Voltage Drop Calculator
Find the voltage drop along a wire in volts and as a percent of the supply voltage, and the voltage left at the load. Choose the wire size and material, then enter the voltage, current, and one-way length of a DC, single-phase, or three-phase circuit.
- Choose US for AWG and kcmil sizes or Metric for mm² sizes.
- Choose the System: DC, Single-phase AC, or Three-phase AC.
- Enter the Voltage of the circuit and the Current the load draws. For three-phase, enter the line-to-line voltage.
- Enter the One-way length, the distance from the power source to the load along the wire. Don't double it. The calculator already counts the return path.
- Choose the Wire size and Wire material. To compare wire sizes, change the size and watch the result.
Voltage and current accept whole numbers and decimals, such as
120 or 13.8. The length also accepts fractions,
such as 75 1/2.
How to calculate voltage drop
In a DC or single-phase circuit, the current flows out on one wire and back on the other, so the drop counts the wire twice:
Voltage drop = 2 × one-way length × current × wire resistance per length
In a balanced three-phase circuit, the drop between two lines is √3 times the drop along one wire:
Voltage drop = √3 × one-way length × current × wire resistance per length
Percent drop = voltage drop ÷ voltage × 100
Voltage at load = voltage − voltage drop
For example, 12 AWG copper has a resistance of about 1.9706 Ω per 1,000 ft at 75 °C. At 16 A over 100 ft one way, the drop is 2 × 100 × 16 × 1.9706 ÷ 1,000 = 6.31 V, which is 5.25% of 120 V. The calculator uses the unrounded resistance.
How the wire resistance is found
The calculator works out the DC resistance of each wire size from its cross-sectional area and the resistivity of the metal:
Resistance per length = resistivity at 20 °C × (1 + temperature coefficient × (75 − 20)) × 1.02 ÷ area
- Copper is the International Annealed Copper Standard, 1/58 Ω·mm²/m at 20 °C, with a temperature coefficient of 0.00393 per °C, from the NBS (now NIST) Handbook 100, Copper Wire Tables.
- Aluminum has 61% of the conductivity of that copper, with a temperature coefficient of 0.00403 per °C, from the NBS Handbook 109, Aluminum Wire Tables.
- AWG diameters come from table 1 of Handbook 100. The area in circular mils is the diameter in mils squared, and 1 kcmil is 1,000 circular mils, about 0.5067 mm². Metric sizes use the nominal area.
- Stranded wire has about 2% more resistance than solid wire of the same area, so the calculator multiplies by 1.02 for every size. Solid wire gives about 2% less drop.
- 75 °C is the temperature used for the wire, since wire carrying current runs warmer than the room. A cold wire has less resistance and a slightly smaller drop. At 20 °C, the drop is about 18% less.
AC circuits and shared neutrals
For AC circuits, the calculator uses the DC resistance of the wire only. It treats the power factor as 1 and leaves out the reactance of the wire. That's close for small wire and resistive loads like heaters and incandescent lights. For large wire, about 1/0 AWG or 50 mm² and up, or for motor loads, the actual drop can be different from the result.
A 120/240 V circuit with a shared neutral isn't one of the choices. Enter a 240 V load as Single-phase AC at 240 V, since its current goes out on one hot wire and back on the other. Enter a 120 V load as Single-phase AC at 120 V, since its current goes out on a hot wire and back on the neutral.
Recommended voltage drop
The NEC (NFPA 70) has an informational note that a voltage drop of no more than 3% on a branch circuit, and no more than 5% on the feeder and branch circuit together, gives reasonable efficiency of operation. It's an explanation, not a rule. The calculator shows this for reference only and doesn't check whether your circuit meets the code.
Examples
120 V branch circuit
A 12 AWG copper circuit at 120 V carries 16 A over 100 ft one way. The drop is 6.31 V (5.25%), leaving 113.69 V at the load. With aluminum wire of the same size, the drop is 10.38 V (8.65%).
240 V circuit
A 6 AWG copper circuit at 240 V carries 50 A over 150 ft one way. The drop is 7.35 V (3.06%), leaving 232.65 V at the load.
12 V DC run
A 10 AWG copper run at 12 V DC carries 20 A over 25 ft one way. The drop is 1.24 V (10.32%), leaving 10.76 V at the load. Low voltage circuits lose a larger share of their voltage over the same wire.
Three-phase feeder
A 4/0 AWG aluminum feeder at 480 V three-phase carries 150 A over 300 ft one way. The drop is 7.80 V (1.63%), leaving 472.20 V.
Metric circuit
A 2.5 mm² copper circuit at 230 V carries 16 A over 30 m one way. The drop is 8.21 V (3.57%), leaving 221.79 V at the load.
FAQ
The one-way length. The calculator multiplies by 2 for DC and single-phase circuits to count the wire back to the source, so entering the round trip doubles the drop.
The line-to-line voltage, such as 208, 400, or 480 V. The three-phase formula uses √3 to give the drop between two lines, so compare it with the line-to-line voltage.
The NEC (NFPA 70) notes that 3% on a branch circuit, and 5% on the feeder and branch circuit together, gives reasonable efficiency. A larger drop wastes power as heat in the wire, and motors and lights may not run as well. Your local code and the equipment's instructions decide what's allowed.
Aluminum conducts about 61% as well as copper, so the same size of aluminum wire has about 1.65 times the resistance and drop. 12 AWG aluminum has 3.245 Ω per 1,000 ft at 75 °C, compared with 1.971 Ω for copper.
Related calculators
- Solar Panel Calculator
How many solar panels and what system size you need from your electricity use and peak sun hours, with yearly production.
- Generator Size Calculator
What size generator you need, from the running and starting watts of the appliances you want to run.
- Pool Shock Calculator
How much cal-hypo, dichlor, or liquid chlorine to add to shock a pool, from its volume and the free chlorine you need.
- Pool Salt Calculator
How much salt to add to a saltwater pool from its volume and salt level, and how many bags to buy.
- Pool Volume Calculator
How many gallons or liters your pool holds, from its shape, size, and shallow and deep end depths.
- BTU Calculator
Air conditioner size in BTU/h and tons for each room, with a heating estimate from your climate zone.