Quick answer: Voltage drop is the loss of voltage as current flows through a cable's resistance and reactance. For a three-phase circuit: Vd = √3 × I × L × (R cosθ + X sinθ), and the percentage drop is Vd% = (Vd ÷ V) × 100. Keep it within 3% for feeders and 5% for the whole run (source to load).
What causes voltage drop?
Every conductor has resistance. As current flows, some voltage is "used up" pushing current through that resistance (and, for AC, the cable's reactance). The longer the cable and the higher the current, the larger the drop — which is why long runs so often force a bigger cable than ampacity alone would suggest.
The voltage drop formula
Three-phase: Vd = √3 × I × L × (R·cosθ + X·sinθ)
Single-phase: Vd = 2 × I × L × (R·cosθ + X·sinθ)
Percentage: Vd% = (Vd / V) × 100
Simplified (using cable tables):
Vd = (mV/A/m × I × L) / 1000Where I = current (A), L = one-way length (m), R and X = resistance and reactance per metre, and mV/A/m is the manufacturer's voltage-drop figure for that cable.
How to calculate voltage drop — step by step
- Find the load current (A) for the circuit.
- Measure the one-way cable length (m) from source to load.
- Look up the cable's mV/A/m value from the datasheet.
- Multiply: Vd = (mV/A/m × I × L) ÷ 1000 to get volts dropped.
- Convert to %: divide by the nominal voltage and check against the limit.
Acceptable voltage drop limits
| Circuit section | Recommended max drop | Standard |
|---|---|---|
| Lighting circuits | 3% | IEC 60364 / IS 732 |
| Power / feeder circuits | 3% | NEC 210.19 (recommended) |
| Total (source to final load) | 5% | NEC / IEC |
| Motor starting (transient) | 10–15% | Design practice |
Enter current, length, cable size and phase — get volts dropped, percentage, and a pass/fail against code limits instantly.
Open the Voltage Drop Calculator →Worked example
A 30 A, three-phase, 415 V load fed by 25 m of 6 mm² copper cable (mV/A/m ≈ 7.3):
- Vd = (7.3 × 30 × 25) ÷ 1000 = 5.48 V
- Vd% = (5.48 ÷ 415) × 100 = 1.32% ✅ well within the 3% limit.
If the run were 80 m instead, the drop would rise to ≈4.2% — over the limit — and you would step up to 10 mm² to fix it.
How to reduce voltage drop
- Increase the cable cross-sectional area (the most common fix).
- Shorten the cable run or relocate the distribution board closer to the load.
- Raise the distribution voltage for very long runs.
- Improve the power factor to reduce current.
Related MEP calculators
Use alongside the Cable Size Calculator and the Power Factor Correction Calculator.
Frequently Asked Questions
What is the formula for voltage drop?
For three-phase: Vd = root3 x I x L x (R cos-theta + X sin-theta). Using cable tables it simplifies to Vd = (mV/A/m x I x L) / 1000. Percentage drop = (Vd / V) x 100.
What is the acceptable voltage drop percentage?
Keep voltage drop within 3% for feeder and lighting circuits and within 5% total from source to the final load, per IEC 60364 and NEC recommendations.
How do I reduce voltage drop?
Increase the cable cross-sectional area, shorten the run, relocate the distribution board closer to the load, raise the voltage, or improve the power factor.
Does voltage drop depend on cable length?
Yes. Voltage drop is directly proportional to cable length and current, so long runs often need a larger cable than ampacity alone would require.