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 × K × I × L ÷ CM for single-phase, or √3 for three-phase. K = 12.9 for copper, 21.2 for aluminium; L in feet; CM is the circular-mil area.
Worked example
With these inputs:
- Supply
- 240 V single-phase
- Load current (A)
- 30
- One-way cable length
- 100 ft
- Conductor
- Copper
- Conductor size
- 10 AWG
- Allowed drop (%)
- 3
the result is:
- Voltage drop
- 7.46 V (3.11%)
- Voltage at load
- 232.5 V
- Smallest size for ≤3% drop
- 8 AWG
Tips from the trade
- NEC informational notes suggest 3% for a branch circuit and 5% total.
- 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 local building codes before ordering.