Voltage Drop Calculator
Voltage Drop Calculator
Instant, Free & Accurate Electrical Tool for All Cable Types
Introduction
Voltage drop is a critical consideration in any electrical installation, whether it's household wiring, industrial cabling, or long-distance power runs. If cables are too long or too thin for the current they carry, voltage drop can cause dim lighting, overheating equipment, and inefficient power transmission. The Voltage Drop Calculator helps electricians, engineers, and DIY enthusiasts instantly calculate how much voltage is lost over a given cable length, so they can choose the right wire gauge and avoid costly or unsafe installations.
How to Use This Calculator
The Voltage Drop Calculator needs five simple inputs:
- Supply Voltage (V): Enter the source voltage, for example 230V.
- Current (Amps): Enter the current flowing through the circuit, for example 10A.
- Cable Length (Meters): Enter the one-way length of the cable run, for example 50 meters.
- Wire Resistance (Ω/km): Enter the resistance per kilometer of the wire, for example 0.0175 Ω/km for copper.
- Phase Type: Select Single Phase or Three Phase depending on your electrical system.
- Click Calculate to see the voltage drop instantly.
The Formula Behind It
For single-phase circuits, the calculator uses:
Voltage Drop = 2 × I × R × L / 1000
For three-phase circuits, the formula changes slightly because of the phase relationship between conductors:
Voltage Drop = √3 × I × R × L / 1000
Where I is current in amps, R is resistance in ohms per kilometer, and L is the cable length in meters. The factor of 2 in single-phase accounts for both the outgoing and return conductor, while three-phase uses the √3 (approximately 1.732) factor due to the phase angle relationships between the three conductors.
Worked Example
For a single-phase circuit with a supply voltage of 230V, current of 10A, cable length of 50 meters, and wire resistance of 0.0175 Ω/km:
- Voltage Drop = (2 × 10 × 0.0175 × 50) / 1000
- Voltage Drop = 17.5 / 1000 = 0.0175V
Note: Since resistance is entered per km while the cable length is in meters, the calculator converts units internally before applying the formula, so always check your wire's datasheet to confirm whether its rated resistance is per km or per meter. As a percentage of supply voltage, this drop is well within the commonly recommended limit of under 3-5% for most electrical installations, confirming the cable size is adequate for this run.
Practical Context and Uses
This calculator is essential for electricians planning wiring for homes, offices, and factories, especially when cable runs are long, such as from a main distribution board to an outbuilding or borewell pump far from the house. Solar panel installers use voltage drop calculations to size DC cabling correctly between panels, inverters, and batteries, since excessive drop can meaningfully reduce system efficiency. Industrial engineers rely on this calculation when planning motor feeder cables, where undersized wiring can cause motors to underperform or overheat. It's also useful for anyone setting up generators, workshops, or outdoor lighting circuits where cable runs exceed typical indoor wiring distances.
Frequently Asked Questions
Q1: What is an acceptable voltage drop percentage?
Most electrical codes recommend keeping voltage drop under 3% for lighting circuits and under 5% for power circuits, though specific limits can vary by country and application.
Q2: Why does cable length matter so much?
Longer cables have higher total resistance, and since voltage drop is directly proportional to both current and cable length, doubling the length doubles the voltage drop for the same current and wire size.
Q3: How do I find my wire's resistance per km value?
This value is provided by wire manufacturers in cable datasheets, and it varies based on the conductor material (copper vs aluminum) and cross-sectional area of the wire.
Q4: Does upgrading to a thicker wire reduce voltage drop?
Yes, thicker wires have lower resistance per unit length, which directly reduces voltage drop for the same current and cable length.
Q5: Is the formula different for three-phase systems?
Yes, three-phase systems use a √3 factor instead of the factor of 2 used in single-phase calculations, reflecting the different conductor and phase relationships in three-phase power distribution.