Voltage Drop Calculator
Estimate the voltage lost over a length of wire carrying current.
Formula
Vdrop = I × R/km × (length_km × 2)
Inputs
- Load current (A)
- Wire resistance (Ω/km)
- One-way run length (m)
- Supply voltage (V)
Notes and assumptions
- Assumes a round-trip (out and back) conductor length, as is standard for single-phase/DC circuits.
What it does
Estimates how much voltage is lost along a cable carrying current, counting both the outgoing and the return conductor.
Where it's used
Choosing cable sizes for long runs, outdoor lighting, pumps and DC solar wiring.
How to use it
- Enter the load current.
- Enter the cable’s resistance per km from its datasheet.
- Enter the one-way length.
- Enter the supply voltage to see the drop as a percentage.
The formula explained
- V_drop
- I × (R/km) × 2 × length(km)
- ×2
- current flows out and back
Worked example
16 A over 40 m of 2.5 mm² copper (≈7.41 Ω/km).
Inputs:
- Load current: 16 A
- Wire resistance: 7.41 Ω/km
- One-way run length: 40 m
- Supply voltage: 230 V
The calculator gives:
- Result: 9.4848 V
- Loop resistance: 0.5928 Ω
- % of supply voltage: 4.12%
Tips and common mistakes
- Many wiring rules limit drop to around 3–5 %; check your local code.
- Low-voltage DC (12/24 V) suffers far more from the same drop in volts.
- Cable resistance rises when it gets warm.
Questions people ask
How do I reduce voltage drop?
Use a thicker cable, shorten the run, or use a higher supply voltage. In the example, going from 2.5 mm² to 4 mm² copper (about 4.61 Ω/km) cuts the drop from 4.1 % to about 2.6 %.
Does this work for three-phase circuits?
The calculator counts the outgoing and return conductors, as in a single-phase or DC circuit. For a balanced three-phase load, the line-to-line drop is √3 × I × R per conductor instead of 2 × I × R.
Why is voltage drop a bigger problem at 12 V?
The same loss in volts is a much bigger share of a small voltage. A 1 V drop is 0.4 % of 230 V but 8.3 % of 12 V — enough to dim lights or stop a pump starting. Low-voltage runs need much thicker cable.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.