Electrical Voltage Drop Calculator and Formula

An electrical voltage drop calculator estimates how much voltage is lost along a conductor as current flows through it. Enter the conductor’s one-way length, current, conductor resistance per unit length, and system type; the calculator returns the drop in volts and its percentage of the source voltage. Use the result as an estimate, not as a wire-sizing or safety approval.

Formula

The calculator uses a resistance-based estimate:

Voltage drop, ΔV = I × R × L × k

• ΔV is the estimated voltage drop in volts (V).
• I is current in amperes (A).
• R is conductor resistance in ohms per kilometre (Ω/km).
• L is the one-way conductor length in metres (m); dividing by 1,000 converts it to kilometres.
• k is 2 for a direct-current or single-phase two-conductor circuit, accounting for the outgoing and return paths. It is √3, approximated here as 1.732, for a balanced three-phase circuit using line-to-line voltage and the appropriate per-conductor resistance.

The percentage is ΔV ÷ source voltage × 100. Estimated receiving-end voltage is source voltage minus the calculated drop. The equations assume the supplied resistance represents the conductor under relevant operating conditions and that reactance and other effects can be ignored.

Worked Example

Using the calculator’s defaults: one-way length 30 m, current 20 A, resistance 3.28 Ω/km, source voltage 230 V, and a DC or single-phase circuit:

1. Convert length to kilometres: 30 ÷ 1,000 = 0.03 km.
2. Calculate the two-conductor loop resistance: 3.28 × 0.03 × 2 = 0.1968 Ω.
3. Calculate voltage drop: 20 × 0.1968 = 3.936 V, which rounds to 3.94 V.
4. Calculate percentage drop: 3.936 ÷ 230 × 100 = 1.7113%, which rounds to 1.71%.
5. Estimate receiving-end voltage: 230 − 3.936 = 226.064 V, which rounds to 226.06 V.

When to Use a Voltage Drop Calculator

Voltage drop is the reduction in electrical potential between a source and a load caused by impedance in the conductors. A calculator is useful for understanding how a run’s length, current, or conductor resistance affects the voltage available at the receiving end. It can help compare scenarios, such as a shorter route with the same conductor data or a lower current on the same run.

Length matters because a longer conductor has more resistance. Current matters because the voltage drop across a given resistance rises in proportion to current. Resistance depends on the conductor material, cross-sectional area, temperature, and construction. For a basic estimate, use a resistance value that matches the conductor and the conditions as closely as possible.

For this calculator, length means the distance from the source to the load in one direction. The circuit factor accounts for the return path in DC and single-phase circuits. Do not enter the combined outgoing-and-return length and also use the factor of 2, or the result will be doubled. The three-phase option is intended for a balanced circuit calculation; it is not a substitute for analysis of an unbalanced system.

Choosing Resistance and Circuit Type

Resistance values may be listed in ohms per kilometre, ohms per metre, or ohms per 1,000 feet. Convert the value to Ω/km before entering it. For example, a value in Ω/m must be multiplied by 1,000. A value in Ω/1,000 ft must be multiplied by approximately 3.28084 to express it in Ω/km.

Conductor resistance increases as temperature rises. Manufacturer data or applicable engineering references may provide resistance at a stated temperature; use a value appropriate to the intended calculation. If you only have a nominal or room-temperature value, the result may understate the drop under warmer operating conditions. AC circuits can also have reactance, and power factor can affect the voltage-drop calculation, especially for longer runs or larger conductors. This simple calculator does not include those terms.

• Select DC or single-phase for a two-conductor loop when using the stated resistance convention.
• Select three-phase for the balanced three-phase equation and use line-to-line source voltage.
• Enter the resistance of one conductor, not the combined loop resistance, because the calculator applies the circuit factor.
• Check that length, resistance, current, and voltage refer to the same circuit and operating case.

How to Read the Result and Its Limits

The voltage-drop output is an estimated difference in volts. The percentage output shows that difference relative to the entered source voltage, which makes comparisons across different nominal voltages easier. The receiving-end result is a simple subtraction; it is not a measurement or a guarantee that a load will operate correctly.

Real circuits can depart from this estimate because conductor temperature, AC reactance, power factor, connections, source impedance, and changing load current affect performance. The equation also treats current as a single value throughout the run. A circuit with varying loads, multiple branches, or a nonstandard configuration may need a more detailed method.

There is no universal acceptable percentage for every circuit or purpose. Applicable electrical codes, equipment requirements, installation conditions, and design objectives determine what is suitable. The output must not be used by itself to choose conductor size, protective devices, or installation methods. Have a qualified electrical professional verify design and code requirements before installation or modification.

Common Input Mistakes

• Entering total loop length: For a two-wire circuit, enter one-way length because the factor of 2 includes the return conductor.
• Mixing units: The length input is metres and resistance is Ω/km. A unit mismatch can change the answer by a factor of 1,000 or more.
• Using the wrong voltage: For the percentage calculation, enter the source voltage corresponding to the circuit and load being evaluated. The three-phase formula assumes line-to-line voltage.
• Using an unsuitable resistance figure: Confirm whether a reference value applies to the conductor’s material, size, and temperature.
• Treating rounded results as exact: The displayed values are rounded for readability. Calculations and design checks should retain appropriate precision and use the required engineering method.

Frequently Asked Questions

How do I calculate voltage drop manually?
For this estimate, multiply current by conductor resistance and one-way length in compatible units, then apply the circuit factor: 2 for DC or single-phase, or approximately 1.732 for balanced three-phase. Divide the result by source voltage and multiply by 100 for the percentage.

Should I enter one-way length or round-trip length?
Enter one-way length in this calculator. Its factor of 2 already accounts for the outgoing and return conductors in a DC or single-phase circuit. Entering a round-trip length as well would count the return path twice.

Does this calculator work for three-phase power?
It provides a basic balanced three-phase estimate when you select that option, enter line-to-line voltage, and use the resistance of one conductor. Unbalanced loads, significant reactance, or other complexities require a suitable engineering calculation.

Why does voltage drop increase with temperature?
For common metallic conductors, resistance generally rises as temperature increases. Since the basic equation multiplies current by resistance, a higher resistance produces a larger estimated drop at the same current and length.

Can I use the result to choose a wire size?
No. This calculation alone does not account for all code, installation, ampacity, fault-current, temperature, or equipment requirements. A qualified professional should select and verify conductors and protective devices for the specific installation.

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in l | One-way conductor length | m | 30 | 0
in i | Current | A | 20 | 0
in r | Conductor resistance | ohm/km | 3.28 | 0
in v | Source voltage | V | 230 | 0
sel s | Circuit type | DC or single-phase=2, Three-phase=1.732
out drop | Voltage drop | V | i * r * l / 1000 * s | 2
out pct | Voltage drop | % | drop / v * 100 | 2
out end | Estimated receiving-end voltage | V | v - drop | 2