Every electrical cable has a limit to the current it can carry safely. When that limit is exceeded, the cable heats up, insulation degrades and the risk of fire rises. Choosing the correct cable size — and matching it with the right breaker — is one of the most important decisions in any wiring job.
Why cable size matters
Cable size is described by the cross-sectional area of the copper conductor, usually in square millimetres (mm²). A larger conductor has lower resistance, carries more current and heats up less. An undersized cable may work for months before insulation begins to fail, which is why the problem often stays hidden until something goes wrong.
The factors an electrician considers
1. The load
The total current the circuit will carry, based on the appliances connected and how they are used at the same time.
2. The protective device
The breaker must protect the cable. A cable should never be protected by a breaker rated higher than the cable can safely carry.
3. Cable length and voltage drop
Long cable runs lose voltage along the way. For distant rooms, outdoor buildings or long appliance runs, a larger cable may be needed to keep voltage within acceptable limits.
4. Installation method
Cables bunched together in conduit, buried in insulation or run through hot areas cannot release heat as easily, reducing their safe capacity.
5. Ambient temperature
Higher surrounding temperatures reduce how much current a cable can carry safely — an important consideration in hot climates and roof spaces.
Typical circuit types in a home
- Lighting circuits carry relatively low loads and use smaller cable.
- General socket circuits need larger cable to handle a mix of everyday appliances.
- Air conditioners, water heaters and cookers usually need dedicated circuits with cable and breakers sized for their specific rating.
Exact sizes depend on the appliance rating, the cable type and the installation conditions, so each circuit should be calculated rather than guessed.
Warning signs of undersized cable
- Sockets, plugs or switch plates that feel warm.
- Breakers that trip when a particular appliance runs for a while.
- Lights dimming noticeably when heavy loads start.
- Discoloured cable insulation near terminals.
Quality matters too
Two cables marked with the same size are not always equal. Low-quality cable may contain less copper than stated or use insulation that degrades quickly. Use cable from reputable manufacturers and buy from trusted suppliers.
Get it calculated properly
When we plan new wiring or a rewire, every circuit is sized for its load, length and installation method, then tested before hand-over. If you are adding a new air conditioner or heater, our installation service includes checking that the supply cable is suitable.
Adding a heavy appliance? Ask us to check the circuit before you plug it in.