How to use this calculator
- Enter or choose system.
- Enter or choose supply voltage.
- Enter or choose load current.
- Enter or choose one-way cable length.
- Enter or choose length in.
- The voltage drop updates as you type. Use Copy result or Share to keep it.
Method
R = ρ × L ÷ A × (1 + α(T − 20 °C)); DC and single-phase: Vd = 2 × I × R; three-phase: Vd = √3 × I × R
- ρ
- resistivity at 20 °C: copper 0.017241 Ω·mm²/m (1/58, 100% IACS), aluminium 0.028265 (61% IACS)
- L
- one-way length (m)
- A
- cross-section (mm²); AWG uses d = 0.127 mm × 92^((36 − n)/39)
- α
- temperature coefficient: copper 0.00393, aluminium 0.00403 per °C
- I
- load current (A)
Worked example
- Resistance of one conductor: 0.017241 × 30 ÷ 2.5 = 0.2069 Ω.
- Single-phase current flows out and back: Vd = 2 × 16 × 0.2069 = 6.62 V.
- 6.62 ÷ 230 = 2.88% of the supply, leaving 223.38 V at the load.
16 A over 30 m of 2.5 mm² copper drops about 6.6 V (2.9%) on a 230 V single-phase circuit at 20 °C.
What the result means
Wiring rules set maximum drops, often a few per cent depending on the country and the type of circuit. If the result is close to your limit, use a larger conductor or a shorter route, and repeat the calculation at the cable’s operating temperature.
Assumptions and limits
- Resistive drop only: cable reactance and load power factor are ignored, which is accurate for small and medium cables at mains frequency.
- Three-phase loads are balanced.
- Cable sizing must also meet current-carrying capacity, fault protection and installation rules set by your local wiring regulations.
- Very large conductors (above about 50 mm²) need reactance included; use the manufacturer’s mV/A/m data.
Questions
Why multiply by 2 for single-phase?
Current goes out along one conductor and returns along the other, so the cable length is effectively doubled. In a balanced three-phase circuit the factor is √3 instead.
Which temperature should I use?
For a quick check use 20 °C. For a design check use the cable’s maximum operating temperature (for example 70 °C for PVC insulation), which raises copper resistance by about 20%.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.