Voltage Drop Calculator
Voltage drop in volts and percent along a copper cable, for single-phase and three-phase circuits.
Results
What this tool does
Every metre of cable has resistance, and on a long run the voltage arriving at the load can be noticeably lower than the voltage leaving the board. Most national rules limit the drop to a few percent; this gives you both the volts and the percentage so you can check against your local limit.
Formula
ΔU₁φ = 2 × ρ × L × I ÷ S · ΔU₃φ = √3 × ρ × L × I ÷ S
Variables
| Symbol | Meaning | Unit |
|---|---|---|
i | Current | A |
l | One-way cable length | m |
s | Conductor cross-section | mm² |
u | Voltage | V |
ro | Resistivity | Ω·mm²/m |
d1 | Voltage drop | V |
p1 | Voltage drop | % |
d3 | Voltage drop | V |
p3 | Voltage drop | % |
Worked example
- Current20 A
- One-way cable length35 m
- Conductor cross-section4 mm²
- Voltage230 V
- Resistivity0.0175 Ω·mm²/m
- Voltage drop6.125 V
- Voltage drop2.663 %
- Voltage drop5.304 V
- Voltage drop2.306 %
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
Frequently asked questions
Why is there a 2 in the single-phase formula?
Because the current travels out along one conductor and back along the other, so it passes through twice the length you measured. Enter the one-way distance from the board to the load: the calculator doubles it for you. Three-phase circuits use √3 instead, because the return path is shared between phases.