Voltage drop calculator
Work out the voltage drop on a cable, the smallest cross-section that meets your limit, and the longest run you can use. Single- or three-phase, copper or aluminium.
How voltage drop is calculated
The calculator uses the IEC 60364-5-52 method, which includes cable reactance:
ΔU = b · I · L · (ρ / S · cos φ + λ · sin φ) ΔU% = ΔU / U · 100
- b = 2 for single-phase, √3 ≈ 1.732 for three-phase
- I = load current (A), L = route length (m), S = cross-section (mm²)
- ρ = resistivity at operating temperature: 0.0225 Ω·mm²/m copper, 0.036 aluminium
- λ = cable reactance, about 0.08 mΩ/m
- U = 230 V single-phase (line–neutral), 400 V three-phase (line–line)
Worked example
A 16 A socket circuit, 30 m from the board, 2.5 mm² copper, resistive load (cos φ = 1):
ΔU = 2 · 16 · 30 · 0.0225 / 2.5 = 8.64 V ΔU% = 8.64 / 230 · 100 = 3.76%
That passes the 5% limit for sockets but would fail 3% for lighting. With 4 mm² the drop falls to 2.35%.
Typical voltage drop limits
| Supply | Lighting | Other uses |
|---|---|---|
| Public low-voltage network | 3% | 5% |
| Private LV supply (own transformer) | 6% | 8% |
Values from IEC 60364-5-52, Annex G. Your national wiring rules may set different limits.
Maximum run lengths (single-phase 230 V, copper, cos φ = 1)
| Current | Section | Max at 3% | Max at 5% |
|---|---|---|---|
| 10 A | 1.5 mm² | 23 m | 38 m |
| 16 A | 2.5 mm² | 23 m | 39 m |
| 20 A | 4 mm² | 30 m | 51 m |
| 25 A | 6 mm² | 36 m | 61 m |
| 32 A | 6 mm² | 28 m | 47 m |
| 32 A | 10 mm² | 47 m | 79 m |
| 40 A | 10 mm² | 38 m | 63 m |
Frequently asked questions
What voltage drop is allowed?
IEC 60364-5-52 recommends 3% for lighting and 5% for other uses when supplied from the public low-voltage network, measured from the origin of the installation to the load. National rules may differ, so check yours.
Do I enter the round-trip length?
No. Enter the one-way route length from the board to the load. The calculator multiplies by 2 for single-phase and by √3 for three-phase.
Is a cable that passes the voltage-drop check big enough?
Not necessarily. It must also carry the load current without overheating for its installation method, and be coordinated with the protective device.
Why does aluminium have a larger drop?
Aluminium has roughly 60% higher resistivity than copper, so at the same cross-section it loses more voltage. A size one or two steps larger is typical.