Calculating your voltage drop is straightforward once you know the Voltage Drop formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Voltage Drop Calculator.
What is Voltage Drop?
The Voltage Drop calculation tells you your voltage drop from a few simple inputs. The figure you are solving for here is the voltage drop.
The Voltage Drop formula
The core formula is:
Voltage drop = 2 × Current × Resistivity (Ω·mm² ÷ m) × One-way cable length ÷ Conductor areaHere is what each input means:
- Current — a value measured in A. Example: 10 A.
- One-way cable length — a value measured in m. Example: 20 m.
- Conductor area — a value measured in mm². Example: 2.5 mm².
- Resistivity (Ω·mm²/m) — a number. Example: 0.0175.
How to calculate it step by step
- Write down the current (for example, 10 A).
- Write down the one-way cable length (for example, 20 m).
- Write down the conductor area (for example, 2.5 mm²).
- Write down the resistivity (Ω·mm²/m) (for example, 0.0175).
- Apply the formula above to get your voltage drop.
- Double-check the result with the Voltage Drop Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Current | 10 A |
| One-way cable length | 20 m |
| Conductor area | 2.5 mm² |
| Resistivity (Ω·mm²/m) | 0.0175 |
| Voltage drop | 2.800 |
With current of 10 A, one-way cable length of 20 m, conductor area of 2.5 mm² and resistivity (Ω·mm²/m) of 0.0175, the voltage drop works out to 2.800.
Example 2
With current of 20 A, one-way cable length of 20 m, conductor area of 2.5 mm² and resistivity (Ω·mm²/m) of 0.0175, the voltage drop works out to 5.600.
| Result | Value |
|---|---|
| Voltage drop | 5.600 |
Example 3
With current of 5 A, one-way cable length of 20 m, conductor area of 2.5 mm² and resistivity (Ω·mm²/m) of 0.0175, the voltage drop works out to 1.400.
| Result | Value |
|---|---|
| Voltage drop | 1.400 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Voltage Drop Calculator does it instantly, for free, with the formula and a worked example built in.
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