Voltage Drop Calculator

TL;DR: To calculate voltage drop, use the formula V_drop = (2 × L × I) ÷ (S × K), where L is the one-way cable length in meters, I is the current in amps, S is the cross-section in mm², and K is the conductor conductivity (56 for copper, 35 for aluminum); then divide the result by the system voltage and multiply by 100 to get the percentage drop.

What Is the Voltage Drop Calculator?

A voltage drop calculator is an essential electrical engineering tool that determines how much voltage is lost as electricity travels through a cable. Every conductor has inherent resistance, and when current flows through that resistance, a portion of the voltage is converted into heat. This reduction in voltage between the source and the load is the voltage drop. The calculator uses the cable's cross-sectional area, length, current, and material conductivity to compute this loss precisely.

This tool is critical for electrical installers, building services engineers, maintenance technicians, and DIY enthusiasts wiring anything from a garden shed to an industrial plant. Undersized cables cause excessive voltage drop, leading to equipment malfunction, dim lighting, overheating motors, and wasted energy. National and international wiring regulations—such as the IEC 60364 series and the UK's BS 7671—impose strict limits on permissible voltage drop, typically 3% for lighting circuits and 5% for power circuits. Using a calculator ensures compliance and prevents costly rework.

The calculator on this page specifically implements the standard DC/single-phase formula using copper conductivity of 56 m/(Ω·mm²). It takes four primary inputs—cable cross-section, one-way length, load current, and nominal system voltage—and outputs the absolute voltage drop in volts, the percentage drop, the maximum allowable drop in volts, and a clear pass/fail verdict against the 3% and 5% thresholds.

How to Use the Calculator

Using the voltage drop calculator is straightforward. Follow these steps in order:

1. Enter the cable cross-section (seccion_mm2): Input the conductor size in square millimetres. Common residential sizes are 1.5 mm², 2.5 mm², 4 mm², 6 mm², and 10 mm². If using aluminum, ensure you know the exact size, as aluminum requires a larger cross-section than copper for the same current.
2. Enter the one-way cable length (length_m): Type the distance from the supply source (e.g., consumer unit) to the load in meters. This must be the one-way distance, not the total there-and-back length. The calculator automatically doubles it in the formula.
3. Enter the load current (corriente_a): Input the expected current draw in amperes. For a single appliance, use its nameplate rating. For multiple loads on one circuit, sum the currents of all devices that may operate simultaneously.
4. Enter the nominal system voltage (tension_v): Input the standard voltage of your supply. Typical values are 230V (most of Europe, Australia, UK single-phase), 120V (USA/Canada single-phase), or 400V (three-phase). The calculator uses this to compute the percentage drop and the maximum allowed volts.
5. Select the conductor material: Choose between copper and aluminum. The calculator applies conductivity of 56 m/(Ω·mm²) for copper and 35 m/(Ω·mm²) for aluminum.
6. Click "Calculate": The tool instantly displays the voltage drop in volts, the percentage drop, the maximum allowable drop, and a compliance status indicating whether the cable size meets the standard limits.

Formula and Calculation Method

The voltage drop calculator uses a simplified version of Ohm's Law applied to a two-conductor circuit. For a single-phase or DC system, the current flows out through one conductor and returns through the other, so the total wire length is twice the one-way distance. The formula is:

V_drop = (2 × L × I) ÷ (S × K)

• L = one-way cable length in meters
• I = load current in amperes
• S = cross-sectional area of the conductor in mm²
• K = conductivity of the conductor material (56 for copper, 35 for aluminum) in m/(Ω·mm²)

The product (S × K) represents the conductance of the cable per meter. Dividing the current by this conductance gives the volts per meter, and multiplying by 2L gives the total drop for the out-and-return path.

