Voltage Drop Calculator
The Voltage Drop Calculator estimates how much voltage is lost along an electrical cable due to its resistance. It is essential for electricians, engineers, and DIY installers who need to ensure appliances receive enough voltage to operate correctly.
Voltage Drop Calculator
A
m
Ω/m
What the result means
The voltage drop is the product of the current, the total cable length (there and back, hence the factor of 2), and the resistance per meter. A larger drop means the load receives less voltage, which can cause dim lights, slow motors, or equipment malfunction. The percentage is calculated against a 230 V reference supply.
How to use this calculator
- 1Enter the current flowing through the cable in amperes (A).
- 2Enter the one-way cable length in meters (m).
- 3Enter the resistance per meter of the cable in ohms per meter (Ω/m).
- 4Press Calculate to see the total voltage drop in volts and as a percentage of a 230 V supply.
- 5Compare the result with the recommended maximum drop (usually 3–5% for lighting and 5% for power circuits).
The formula
The calculation uses a standard, verifiable formula. Here it is in its simplest form.
Voltage Drop = 2 × I × L × R
Where:
I = Current (A)
L = One-way cable length (m)
R = Resistance per meter (Ω/m)
The factor of 2 accounts for the outbound and return conductors.
What each variable means
| Symbol | Name | Description |
|---|---|---|
| I | Current | The current flowing through the cable, measured in amperes. |
| L | Length | The one-way length of the cable run, measured in meters. |
| R | Resistance per meter | The resistance of the cable per meter of length, measured in ohms per meter. |
Step-by-step example
Example: 10 A over 50 m of cable at 0.01 Ω/m
Current:10 ALength:50 mResistance per meter:0.01 Ω/m
- 1Voltage drop = 2 × 10 × 50 × 0.01
- 2= 2 × 10 × 0.5
- 3= 10 V
- 4As a percentage of 230 V: (10 ÷ 230) × 100 = 4.3%
Result
10 V drop (4.3% of 230 V)
What changes the result
- Longer cable runs produce larger voltage drops.
- Thicker cables (lower resistance per meter) reduce voltage drop.
- Higher currents increase the voltage drop proportionally.
- The factor of 2 accounts for both the live and return conductors in a single-phase circuit.
Edge cases to be aware of
Unusual situations handled correctly
- If current or length is zero, the voltage drop is zero.
- If resistance per meter is zero (superconductor), there is no drop.
- Three-phase circuits have a different factor (√3 instead of 2) — this calculator assumes single-phase.
Common mistakes
Avoid these errors
- Forgetting the factor of 2 for the return conductor.
- Using the total cable length instead of the one-way length.
- Entering resistance in ohms instead of ohms per meter.
- Comparing the drop against the wrong supply voltage.
Assumptions
- The circuit is single-phase AC or DC.
- The cable resistance is uniform along its length.
- The supply voltage is 230 V for the percentage calculation.
- Temperature effects on resistance are ignored.
Limitations
- Does not account for three-phase circuits (which use a √3 factor).
- Ignores the effect of temperature on cable resistance.
- Assumes a fixed 230 V reference; other supply voltages will give different percentages.
- Does not include connection resistance or other losses in the circuit.
Frequently asked questions
What is an acceptable voltage drop?+
For most installations, a voltage drop of 3% or less for lighting and 5% or less for power circuits is considered acceptable. Higher drops can cause equipment to underperform or fail.
Why is the factor 2 used in the formula?+
In a single-phase circuit, current travels out along the live conductor and returns along the neutral conductor. Both conductors contribute to the total resistance, so the cable length is counted twice.
How can I reduce voltage drop?+
Use a thicker cable with lower resistance per meter, shorten the cable run, or reduce the current. Any of these will lower the voltage drop.
Does this calculator work for three-phase circuits?+
No. Three-phase circuits use a factor of √3 instead of 2. This calculator is designed for single-phase or DC circuits.