What is the maximum voltage drop allowed as per IS 732?

IS 732 limits voltage drop to 3% of the nominal supply voltage on any branch circuit. For a 230V single-phase supply that is 6.9V, and for a 415V three-phase supply it is 12.45V.

What is the voltage drop formula for a single-phase circuit?

For single phase: Vd = (2 × L × I × ρ) / A. Where L is one-way cable length in metres, I is the design current in amperes, ρ is the resistivity of the conductor (0.0172 Ω·mm²/m for copper), and A is the cable cross-section in mm².

How do I reduce voltage drop in a long cable run?

Increase cable cross-section (larger mm²), use copper instead of aluminium, shorten the cable route, split the load across two circuits, or relocate the distribution board closer to the loads.

Does voltage drop affect motors?

Yes. A 3% voltage drop reduces motor torque by approximately 6% because torque is proportional to the square of voltage. Sustained low voltage reduces motor efficiency and significantly shortens its service life.

What resistivity value should I use for copper cable?

IS 732 uses 0.0172 Ω·mm²/m for copper at 20°C. At operating temperature use 0.0196 for PVC-insulated and 0.0206 for XLPE-insulated copper cables to account for temperature rise.

Voltage Drop Calculation as per IS 732 — Step-by-Step Guide

Voltage drop is one of the most important checks every electrical engineer must perform during the design of low-voltage (LV) cable systems. When current flows through a conductor, it encounters resistance and causes a reduction in voltage at the load end. If this drop is too large, sensitive equipment malfunctions, motors overheat, and lighting flickers. IS 732 — the Indian Standard for Electrical Wiring Installations — mandates that voltage drop on any branch circuit must not exceed 3% of the nominal supply voltage.

This guide walks you through the complete voltage drop calculation process, from the formula derivation to two fully worked examples — one single-phase and one three-phase — so you can confidently size cables on your next project.

Why Voltage Drop Matters in LV Circuits

The supply voltage at the source is typically 230V (single-phase) or 415V (three-phase) in India. As you run a cable from the distribution board to a load — say, 80 metres to a distant motor — the cable's resistance causes a voltage drop. The load only sees the reduced voltage.

Motors: Torque is proportional to voltage squared. A 5% drop causes a 10% torque reduction, stalling motors and increasing current draw.
LED drivers and electronics: Under-voltage causes flickering, reduced output, and premature failure.
UPS systems: Low input voltage forces UPS units onto battery unnecessarily, shortening battery life.
Incandescent and halogen lamps: Visible dimming occurs at just 3–5% drop.

IS 732 sets the 3% limit on branch circuits to protect equipment and ensure an acceptable quality of supply at the point of use. Some designers also apply an overall feeder + branch limit of 5% — but each branch must individually pass the 3% check.

The Voltage Drop Formula (IS 732)

IS 732 uses the Ohm's Law derivation based on conductor resistivity. The formulas are:

Single Phase: Vd = (2 × L × I × ρ) / A Three Phase: Vd = (√3 × L × I × ρ) / A Where: Vd = Voltage drop (Volts) L = One-way cable length (metres) I = Design current (Amperes) ρ = Resistivity of conductor (Ω·mm²/m) Copper: 0.0172 Aluminium: 0.0282 A = Cable cross-sectional area (mm²) Voltage Drop Percentage: Vd% = (Vd / Vsupply) × 100 Must be ≤ 3% as per IS 732

The factor of 2 in the single-phase formula accounts for the both the live and neutral conductors — current flows through both. In three-phase systems, √3 (≈ 1.732) replaces 2 because of the phase geometry.

Worked Example 1 — Single-Phase Circuit

Problem

A 3kW, 230V single-phase pump is located 60m from the distribution board. The design current is 13.5A. A 4mm² copper cable (PVC-insulated) is proposed. Check whether this satisfies the IS 732 3% voltage drop limit.

