Engineering · Electrical
Voltage Drop Calculator — Cable Size, Length & Power Loss
Work out the voltage drop along a cable from the load, the run length and the wire size: the volts lost, the percentage of the supply, the voltage at the load and the power wasted as heat. It corrects resistance for conductor temperature, handles parallel conductors, and finds the smallest standard size (mm² or AWG) and the longest run that stay within your limit.
Your inputs
Quick start — try a typical case
230 V single-phase or 415 V three-phase (line-to-line) in India; 120/240 V in the US; 12/24/48 V for solar and batteries.
Distance from the supply to the load — the calculator counts the return path.
3 % is the usual target: the NEC recommends 3 % for a branch circuit (5 % overall), and 3 % at the consumer's terminals is the commonly cited IS 732 figure.
Your result Live
Voltage Drop
6.19 V (2.69%)
Within your 3% limit — 223.8 V at the load
- Voltage at the Load
- 223.8 V
- Power Lost in the Cable
- 123.8 W (2.69% of the load)
- Longest Run Within the Limit
- 33 m with 4 mm²
- Smallest Size Within the Limit
- 4 mm²
- Conductor Resistance
- 5.157 Ω per km at 70 °C
This checks voltage drop only. The cable must also carry the current safely (its current rating for the installation method) and be protected by the right breaker — follow the wiring code and a licensed electrician.
Cable reactance is ignored, which is accurate for small cables; for large cables on long AC runs the drop is a little higher.
How this is calculated
Vd = k × L × I × ρ ÷ A · k = 2 (single-phase, DC) or √3 (three-phase) · ρ_T = ρ₂₀ × (1 + α(T − 20)) · % = Vd ÷ V × 100
- ρ
- Resistivity: copper 0.01724, aluminium 0.02826 Ω·mm²/m at 20 °C
- A
- Conductor cross-section, mm²
- L, I
- One-way length (m) and load current (A)
Current flowing through a cable's resistance loses voltage along the way. In a single-phase or DC circuit the current goes out and back, so the drop is twice the one-way length times current times resistance per metre. In a balanced three-phase circuit the line-to-line drop uses √3 instead of 2. Resistance depends on the metal, the cross-section and the temperature — a hot copper cable at 70 °C has about 20 % more resistance than at 20 °C.
Assumptions & sources
- Resistive drop only; reactance ignored.
- Balanced load on three-phase circuits.
- Length is one way; the return conductor is the same size.
- 1IEC 60028 — International standard of resistance for copper (1/58 Ω·mm²/m at 20 °C) — International Electrotechnical Commission
- 2NFPA 70 (National Electrical Code) 210.19 and 215.2 informational notes — 3 % branch, 5 % total voltage drop — National Fire Protection Association
- 3IS 732 — Code of practice for electrical wiring installations — Bureau of Indian Standards
Calculator ID ENG-VDROP-001 · version 1.0.0
Common questions
What is an acceptable voltage drop?
About 3 % from the distribution board to the furthest point, and no more than 5 % in total from the supply — the NEC's recommendation and the usual design target in India too.
Why does a longer cable need a thicker wire?
Resistance grows with length, so the voltage lost grows too. A larger cross-section lowers the resistance per metre and brings the drop back within the limit.
Is aluminium wire OK?
It works but has about 1.6 times the resistance of copper, so it needs a larger size for the same drop.