Voltage drop is one of the most under-appreciated causes of equipment malfunction in Indian electrical installations. A motor that trips on under-voltage protection, an LED driver that flickers, a control relay that fails to pick up reliably — these faults often trace back not to the equipment itself but to excessive resistance in the wiring feeding it. Understanding voltage drop, calculating it correctly, and measuring it in the field are essential skills for any electrician or electrical engineer working on LT installations.

What Is Voltage Drop?

Every conductor has resistance. When current flows through that resistance, energy is lost as heat and the voltage at the far end of the cable is lower than the voltage at the source end. This difference — the voltage that is "used up" by the cable rather than delivered to the load — is voltage drop.

A simple analogy: imagine water flowing through a long, narrow pipe. Friction in the pipe resists the flow and the water pressure at the tap end is lower than at the pump. Electrical voltage drop works the same way — current (flow) through resistance (friction in the conductor) produces a pressure loss along the cable run.

Voltage drop — single phase circuit

Common Causes of Excessive Voltage Drop

Undersized cable

The most common cause. A cable selected for a lower load, then subjected to a higher current as the load grows, will see voltage drop increase as the square of current.

Excessive cable length

Voltage drop is directly proportional to length. Long sub-main runs to distant DB boards or motors at the far end of a factory are frequent offenders.

Loose or corroded connections

A poorly terminated lug, a corroded busbar joint, or a loose terminal adds resistance at that point — often enough to cause significant drop at full load.

High ambient temperature

Copper resistance increases approximately 0.4% per °C. Cable running through a hot area (engine room, above furnaces) has higher resistance than the same cable in a cool environment.

Harmonics and skin effect

High harmonic content in the load current increases the effective resistance of the conductor, particularly in larger cable sizes.

The Voltage Drop Formula

The standard formula for voltage drop in a single-phase circuit (live + neutral, both conductors contribute resistance):

Single-phase voltage drop

VD = 2 × ρ × L × I / A

VD = Voltage drop (Volts)

ρ = Resistivity of copper = 0.0175 Ω·mm²/m (at 20°C)

L = One-way cable length (metres)

I = Load current (Amperes)

A = Cable cross-sectional area (mm²)

Alternatively: VD = I × R × L × 2 where R is resistance per metre from cable tables

Worked Example

Scenario: A 10 A load is connected 45 m from the distribution board via 4 mm² copper cable.

VD = 2 × 0.0175 × 45 × 10 / 4

VD = 2 × 0.0175 × 450 / 4

VD = 2 × 1.969 = 3.94 V

% VD = (3.94 / 230) × 100 = 1.71% — within IS 732 limit

Check: If cable were 2.5 mm², VD = 6.3 V = 2.74% — still acceptable but closer to the limit. Going to 50 m run would push it to 3.0% — borderline.

IS 732 Voltage Drop Limits

IS 732 (Code of Practice for Electrical Wiring Installations) sets the permissible voltage drop limits for LT installations in India. These limits apply from the origin of the installation (typically the energy meter / service entrance) to any point of utilisation:

Installation type	Max voltage drop	Notes
Lighting circuits	3%	From origin to lamp; sensitive to flicker
Power circuits (general)	5%	From origin to socket/fixed equipment
Motor starting (transient)	Up to 15%	Transient only — steady-state still ≤ 5%
Sub-mains (main to sub-DB)	Included in overall 5%	No separate limit; part of total budget

IS 732 vs IEC 60364

IS 732 is substantially aligned with IEC 60364. Some project specifications — particularly for industrial plants, data centres, or export-oriented facilities — reference IEC 60364-5-52 directly, which also specifies 3% for lighting and 5% for other circuits. Always check project specifications before finalising cable sizing.

How to Measure Voltage Drop in the Field

Calculating voltage drop during design is good practice. Measuring it on an installed circuit confirms whether the installation actually meets the limits under real load conditions — which accounts for actual cable routing, connection quality, and operating temperature.

Energise the circuit under full load

Voltage drop must be measured under the actual operating current. Measuring at no load gives zero drop, which tells you nothing.

Measure voltage at the supply end

Place your multimeter probes at the source terminals — the incoming terminals of the MCB or fuse feeding the circuit.

Measure voltage at the load end

Without changing the load condition, measure voltage at the load terminals — the motor terminal box, socket outlet, or final connection point.

Calculate the difference

VD = V_source − V_load. Express as a percentage of nominal: (VD / 230) × 100 for single phase.

Measure load current simultaneously

Use a clamp meter to record current during the voltage measurement. This allows you to cross-check against your calculated design values.

Use a True RMS meter for accurate results

Non-linear loads (VFDs, rectifiers, UPS systems, LED drivers) produce distorted current waveforms that cause additional heating in cables — and therefore additional voltage drop. A True RMS multimeter accurately captures the heating effect of distorted waveforms; an average-responding meter will underread on these loads.

CIE's True RMS digital multimeters and clamp meters are well-suited to voltage drop measurement in both residential and industrial installations. For cable sizing calculations, fault level verification, and commissioning support, explore our instruments range or speak to our technical team.

Voltage Drop: What It Is, Why It Matters, and How to Calculate It

What Is Voltage Drop?

Common Causes of Excessive Voltage Drop

The Voltage Drop Formula

IS 732 Voltage Drop Limits

How to Measure Voltage Drop in the Field