Wire Voltage Drop Calculator: Formula and Guide

A wire voltage drop calculator estimates how much voltage is lost along a conductor when current flows through it. Enter the wire length, current, conductor material, and wire size to calculate the drop in volts and as a percentage of the supply voltage.

Formula

For a two-wire direct-current circuit, the calculator uses the resistance relationship V_drop = I × R, with conductor resistance estimated as R = ρ × L_loop / A. Combined, the formula is V_drop = 2 × ρ × L × I / A, where the factor 2 accounts for the outgoing and returning conductors.

• V_drop is the voltage lost in the wire, in volts (V).
• I is circuit current, in amperes (A).
• ρ is conductor resistivity, in ohm·mm²/m. The default, 0.01724, is a commonly used approximate value for copper near 20°C.
• L is the one-way route length, in feet. The formula converts it to metres using 0.3048 m per foot.
• A is the conductor’s cross-sectional area, in square millimetres (mm²).
• V is the supply voltage, used to find percentage drop and estimated load voltage.

The calculator treats the conductors as having the same material, size, and length. It estimates resistance from conductor dimensions and does not account for every installation condition.

Worked Example

Using the defaults—120 V supply, 10 A current, 50 ft one-way length, 2.08 mm² conductor area, and copper resistivity of 0.01724 ohm·mm²/m:

1. Convert route length: 50 ft × 0.3048 = 15.24 m.
2. Account for both conductors: loop length = 2 × 15.24 = 30.48 m.
3. Estimate loop resistance: 0.01724 × 30.48 / 2.08 ≈ 0.2526 ohm.
4. Calculate the drop: 10 A × 0.2526 ohm ≈ 2.53 V.
5. Calculate percentage drop: 100 × 2.53 / 120 ≈ 2.11%.
6. Estimate voltage at the load: 120 − 2.53 = 117.47 V.

Displayed to two decimal places, the results are 2.53 V drop, 2.11% drop, and 117.47 V at the load.

How to Use the Calculator

• Supply voltage: Enter the nominal voltage at the source, such as 120 V or 230 V. This does not change the absolute drop calculated from current and wire resistance; it changes the percentage and estimated load voltage.
• Current: Enter the current expected to flow in the circuit, in amperes. Use a realistic operating current, not a device’s power rating unless you have converted power to current appropriately.
• One-way length: Measure the route from the source to the load, in feet. The formula doubles it internally to include the return path. Do not enter the total out-and-back distance as the one-way length.
• Wire area: Enter the conductor cross-sectional area in mm². The default 2.08 mm² is approximately the area of 12 AWG copper; this is a conversion for the example, not a recommendation for a particular circuit.
• Resistivity: Use a value that matches the conductor material and the units shown. Copper is about 0.01724 ohm·mm²/m near 20°C; aluminum is higher, roughly 0.0282 in the same units. Actual resistance varies with temperature and conductor construction.

For metric measurements, convert route length to feet before entering it, or use a calculation with metres directly: V_drop = 2ρLI/A when L is in metres and ρ and A use the units above. One metre is about 3.281 ft; one square millimetre is about 1,550 circular mils, though wire tables generally identify sizes by AWG or mm² rather than requiring manual area conversion.

What the Result Means

Voltage drop is the difference between the source voltage and the voltage remaining at the load due to wire resistance. The percentage is the drop divided by the entered supply voltage. In the example, the estimated load receives about 117.47 V from a 120 V source under the stated assumptions.

Longer wire, higher current, and higher-resistivity material increase the drop. A larger conductor area reduces it. For a fixed conductor and route, doubling the current approximately doubles the drop; doubling the area approximately halves it. These relationships help explain why a small load on a short cable behaves differently from a high-current load at the end of a long run.

For alternating-current circuits, this simple calculation is often a useful approximation at ordinary low frequencies and short-to-moderate distances. AC voltage drop can also depend on conductor reactance, power factor, phase arrangement, and the circuit’s actual configuration. The calculator is not a substitute for an AC circuit method where those factors matter.

Limits, Mistakes, and Safety

• Length counted twice: This calculator asks for one-way length and doubles it. Entering the complete loop length would overestimate the drop by a factor of two.
• Confusing wire-size systems: AWG numbers run in the opposite direction to cross-sectional area: a smaller AWG number means a thicker conductor. Look up the area for the exact conductor rather than treating the gauge number as an area.
• Ignoring temperature: Metal resistance rises as it warms. A calculation based on room-temperature resistivity can understate the drop in a hot conductor.
• Using a nominal or incorrect current: Drop is proportional to current. A result based on the wrong load current will also be wrong.
• Treating a result as wire-sizing approval: This is arithmetic, not a safety or code-compliance determination. Required conductor size also depends on installation method, insulation temperature rating, ambient conditions, terminations, protective devices, and applicable electrical rules.

Do not select wire, breakers, or other equipment solely from this estimate. Electrical installations can create fire and shock hazards; have a qualified electrician or other appropriate professional verify conductor sizing and code requirements. A calculated voltage drop does not establish that a circuit is safe or permitted.

Frequently Asked Questions

How do I calculate voltage drop in a wire?
For a basic two-wire DC circuit, multiply current by the total resistance of the outgoing and return conductors. With resistivity and area inputs, use V_drop = 2ρLI/A, making sure length and units match the formula.

Do I enter one-way wire length or the total circuit length?
Enter one-way length in this calculator. It includes both the outgoing and return paths by multiplying the length by two.

Does wire voltage drop increase with distance?
Yes. If current, material, and conductor area stay the same, resistance and voltage drop are approximately proportional to wire length.

How much voltage drop is acceptable?
There is no single answer for every circuit or installation. Applicable electrical rules, equipment requirements, system type, and design conditions determine what is acceptable. Consult the relevant standards and a qualified professional rather than relying on a generic percentage.

Can I use this for aluminum wire?
Yes, as a rough resistance estimate if you enter an appropriate aluminum resistivity in the specified units—about 0.0282 ohm·mm²/m near 20°C. Real installation conditions and AC effects can change the result.

Why does measured voltage drop differ from the calculator?
The actual circuit may have warmer conductors, different wire dimensions, splice or terminal resistance, changing load current, or AC impedance effects not included in the simplified formula. Measurement conditions and meter accuracy can also affect the comparison.

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in v | Supply voltage | V | 120 | 0
in i | Current | A | 10 | 0
in l | One-way wire length | ft | 50 | 0
in a | Wire area | mm² | 2.08 | 0
in rho | Resistivity | ohm·mm²/m | 0.01724 | 0
out drop | Voltage drop | V | 2 * rho * (l * 0.3048) * i / a | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Voltage at load | V | v - drop | 2