Wire Size Calculator

It is tempting to think wire sizing is one sum: take the appliance wattage, divide by voltage for the current, and read off a gauge. Real Indian residential wiring does not work that way, and neither does the code. IS 732:2019 sets a minimum cross-section for each category of circuit — lighting, sockets, dedicated appliance points, feeders — because the category already encodes what matters: the expected load, how many points share the run, the rating of the protective device in front of it, and the safety margin the standard demands.

A lighting circuit and a fridge socket might, on paper, pull similar wattage. But a socket has to survive whatever someone plugs into it next year — an iron, a heater, a vacuum — so the code floors it at 2.5 sqmm behind a 16A MCB, while lighting stays at 1.5 sqmm behind a 6A. The wire is matched to the protective device and the worst-case role, not to today’s bulb. That is why this tool asks what you are wiring, and returns a category minimum, exactly as our ElectroPro estimator does.

The other half of the answer is the protective device. A wire and its MCB are a pair: the MCB must trip before the cable overheats. Size the wire from the circuit’s role and you get a conductor that matches its breaker. Size it from a single appliance’s wattage and you can end up with a cable that its own MCB will never protect.

An undersized conductor carries more current than it is rated to. Copper has resistance, so that current turns into heat inside the wire — and because the cable is buried in a wall chase or bunched in a conduit, the heat cannot escape. Three things follow, in order:

• The insulation cooks. PVC insulation degrades with sustained heat. Over months it hardens, cracks, and stops insulating — long before anything visibly fails.
• Nuisance tripping — or worse, none. If the MCB is correctly rated for the load it trips repeatedly; if someone “fixes” that by fitting a bigger MCB, the wire is now unprotected and can run hot indefinitely.
• Char, then fire. The hottest point — usually a socket terminal or a joint — browns, then chars, then ignites the surrounding material. Most residential electrical fires trace back to an undersized or loose conductor.

IS 694:2010 adds a second trap: counterfeit wire. A cable stamped “2.5 sqmm” with a thin copper core is effectively undersized from the day it is laid. Insist on ISI-marked wire and verify the core, not just the print on the sheath.

Say you are wiring a 2,000 W storage geyser in a bathroom. A naive wattage sum says: 2000 W ÷ 230 V ≈ 8.7 A, and a 2.5 sqmm wire is rated comfortably above that — so 2.5 sqmm “works.” The code says otherwise.

AC / Geyser (2 kW+) → 4.0 sqmm minimum (IS 732:2019 Cl 6.2), dedicated 20A circuit

A geyser is a continuous, high-current load on its own dedicated circuit, and in a bathroom it must sit behind a 30 mA RCCB per IS 3043:2018. IS 732:2019 floors that dedicated appliance circuit at 4.0 sqmm — not because 8.7 A needs 4.0 sqmm in isolation, but because the category (a 2 kW+ point drawing near-continuous current, protected by a 20A MCB, often on a long run to the DB) demands that margin. Pick “AC / Geyser” above and the tool returns exactly this: 4.0 sqmm, the same figure ElectroPro puts on its DB schedule.