About this calculator
Voltage drop on a branch or sub-main cable for single- and three-phase supplies, plus the smallest conductor that keeps you within your limit.
How it’s calculated
VD = 2 × ρ × L × I ÷ A for single-phase, or √3 for three-phase. ρ ≈ 0.0225 Ω·mm²/m for copper at operating temperature, 0.036 for aluminium.
Worked example
With these inputs:
- Supply
- 230 V single-phase
- Load current (A)
- 30
- One-way cable length
- 30 m
- Conductor
- Copper
- Conductor size
- 6 mm²
- Allowed drop (%)
- 5
the result is:
- Voltage drop
- 6.75 V (2.93%)
- Voltage at load
- 223.3 V
- Smallest size for ≤5% drop
- 4 mm²
Tips from the trade
- BS 7671 Appendix 4 typically allows 3% for lighting and 5% for other uses on a public supply.
- Long runs to outbuildings and EV chargers are where voltage drop usually decides the cable size.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.