Choosing the right wire size is one of the most important decisions in any electrical installation — and one of the most commonly overlooked. Many homeowners and even some electricians default to whatever wire is cheapest or most readily available, without considering whether it is actually rated for the load it will carry. This is a mistake that can have serious consequences.
When a wire is undersized for its load, it overheats. The insulation degrades, becomes brittle, and eventually cracks or melts. This is one of the leading causes of electrical fires in Indian homes. Beyond fire risk, undersized wires cause voltage drop — your appliances run at lower voltage than they are designed for, reducing efficiency and shortening their lifespan.
The good news is that wire sizing is not complicated once you understand the basic principles. This guide walks you through everything you need to know to make the right choice for your home or project.
Wire size in India is expressed in square millimetres (sq mm), which refers to the cross-sectional area of the copper conductor inside the wire. This is different from wire diameter — a common source of confusion.
Cross-sectional area determines how much current a wire can carry. A larger cross-section means more copper, lower resistance, and a higher current-carrying capacity. Here is what the most common sizes look like in practice:
1.0 sq mm — approximately 1.13 mm conductor diameter — used for light circuits
1.5 sq mm — approximately 1.38 mm conductor diameter — the most common household wire
2.5 sq mm — approximately 1.78 mm conductor diameter — heavier sockets and power circuits
4.0 sq mm — approximately 2.26 mm conductor diameter — AC and geyser circuits
6.0 sq mm — approximately 2.76 mm conductor diameter — large AC units and heavy appliances
10.0 sq mm — approximately 3.57 mm conductor diameter — sub-distribution and main feeds
Note that these are cross-sectional areas of the copper conductor only — the actual cable will be thicker because of the PVC insulation layer around it.
The current carrying capacity of a wire is the maximum continuous current it can safely carry without overheating. The figures below are approximate values for single-core PVC insulated copper wires installed in conduit at normal Indian ambient temperatures (30–35°C). Actual capacity may vary slightly based on installation conditions, ambient temperature, and bundling.
1.0 sq mm — up to 10A — Suitable for: Lighting points, small switches
1.5 sq mm — up to 15A — Suitable for: Fans, lighting circuits, small 5A sockets
2.5 sq mm — up to 20A — Suitable for: General power sockets (15A), TV, refrigerator
4.0 sq mm — up to 25A — Suitable for: 1-ton and 1.5-ton AC units, geysers (3–5 kW)
6.0 sq mm — up to 32A — Suitable for: 2-ton AC units, large water heaters, heavy appliances
10.0 sq mm — up to 45A — Suitable for: Sub-distribution boards, heavy industrial equipment
These figures assume standard PVC insulated copper wire (IS 694) installed in conduit. For wires running through walls without conduit (direct embedding), you may need to derate by 10–15% due to reduced heat dissipation. Always check with a qualified electrician for your specific installation.
IS 694 — the Bureau of Indian Standards specification for PVC insulated cables up to 1100V — sets binding requirements for wire quality that directly relate to wire sizing. Understanding these requirements helps you recognise a genuinely compliant wire from one that only appears compliant.
For each wire cross-section, IS 694 specifies:
Maximum conductor resistance per kilometre — ensuring the copper is of the right purity and cross-section (higher resistance = less copper than claimed)
Minimum insulation thickness — the PVC jacket must be thick enough for the voltage rating
Minimum conductor cross-section tolerance — actual cross-section cannot be more than 3% below the nominal value
Voltage withstand — the insulation must pass a high-voltage test without breakdown
A common trick used by sub-standard manufacturers is to reduce the actual conductor cross-section while labelling the wire at the higher nominal size. For example, a wire labelled "2.5 sq mm" may actually contain only 2.2 sq mm of copper — reducing its current capacity significantly while appearing the same from the outside. ISI certified wires are tested against these parameters, which is why the ISI mark provides genuine assurance.
Even if a wire is correctly rated for the load current, voltage drop can be an issue on long cable runs. Voltage drop occurs because every conductor has resistance, and current flowing through resistance causes a voltage reduction along the cable length.
In India, the standard supply voltage is 230V (single phase). BIS and electrical installation codes generally recommend that voltage drop in internal wiring should not exceed 2–3% of the supply voltage — that is, no more than 5–7 volts from the distribution board to the appliance point.
Practical guidance for long runs:
For runs longer than 20 metres carrying significant load, go one size up from the calculated minimum
Example: If 2.5 sq mm is adequate for current at 10 metres, use 4.0 sq mm for the same load at 30+ metres
Long sub-distribution runs from main board to a sub-board should use 10 sq mm or larger
Submersible pump cables are especially vulnerable to voltage drop — always size generously for pump wiring
Use the formula: Voltage Drop = (2 × Length × Current × Resistance per metre) to calculate precisely
Here is a practical room-by-room reference for a typical Indian home, covering the most common circuits. These are starting points — your electrician should calculate based on your actual load list.
Bedroom — lighting and fan circuit: 1.5 sq mm | power sockets (5A/15A): 2.5 sq mm
Living room — lighting and fan: 1.5 sq mm | TV, music system, general sockets: 2.5 sq mm
Kitchen — general sockets (mixer, toaster): 2.5 sq mm | microwave/OTG (dedicated circuit): 4.0 sq mm
AC point — 1-ton or 1.5-ton split AC: 4.0 sq mm (dedicated circuit) | 2-ton AC: 6.0 sq mm (dedicated circuit)
Geyser / water heater — 3 kW to 5 kW geyser: 4.0 sq mm (dedicated circuit, separate MCB)
Washing machine — 2.5 sq mm minimum, dedicated socket preferred
Refrigerator — 2.5 sq mm dedicated socket recommended (continuous load)
Main distribution board to sub-board — 10 sq mm to 16 sq mm depending on total load
Submersible pump — size based on motor kW rating; typically 4.0–6.0 sq mm for domestic pumps
One important principle: always install separate dedicated circuits for high-load appliances like AC units and geysers. Sharing one circuit between an AC and other sockets is a common cause of MCB tripping and, in older wiring without MCBs, overheating.
These are the most frequent wire sizing errors we see in Indian residential and commercial installations:
Using 1.5 sq mm wire for AC units — 1.5 sq mm is not rated for the 15–20A draw of even a small AC; always use 4.0 sq mm
Using the same wire for lighting and power on the same circuit — separate these circuits for safety and flexibility
Mixing wire sizes on the same circuit — never join a thinner wire mid-circuit; the weakest section determines the whole circuit's safe capacity
Not accounting for load growth — if you plan to add a room AC in two years, size the wiring for it now; rewiring is far more expensive
Using non-ISI wire to save cost — undersized conductors in non-ISI wires mean actual capacity is lower than labelled
Ignoring ambient temperature — wires in hot roof spaces or near cooking areas derate; go one size up in such locations
Not installing dedicated circuits for heavy appliances — every AC, geyser, and oven should have its own circuit and MCB
Getting wire sizing right is one of the most important investments you can make in your home's electrical system. Correct sizing prevents overheating, reduces voltage drop, protects your appliances, and significantly reduces the risk of electrical fire.
This guide provides a solid foundation, but every installation is different. We always recommend working with a licensed electrical contractor who can calculate your specific load requirements, verify your existing wiring, and ensure the installation complies with the National Electrical Code (NBC) and local regulations.
Elmeck Wires & Cables manufactures the full range of ISI certified FR and FRLS house wires from 1.0 sq mm through 16 sq mm — all compliant with IS 694 and available through our nationwide dealer network. If you have questions about which product is right for your project, reach us on WhatsApp at +91 93111 52255.
