Voltage Drop Calculator: Formula and Practical Guide

A voltage drop calculator estimates how much voltage is lost along a circuit’s conductors, based on current, conductor length, resistance, and—in AC circuits—power factor and phase. Enter the current, one-way distance, conductor resistance per unit length, and circuit type to get the estimated drop in volts and as a percentage of supply voltage.

Formula

For this simplified calculator, voltage drop equals current multiplied by the total resistance of the conductors in the circuit. It uses the following expressions:

• DC or single-phase: ΔV = I × (2L) × R
• Balanced three-phase: ΔV = I × (√3 × L) × R
• Percentage drop: drop (%) = 100 × ΔV / V
• Estimated load voltage: V_load = V − ΔV

Here, ΔV is voltage drop in volts (V); I is circuit current in amperes (A); L is the one-way distance from source to load in metres (m); R is conductor resistance in ohms per kilometre (Ω/km); and V is the nominal supply voltage in volts. The circuit selector applies a factor of 2 for a DC or single-phase circuit, accounting for the outgoing and return path, and √3 (approximately 1.732) for a balanced three-phase circuit. The entered power factor is included to identify a relevant AC parameter, but this simplified resistance-only calculation does not use it. For reactance or power-factor effects, use an appropriate AC impedance calculation instead.

Resistance must describe the conductor material and size at the relevant operating temperature. The calculator converts the one-way distance in metres to kilometres before multiplying by resistance in Ω/km.

Worked Example

Using the calculator’s default values—10 A, a one-way length of 30 m, resistance of 5.21 Ω/km, supply voltage of 230 V, DC or single-phase, and power factor 1—the round-trip conductor length is 2 × 30 = 60 m, or 0.06 km.

1. Calculate the drop: 10 A × 0.06 km × 5.21 Ω/km = 3.126 V.
2. Rounded to the calculator’s two decimal places, the voltage drop is 3.13 V.
3. Calculate the percentage: 100 × 3.126 / 230 = 1.359…%, which rounds to 1.36%.
4. Estimate the load voltage: 230 − 3.126 = 226.874 V, which rounds to 226.87 V.

The result is an estimate based on the stated resistance and circuit assumptions, not a conductor or protection-device recommendation.

When to Use a Voltage Drop Calculator

Use this calculation to understand why voltage at a load may be lower than voltage at the source, or to compare how current, distance, and conductor resistance affect the loss. It is useful for a preliminary estimate on DC, single-phase, and balanced three-phase circuits when conductor resistance is known. A longer run, higher current, or higher resistance produces a larger drop.

The input distance is one-way: measure from the source to the load, not the combined length of both conductors. The calculator doubles that distance for DC and single-phase circuits. For three-phase circuits, it uses the standard √3 factor for a balanced system rather than doubling the distance.

Choosing Resistance and Interpreting Results

Resistance depends on conductor material, cross-sectional area, and temperature. Obtain it from a reliable conductor data table or manufacturer documentation for the actual conductor. Do not assume that a resistance figure for one size or material applies to another. Many tables give resistance per kilometre or per 1,000 feet; convert units before entering the value.

The percentage result compares the calculated drop with the supply voltage you entered. For example, a 3 V drop on a 120 V supply is 2.5%, while the same 3 V on a 240 V supply is 1.25%. Percentage is a comparison, not a statement that a circuit is acceptable: applicable requirements and equipment needs depend on the installation and must be checked separately.

In US customary units, a conductor table may give resistance in ohms per 1,000 ft. Either convert the distance to kilometres and resistance to Ω/km, or use a calculator configured consistently for feet and ohms per foot. Never combine metres with Ω/1,000 ft or feet with Ω/km without converting.

Limits, AC Effects, and Common Mistakes

This calculator models conductor resistance only. In AC circuits, conductor impedance can also include reactance, and the contribution of resistance and reactance depends on power factor. The power-factor input here is not applied to the output; it is included as a reminder that a more complete AC calculation may need it. For long runs, larger conductors, or AC systems where accuracy matters, use a method that accounts for conductor impedance, phase configuration, operating temperature, and the actual load.

• Entering total circuit length: For DC or single-phase circuits, enter the one-way distance because the formula already includes the return path.
• Using the wrong circuit factor: Choose three-phase only for a balanced three-phase circuit. Unbalanced loads require a more specific analysis.
• Mixing units: Ensure distance and resistance units match the calculator’s stated inputs.
• Using an unsuitable resistance value: Resistance changes with conductor size, material, and temperature; a nominal or room-temperature value may not represent actual operating conditions.
• Overinterpreting rounded figures: The displayed result is rounded to hundredths. Calculations and design checks should use appropriate underlying precision.

A voltage-drop estimate cannot establish whether a wire, circuit, or installation is safe or compliant. Do not use it by itself to select wire size, breakers, or other protective equipment. Electrical work and final conductor sizing should be assessed against the relevant local rules and installation conditions by a qualified professional.

Frequently Asked Questions

What is voltage drop?
Voltage drop is the reduction in electrical potential between a circuit’s source and load as current passes through conductor impedance. Some drop is inherent; its size depends on the circuit and conductors.

Is the cable length entered one way or round trip?
Enter one-way distance. The calculator accounts for the return conductor in DC and single-phase calculations. For three-phase circuits it uses the balanced-system √3 factor.

Why does a longer cable increase voltage drop?
A longer conductor has more resistance, all else equal. Since the drop is current multiplied by resistance, greater resistance produces a larger voltage loss.

Does this calculator work for three-phase power?
It provides a simplified estimate for balanced three-phase circuits using the √3 factor. It does not model imbalance, detailed conductor impedance, or every system configuration.

Does power factor change the result shown?
No. This calculator’s resistance-only formula does not use the power-factor input. A more complete AC voltage-drop calculation may include power factor and conductor reactance.

Can I use the result to choose wire size?
No. The estimate does not verify ampacity, protective-device coordination, installation conditions, or code requirements. Have conductor sizing and electrical safety checked by a qualified professional.

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in i | Current | A | 10 | 0
in d | One-way conductor length | m | 30 | 0
in r | Conductor resistance | ohm/km | 5.21 | 0
in v | Supply voltage | V | 230 | 0
sel c | Circuit type | DC or single-phase=2, Three-phase=1.732
in p | Power factor | - | 1 | 0 | 1
out drop | Voltage drop | V | i * (d / 1000) * r * c | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Estimated voltage at load | V | v - drop | 2