Solution

Given: L = 60 m I = 13.5 A ρ = 0.0172 Ω·mm²/m (copper, PVC — using 0.0196 at operating temp) A = 4 mm² Vd = (2 × 60 × 13.5 × 0.0196) / 4 = (2 × 60 × 13.5 × 0.0196) / 4 = 31.752 / 4 = 7.94 V Vd% = (7.94 / 230) × 100 = 3.45% Result: FAIL — 3.45% exceeds the IS 732 3% limit.

Corrective Action

Try 6 mm²: Vd = (2 × 60 × 13.5 × 0.0196) / 6 = 31.752 / 6 = 5.29 V Vd% = (5.29 / 230) × 100 = 2.30% Result: PASS — 2.30% is within the 3% limit. Use 6mm² copper cable for this circuit.

Worked Example 2 — Three-Phase Circuit

Problem

A 15kW, 415V three-phase motor is installed 80m from the motor control centre (MCC). The full-load current is 27A (at PF 0.86). Check voltage drop for a 10mm² copper cable.

Given: L = 80 m I = 27 A ρ = 0.0196 Ω·mm²/m (copper PVC at operating temperature) A = 10 mm² Vd = (√3 × 80 × 27 × 0.0196) / 10 = (1.732 × 80 × 27 × 0.0196) / 10 = 73.35 / 10 = 7.34 V Vd% = (7.34 / 415) × 100 = 1.77% Result: PASS — 1.77% is well within the IS 732 3% limit. 10mm² copper cable is acceptable.

Quick Reference — Maximum Cable Length for 3% Drop

For a 230V single-phase circuit using copper cable at full IS 732 3% allowance (Vd = 6.9V), the maximum one-way cable length for common cable sizes and currents is:

Cable Size	Current (A)	Max Length (m)
2.5 mm²	10	49.8 m
4 mm²	16	49.5 m
6 mm²	20	59.9 m
10 mm²	32	62.4 m
16 mm²	40	79.4 m
25 mm²	63	79.4 m

Use our free Voltage Drop Calculator to instantly check IS 732 compliance for any single-phase or three-phase circuit — no manual arithmetic required.

Common Mistakes in Voltage Drop Calculations

Using 20°C resistivity at operating temperature: PVC cables run at 70°C — always use temperature-corrected resistivity (0.0196 for copper PVC, 0.0282 × 1.14 = 0.0322 for aluminium PVC).
Using two-way length instead of one-way: The formula already multiplies by 2 for single-phase. Always input the one-way cable length (source to load).
Not checking feeder drop separately: If your main feeder has 2% drop and branch has 3%, the total at the load is 5%. Design each section so the cumulative drop is acceptable.
Ignoring neutral conductor for single-phase: The return path (neutral) has the same resistance as the live conductor — this is why the formula uses 2 × L.

Voltage Drop vs Voltage Regulation — Key Difference

Voltage drop is the difference in voltage between the sending end and receiving end of a cable under load conditions. It is governed by IS 732 for cable design and is expressed in volts or as a percentage of supply voltage.

Voltage regulation is the percentage change in terminal voltage between no-load and full-load conditions. It is used for transformer and generator sizing, not for cable design. Do not confuse the two — they use different calculations and serve different design purposes.

Aluminium vs Copper — Impact on Voltage Drop

Aluminium has a resistivity of 0.0282 Ω·mm²/m at 20°C — approximately 1.64 times higher than copper. This means for the same cable length and current, aluminium produces 64% more voltage drop than copper of the same cross-section. To achieve the same voltage drop performance, you need to increase the aluminium cable size by approximately one to two standard sizes (e.g., 16mm² aluminium ≈ 10mm² copper for voltage drop purposes).

Conclusion

Voltage drop calculation is a mandatory step in every LV cable design and must be performed alongside current carrying capacity checks. IS 732's 3% limit protects your electrical installation from equipment damage, inefficiency, and safety hazards. By mastering the formula and understanding when to upsize cables, you ensure a reliable, code-compliant electrical system.

For faster results on every project, use the MEPMate Voltage Drop Calculator — it checks IS 732 compliance instantly for both single-phase and three-phase circuits. Also check our Cable Size Calculator which simultaneously sizes cables for both current capacity and voltage drop.