 12 AWG at 120 volts
A 12 AWG wire has a diameter of 2.053 mm and an area of 3.309 square millimeters. With copper at 0.017241 ohm mm2/m, the round trip of 100 feet (30.48 m) has a resistance of 0.15882 ohm. The drop is 2.382 V, which is 1.99% of 120 V, leaving 117.618 V at the load, and 35.73 W is lost as heat.
The same run in 10 AWG drops 1.498 V (1.25%) and loses 22.47 W, and in 14 AWG it drops 3.788 V (3.16%). For a drop of 3%, 12 AWG could run 75.6 feet one way at this current, and the thinnest size that stays within 3% for 50 feet is 13 AWG. AWG 10 has a diameter of 2.588 mm, which checks against Ohm's law done separately.
What voltage drop does
A long thin wire has resistance, and the current flowing through it uses up some of the voltage on the way. Lights dim, motors run hot and electronics may reset when too much is lost. Thicker wire has lower resistance, and a shorter run loses less, which is why the table lets you compare sizes for the same job.
The power lost in the wire turns to heat, and it grows with the square of the current. Doubling the current quadruples the loss in the wire.
What this page does not decide
This is a direct current estimate for two wires at the resistivity you entered. It does not model heat, bundling, temperature, alternating current effects, motor starting current or the rules for choosing a safe wire size and protection for a circuit. Use it to compare options, and rely on the rules and a qualified electrician for real installations. Related tools include the Ohm's Law Calculator, the Voltage Divider Calculator and the Circuit Load Calculator.
More engineering tools
For cross sections, bends and orbits, try the Section Properties Calculator, Sheet Metal Bend Allowance Calculator and Orbital Mechanics Calculator.
Assumptions and limits
- The run has two conductors carrying the same current, so the length is counted twice.
- The resistivity is the value you enter, and it does not change with temperature or heating.
- The load draws a steady current; the effect of alternating current at high frequency is ignored.
- The target drop is an example for comparing options, not a limit that any rule sets for your project.
- The example values are an illustration. Replace them with your own.
Frequently asked questions
Why is the length counted twice?
Current flows out on one wire and back on the other, so both carry the drop. A 50 foot run has 100 feet of wire in the circuit.
Which resistivity should I use?
Copper near room temperature is about 0.0172 ohm mm2/m and aluminum about 0.0282. Use the value for your wire. Resistance rises as wire warms, so hot conditions give a larger drop than shown.
Is a drop of 3% a rule?
It is only an example target on this page. Which limit applies to a job depends on the use and the rules in your area, and this page does not state or check them.
What do sizes like 4/0 mean?
They are the large sizes above 1 AWG, read "four aught". Each step up is thicker: 1/0, 2/0, 3/0 and 4/0.