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
- SANS 10142-1 (6.2.7) limits voltage drop to 5% of the declared voltage from the point of supply to any outlet: 11.5 V on a 230 V single-phase circuit and 20 V on a 400 V three-phase circuit.
- 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 SANS 10400 (the National Building Regulations) and your local municipality before ordering.