What is the maximum allowable voltage drop in an electrical installation?

Per standard regulations, the maximum voltage drop is 3 % for residential and lighting circuits and 5 % for other uses.

What is the minimum cable cross-section for power outlets?

General-purpose outlet circuits use 2.5 mm² cable; large appliance circuits use 4 mm². Lighting circuits use 1.5 mm².

How do I know if I need an additional RCD?

If the circuit feeds wet areas (bathroom, kitchen, outdoor) or equipment with higher contact risk, a high-sensitivity RCD (30 mA) is mandatory.

Voltage Drop Calculator

Last updated: 2026-08-10

Use the Voltage Drop Calculator to get instant, accurate results. Enter your values below.

Inputs

Technical Parameters

Cable section (mmÂ²)

Current (A)

Voltage (V)

Dimensions

Circuit length (m)

Result

Enter values and press Calculate

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Common Sizes — Click to Fill

	Circuit length (m)	Cable section (mmÂ²) (mm²)	Current (A) (A)
Alumbrado (1.5 mm²)	15 m	1.5 mm²	8 A
Tomas salón (2.5 mm²)	25 m	2.5 mm²	16 A
Electrodomésticos (4 mm²)	30 m	4 mm²	25 A
Climatización (6 mm²)	40 m	6 mm²	32 A
Subcuadro exterior (10 mm²)	55 m	10 mm²	40 A

Popular combinations

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When designing or troubleshooting an electrical installation, ensuring that the voltage at the end of a cable run remains within safe and functional limits is critical. Our Voltage Drop Calculator helps you quickly determine the expected voltage loss in a cable based on its length, cross-section, current, and system voltage, using the standard formula for copper conductors. This tool is essential for electricians, engineers, and DIY enthusiasts who need to verify compliance with regulations and avoid undersized cables.

What the Voltage Drop Calculator Does and When to Use It

The calculator solves for the voltage drop (in volts and as a percentage) across a two-wire copper cable circuit. It uses the fundamental DC resistance formula adapted for AC power distribution, assuming a resistive load and a conductor temperature typical for standard installations. The primary output is the percentage drop relative to the supply voltage, which is then checked against a common 3% threshold for branch circuits.

You should use this calculator whenever you are sizing cables for fixed installations, such as:

Running power to a sub-panel or outbuilding.
Wiring long runs for lighting circuits or heavy appliances.
Verifying that an existing cable size is adequate for a new load.
Performing compliance checks for regulations like the REBT (Spain) or similar international standards that limit voltage drop to 3% for lighting and 5% for other loads.

Formula Explained Variable by Variable

The calculator applies the following formula to compute the voltage drop:

dV = (2 × L × I) / (σ × S)

Where:

dV = Voltage drop in volts (the numerical result).
L = longitud_m – One-way length of the cable in meters. For a round trip, the formula doubles this distance.
I = corriente_a – Current flowing through the conductor in amperes.
σ = Conductivity of copper, set to 56 siemens per meter per square millimeter (S·m/mm²). This value accounts for the typical operating temperature of 70°C.
S = seccion_mm2 – Cross-sectional area of the conductor in square millimeters.

After calculating dV, the percentage drop is found by: dV_pct = (dV / V) × 100, where V is the system voltage (tension_v, default 230V). The tool then returns "SI" (Yes) if dV_pct is 3% or less, and "NO (>3%)" if it exceeds the limit.

Two Worked Examples with Concrete Numbers

Example 1: Garden Lighting Circuit

Scenario: You are installing a 30-meter run of cable from a 230V supply to power four 50W LED floodlights (total 200W, roughly 0.87A). You plan to use 1.5 mm² cable.

Inputs:

seccion_mm2: 1.5
longitud_m: 30
corriente_a: 0.87
tension_v: 230

Calculation:
dV = (2 × 30 × 0.87) / (56 × 1.5) = 52.2 / 84 = 0.62 V
dV_pct = (0.62 / 230) × 100 = 0.27%

Result: The voltage drop is only 0.27%, well under the 3% limit. The 1.5 mm² cable is more than adequate for this load.

Example 2: Heavy Equipment in a Workshop

Scenario: A 7.5 kW single-phase motor (32A at 230V) needs to be supplied from a distribution board 45 meters away. You are considering 6 mm² cable.

Inputs:

seccion_mm2: 6
longitud_m: 45
corriente_a: 32
tension_v: 230

Calculation:
dV = (2 × 45 × 32) / (56 × 6) = 2880 / 336 = 8.57 V
dV_pct = (8.57 / 230) × 100 = 3.73%

Result: The voltage drop of 3.73% exceeds the 3% limit. The calculator would return "NO (>3%)". To fix this, you would need to increase the cable size to 10 mm² or reduce the circuit length.

Common Mistakes When Using the Calculator

Even with a simple tool, users often make errors that lead to incorrect results. Avoid these pitfalls:

Using single-phase length without doubling: The formula automatically accounts for the fact that current must travel to the load and back to the source. Do not enter double the distance manually. Always enter the one-way length.
Confusing cross-section units: The calculator expects the conductor cross-section in square millimeters (mm²). If you have American Wire Gauge (AWG) numbers, you must convert them first. For example, 10 AWG is approximately 5.26 mm², and 12 AWG is about 3.31 mm².
Ignoring the impact of system voltage: The default voltage is 230V (common in Europe, Australia, and many other regions). If your system operates at 120V (USA standard) or 240V, you must change this value. A 2% drop on a 120V circuit is only 2.4V, but it represents the same percentage as on a 230V system.
Using the calculator for aluminum cables: The formula uses a conductivity of 56 for copper. For aluminum, the conductivity is approximately 35 S·m/mm². If you are sizing aluminum conductors, the results will be inaccurate.
Forgetting to round up cable sizes: The calculator gives a pass/fail based on exact numbers. In practice, you cannot use a cable that exactly meets a 3% drop; you must select the next standard size (e.g., 4 mm², 6 mm², 10 mm²).

Frequently Asked Questions

Q1: Why does the formula multiply the length by 2?

A: The factor of 2 accounts for the total circuit length. In a single-phase system, the current flows out through the live wire and returns through the neutral (or the second live wire in a 230V system). Both conductors contribute to the voltage drop. If you only enter the one-way distance, the doubling ensures the calculation reflects the actual round-trip resistance.

Q2: My installation uses 12V DC. Can I still use this calculator?

A: Yes, you can, but with caution. The formula works for DC and AC circuits. However, the 3% pass/fail threshold is typically designed for AC mains voltage (230V/120V). For 12V systems, a 3% drop (0.36V) is extremely tight, and the cable would need to be very thick. For low-voltage systems, a 5% or 10% drop is often acceptable. Simply enter your system voltage and disregard the "Cumple REBT" warning; instead, focus on the absolute voltage drop value.

Q3: What should I do if the calculator says "NO (>3%)"?

A: The most common solution is to increase the cross-sectional area of the cable (use a thicker wire). Other approaches include:

Reducing the length of the cable run (relocate the load or the source).
Splitting the load into two separate circuits.
Raising the system voltage (e.g., stepping up from 120V to 240V for large appliances).

Re-enter the new parameters until you achieve a passing result.

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Written and reviewed by the CalcToWork editorial team. Last updated: 2026-08-10.