House Wiring Cable Size Guide
Cable size is measured by the cross section of the copper core in square millimetres, and picking the right one is the single most important safety decision in home wiring. This guide explains how sq mm relates to current, how to match cable to each circuit and its MCB, and why long runs in Tricity homes sometimes need a size up.
What cable size do I need for house wiring?
Use 1.0 to 1.5 sq mm for lighting, 2.5 sq mm for general sockets, 4 sq mm for geysers and 16A sockets, and 6 sq mm for large air conditioners or hobs. Always size the cable to the circuit and its MCB.
01What sq mm actually measures
The sq mm figure printed on a cable is the total cross sectional area of the copper conductor, not the diameter of the outer sheath. A 2.5 sq mm wire has 2.5 square millimetres of copper carrying the current. The larger that area, the more current the wire can pass before it heats up, because a thicker conductor has lower resistance.
House wire is stranded, meaning the core is made of many fine copper strands twisted together rather than one solid rod. Stranding makes the cable flexible enough to pull through conduit and bend around corners without cracking, while still adding up to the stated sq mm of copper.
Be wary of cable that claims a size but weighs suspiciously little, because a genuine coil has a real, verifiable copper weight. Under gauge cable that is stamped 2.5 sq mm but only holds the copper of a 1.5 sq mm core is common in the cheaper end of the market, and it runs hot at a load it was never able to carry.
02Current carrying capacity by size
Every size has a safe continuous current rating. Push more current than that through it and the insulation slowly cooks, hardens and eventually fails. As a practical guide for enclosed household wiring, 1.0 sq mm suits around 11A, 1.5 sq mm around 14A, 2.5 sq mm around 19A, 4 sq mm around 26A, and 6 sq mm around 33A. Manufacturers print their own precise figures, and running cable bunched together in a hot conduit lowers these numbers.
These ratings assume normal installation. In a warm ceiling void or where several cables share one pipe, derate towards the lower end, because heat cannot escape and the conductor runs hotter for the same current.
03Matching cable to the circuit type
Different circuits draw very different loads, so they need different cable. A lighting circuit only feeds bulbs and fans and can use a slim 1.0 or 1.5 sq mm wire. A general purpose 6A socket circuit is usually run in 2.5 sq mm. A high draw appliance like a geyser, a 16A socket, or a washing machine deserves its own 4 sq mm circuit, and a big air conditioner or an induction hob often needs 6 sq mm.
- Lighting and fans: 1.0 to 1.5 sq mm.
- General 6A sockets: 2.5 sq mm.
- Geyser, 16A socket, washing machine: 4 sq mm.
- Large AC, hob, whole house feeds: 6 sq mm or more.
04Pairing cable with the right MCB
The MCB and the cable are a team, and the golden rule is that the cable must be rated to carry more current than its MCB will ever allow through. The breaker protects the wire, so a 2.5 sq mm circuit takes a 16A or 20A MCB at most, while a 4 sq mm circuit can take a 20A to 25A MCB.
The common and dangerous mistake is fitting a bigger MCB to stop nuisance tripping. That simply lets the cable overheat unnoticed. If a breaker trips repeatedly the answer is to reduce the load or lay a heavier cable, never to fit a larger breaker on the same thin wire.
05Voltage drop on long runs
Electricity loses a little pressure over distance, and on a long cable run that voltage drop can leave an appliance underfed and a wire warm. This matters in larger Tricity houses and in homes where the sub board sits far from the main board. The fix is simple: go one size up on any run longer than roughly fifteen to twenty metres, especially for heavy loads.
For example, a geyser thirty metres from the board is happier on 6 sq mm than on 4 sq mm. The extra copper costs little and keeps the voltage healthy and the cable cool along its whole length.
06Common sizing mistakes to avoid
The most frequent errors are all about cutting corners. Running the whole house on one thin size to save money, feeding a geyser off a 2.5 sq mm socket circuit, and oversizing the MCB to mask a hot cable are the three that cause the most trouble. Each one shifts risk into the walls where you cannot see it.
When in doubt, size up rather than down. Copper is cheap compared to the cost of opening a finished wall, and a slightly generous cable runs cooler, lasts longer and leaves room for tomorrow's appliances.
This margin matters more than ever as homes add load. A kitchen that once had a mixer and a toaster now runs an induction hob, a microwave and an air fryer at once, and a bedroom picks up a second air conditioner. Cable laid a little generously today quietly absorbs that growth instead of forcing a rewire.
Copper cable size, capacity and typical use
| Size (sq mm) | Approx current | Typical MCB | Best used for |
|---|---|---|---|
| 1.0 | About 11A | 6A | Lighting and fan points |
| 1.5 | About 14A | 6 to 10A | Lighting, small sockets |
| 2.5 | About 19A | 16A | General 6A socket circuits |
| 4.0 | About 26A | 20A | Geyser, 16A socket, washer |
| 6.0 | About 33A | 25 to 32A | Large AC, hob, sub feeds |
Frequently asked questions
What cable size do I need for house wiring?
Use 1.0 to 1.5 sq mm for lighting, 2.5 sq mm for general sockets, 4 sq mm for geysers and 16A sockets, and 6 sq mm for large air conditioners or hobs. Always size the cable to the circuit and its MCB.
How is sq mm measured?
It is the total cross sectional area of the copper conductor in square millimetres, not the thickness of the outer sheath. More sq mm means lower resistance and higher safe current.
Can I use a bigger MCB to stop tripping?
No. The MCB protects the cable, so a larger MCB on the same thin wire lets it overheat. Reduce the load or lay a heavier cable instead.
Do long cable runs need a bigger size?
Yes. On runs beyond roughly fifteen to twenty metres, go one size up to limit voltage drop and keep the cable cool, especially for heavy loads.
What size cable for a 2.5 ton AC?
A large AC typically needs a dedicated 6 sq mm copper circuit on its own C-curve MCB. Confirm the exact figure against the appliance rating plate.
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