⚡ ELECTRICAL

Free Voltage Drop Calculator (IS 732)

Calculate voltage drop for single and three-phase LV circuits. Checks IS 732 3% limit instantly. Free tool for MEP engineers.

📐 Standard: IS 732
✅ Free to use
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ℹ️ About This Calculator

Voltage drop is the reduction in electrical potential (voltage) along a conductor as current flows through it. IS 732 — the Indian Standard for electrical wiring installations — limits voltage drop to 3% of the supply voltage on branch circuits to protect equipment performance and efficiency.

Excessive voltage drop causes motors to run hot and stall, LED drivers to flicker, and UPS systems to malfunction. The IS 732 3% limit applies to branch circuits; BS 7671 and IEC 60364 also recommend 3–5% depending on circuit type. Always calculate from the supply point — not from the sub-distribution board — to capture the cumulative drop in long feeders.

📐 Voltage Drop Formula (IS 732)

IS 732

Single Phase: Vd = (2 × L × I × ρ) / A
Three Phase: Vd = (√3 × L × I × ρ) / A

Where:
  Vd = Voltage drop (V)
  L  = One-way cable length (m)
  I  = Design current (A)
  ρ  = Resistivity — Copper: 0.0172 Ω·mm²/m | Aluminium: 0.0282 Ω·mm²/m
  A  = Cable cross-section (mm²)

❓ Frequently Asked Questions

What is the maximum voltage drop allowed as per IS 732? +
IS 732 limits the voltage drop on any branch circuit to 3% of the nominal supply voltage. For a 230V single-phase circuit this means a maximum of 6.9V drop. For a 415V three-phase circuit, the limit is 12.45V.
How do I reduce voltage drop in a long cable run? +
You can reduce voltage drop by increasing the cable cross-section (larger mm²), using copper instead of aluminium, reducing the cable length by relocating the distribution board closer to the load, or splitting the load across two separate circuits.
What resistivity value should I use for copper cable? +
IS 732 uses a resistivity of 0.0172 Ω·mm²/m for copper conductors at 20°C. At operating temperature (70°C for PVC, 90°C for XLPE), the resistance increases — typically use 0.0196 for PVC-insulated and 0.0206 for XLPE-insulated copper cables for conservative design.
Does voltage drop affect three-phase motors differently? +
Yes. A 3% voltage drop causes roughly 6% reduction in motor torque (torque is proportional to voltage squared). IS 325 recommends motors be able to start with supply voltage as low as 85% of rated voltage, but sustained low voltage reduces efficiency and lifespan significantly.
Is voltage drop the same as voltage regulation? +
No. Voltage drop is the volt difference between the sending end and receiving end under load. Voltage regulation is the percentage change between no-load and full-load voltage at the receiving end. Voltage drop is used in IS 732 cable sizing; voltage regulation is used for transformer and generator sizing.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

⚡ Voltage Drop Calculator
Reference: IS 732