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Voltage Drop CalculatorFree online other calculator — instant results, no sign-up

Voltage drop on a copper conductor.

Inputs

V
A
m
mm²
Voltage drop
6.19 V
Drop %5.16%
Voltage at load113.81 V

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About the Voltage Drop Calculator

This calculator determines the voltage loss across a two-wire copper electrical circuit based on distance, load current, and conductor cross-sectional area. Electricians, engineers, and DIYers use it to size wires properly and ensure connected equipment receives sufficient operating voltage.

Voltage
Source supply voltage in volts (V), typically 120 V or 230 V for single-phase branch circuits.
Current
Design or operating current carried by the circuit in amperes (A).
One-way length
Distance from the power source to the load in meters (m). The calculator automatically doubles this distance to account for the return conductor.
Wire area
Cross-sectional conductor area in square millimeters (mm²), such as 1.5 mm², 2.5 mm², or 4.0 mm².

Formula and method

Voltage drop = (2 × L × I × 0.0172) / A
Drop % = (Voltage drop / V) × 100
Voltage at load = V − Voltage drop
  • VSource voltage in volts (V)
  • ICircuit current in amperes (A)
  • LOne-way cable run distance in meters (m)
  • ACross-sectional area of the copper wire in square millimeters (mm²)
  • 0.0172Standard electrical resistivity of copper conductor at 20°C in Ω·mm²/m

The calculator multiplies the round-trip wire length (2 × L) by the load current and the standard resistivity of copper (0.0172 Ω·mm²/m), then divides by the conductor cross-sectional area. It then calculates the percentage drop against the input voltage and subtracts the drop from the source voltage to find load voltage, rounding all values to two decimal places.

Worked example

These are the calculator's starting values and the results it actually produces for them.

Inputs

  • Voltage120 V
  • Current15 A
  • One-way length30 m
  • Wire area2.5 mm²

Results

  • Voltage drop6.19 V
  • Drop %5.16%
  • Voltage at load113.81 V

How to use the Voltage Drop Calculator

  1. 1Multiply the one-way distance by 2 to determine the total conductor loop length in meters.
  2. 2Multiply loop length by load current and the copper resistivity constant (0.0172 Ω·mm²/m).
  3. 3Divide that result by the wire cross-sectional area in mm² to obtain the voltage drop in volts.
  4. 4Divide the voltage drop by the source voltage to find the drop percentage, and subtract the drop from the source voltage to find the remaining voltage at the load.

Reading your result

A voltage drop under 3% to 5% is standard for most branch circuit designs. Higher percentage drops indicate that the conductor is too thin or the distance is too long, which can cause dim lights, motor overheating, or erratic appliance behavior. If the voltage drop exceeds acceptable thresholds, choose a wire with a larger cross-sectional area.

Limitations

The calculation assumes a pure copper conductor operating near room temperature (20°C); it does not account for resistance increases caused by operating heat or insulation thermal ratings. It also models single-phase direct current or purely resistive alternating current, omitting reactance and power factor variations found in heavy inductive AC loads.

Common questions

Why is the one-way length multiplied by 2 in the calculation?+

Current in a complete electrical circuit must flow out to the load along the live conductor and return along the neutral conductor. Multiplying the one-way distance by two accounts for the total resistance of both wires.

What is an acceptable voltage drop percentage?+

Most electrical codes recommend keeping voltage drop to 3% or less on branch circuits, and no more than 5% total across feeder and branch circuits combined.

Does this calculation work for aluminum wire?+

No. The formula uses the resistivity of annealed copper (0.0172 Ω·mm²/m). Aluminum has a higher resistivity (approximately 0.028 Ω·mm²/m) and requires a larger conductor area to achieve the same voltage drop.

Does the calculator save my information?+

Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.

Further reading

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Results are estimates for informational purposes and are not financial, medical or legal advice.