Worked example: Suppose you have a 40-meter run of 6 mm² copper cable carrying 32 amps at 230V. The conductivity of copper is 56, so the calculation proceeds as follows:

• Calculate the numerator: 2 × 40 × 32 = 2560
• Calculate the denominator: 6 × 56 = 336
• Divide to get the absolute drop: 2560 ÷ 336 = 7.62 volts
• Calculate the percentage: (7.62V ÷ 230V) × 100 = 3.31%
• Verify against limits: 3.31% is greater than 3% (fails for lighting) but less than 5% (passes for power circuits)

This demonstrates that while the cable would work for a motor or socket circuit, it is inadequate for a lighting circuit under these conditions. The maximum allowable voltage drop is calculated by multiplying the system voltage by 0.03 (3%) for lighting and 0.05 (5%) for power. For 230V, these are 6.9V and 11.5V respectively.

Practical Examples

Here are three realistic scenarios showing how the calculator guides cable selection:

In the first example, a 30-meter run of 2.5 mm² copper feeding a 16A lighting circuit results in a 2.98% drop. This is just under the 3% threshold, meaning it is technically compliant, but leaves no safety margin for voltage fluctuations. The second example shows a 4 mm² cable comfortably passing both limits. The third example demonstrates why aluminum must be sized up: a 10 mm² aluminum cable at 80 meters already exceeds the 3% limit, whereas the same run with 10 mm² copper would drop only 3% (calculation: 2 × 80 × 20 = 3200; 10 × 56 = 560; 3200 ÷ 560 = 5.71V = 2.48%).

Tips for Accurate Results

• Always use one-way length: The formula automatically accounts for the return conductor. Do not enter the total two-way distance or you will double the voltage drop error.
• Respect the 3% lighting limit strictly: Many experienced electricians accept a 5% drop for everything, but regulatory standards specifically cap lighting at 3% because incandescent and LED lamps noticeably flicker or dim with higher drops. If your calculated value is 2.9%, consider upsizing the cable to 4 mm² for a safety margin.
• Account for grouped cables: The calculator assumes cables in free air at 30°C. When multiple cables are bundled together, they dissipate less heat, increasing their operating resistance and effective voltage drop. Apply a derating factor: for 3–5 grouped cables, add 10% to the calculated drop; for 6–10 cables, add 20%.
• Do not confuse copper and aluminum conductivity: Copper has a conductivity of 56 m/(Ω·mm²), while aluminum is only 35 m/(Ω·mm²). Using the wrong value produces wildly inaccurate results. If your installation uses aluminum, always select the aluminum option or manually adjust the current rating.
• Verify the nominal voltage: Use the actual system voltage, not the nominal supply. A 230V supply can drop to 220V under load. Calculating with a higher voltage masks the true drop percentage.
• Check for mixed loads: If a circuit serves a motor, the starting current can be 5–7 times the running current. Calculate the drop at startup current; if it exceeds 15% during the startup, the motor may not start reliably.

Frequently Asked Questions

What is the maximum allowable voltage drop for lighting and power circuits?

For most low-voltage installations under IEC 60364 and BS 7671, the maximum permissible voltage drop is 3% for lighting circuits and 5% for other (power) circuits. This is measured from the origin of the installation (e.g., the consumer unit) to the final point of use. For a 230V system, 3% equals 6.9 volts and 5% equals 11.5 volts. Some industrial installations have more generous limits, but residential and commercial wiring must adhere to these figures to prevent equipment damage and ensure safety.

How does cable length affect voltage drop?

Voltage drop increases linearly with cable length. If you double the distance, you double the drop. This is because the formula V_drop = (2 × L × I) ÷ (S × K) has the length variable in the numerator. Practically, this means a 50-meter cable run has twice the drop of a 25-meter run with the same current and cable size. For long runs (over 50 meters), you must either increase the cable cross-section, distribute the load, or use a higher voltage supply to keep the drop within acceptable limits.

Why is my calculated voltage drop higher than expected?

There are four common reasons. First, you may have entered the total two-way length instead of the one-way length, doubling the result. Second, the cable temperature affects resistance; at 70°C operating temperature, copper resistance increases by about 20% compared to 20°C. Third, you may have mixed up copper and aluminum conductivity values. Fourth, the actual cable cross-section may be smaller than the nominal size—some low-quality cables are undersized. Verify your inputs, check the ambient temperature, and measure the actual conductor diameter with a gauge if precision matters.