Using a wire that’s too thin for the load is one of the most common causes of overheating and electrical fires at home. This guide explains, in simple terms, how wire size relates to current, what sizes are commonly used for home circuits, and how to estimate your load with a calculator.
In this article
- Why wire size matters
- Wire and breaker must match
- Commonly used wire sizes for home circuits
- Load and wire size estimator
- What wire size for an aircon?
- Water heaters and other heavy loads
- Long runs and voltage drop
- Wire quality and installation
- Signs your wiring may be undersized
- Worked example: a kitchen outlet circuit
- Should you DIY wiring?
- When to upgrade your home wiring
- FAQ
Why wire size matters
Electric current flowing through a wire creates heat. A thicker wire has less resistance, so it carries more current while staying cool. A wire that’s too thin for its load runs hot, its insulation ages faster and can melt, and the risk of fire rises. Thicker wire also reduces voltage drop, so appliances at the end of long runs get proper voltage.
Wire size is usually given as the cross-sectional area in square millimeters (mm²), or in AWG (American Wire Gauge), where a smaller AWG number means a thicker wire.
Wire and breaker must match
The circuit breaker protects the wire. It must trip before the wire gets dangerously hot. That’s why a circuit’s breaker rating is chosen to match the wire size. Never “upgrade” a breaker to a higher rating to stop it from tripping. That removes the protection, and the wire can overheat without the breaker reacting.
Commonly used wire sizes for home circuits
The table below shows sizes commonly used in homes for copper building wire. Actual requirements depend on your local electrical code, wire type, insulation, installation method, length of run and ambient temperature, so always have a licensed electrician confirm.
| Wire size | ~AWG | Common use | Typical breaker |
|---|---|---|---|
| 2.0 mm² | 14 | Lighting circuits | 15 A |
| 3.5 mm² | 12 | General outlets | 20 A |
| 5.5 mm² | 10 | Aircon, larger appliances | 30 A |
| 8.0 mm² | 8 | Bigger aircons, water heaters, ranges | 40 A |
| 14 mm² and up | 6 and up | Main feeders | Per design |
Load and wire size estimator
Estimate the current and a typical wire size
This is a learning tool, not an electrical design. Follow your local electrical code and have a licensed electrician size and install wiring.
What wire size for an aircon?
Aircons should be on a dedicated circuit, meaning their own breaker and wire, not shared with outlets. Smaller units are often on 3.5 mm² or 5.5 mm² wire, while larger units (2 HP and up) may need 5.5 mm² or 8.0 mm² depending on their rated current and the manufacturer’s installation manual. The manual usually lists the recommended breaker and wire size. Follow it.
Water heaters and other heavy loads
Instant water heaters can draw 3,500–5,000 W or more, which is over 20 A. They need a dedicated circuit with an appropriate wire size and breaker, plus ground-fault protection because they’re used in wet areas. See our GFCI and RCBO guide.
Long runs and voltage drop
For long distances, such as a pump far from the house or an outbuilding, a wire that’s fine for the current may still lose too much voltage. Motors and compressors then struggle and heat up. Electricians often choose a size up for long runs.
Wire quality and installation
- Use genuine, certified copper wire with proper markings on the insulation.
- Be wary of very cheap wire; some fakes use thinner copper than labeled.
- Run wires inside conduit, use proper connectors, and avoid tight bends.
- Use the correct colors for line, neutral and ground as required locally.
- Never splice wires without a proper junction box.
Signs your wiring may be undersized
Outlets or switch plates that feel warm, lights that dim when an appliance starts, a burning smell, discolored outlets, or a breaker that trips often are all warnings. Read our guide on why a breaker keeps tripping, and call a licensed electrician to inspect.
Worked example: a kitchen outlet circuit
Imagine a kitchen circuit feeding a rice cooker (650 W), an electric kettle (1,500 W) and a microwave (1,000 W). If all run at once, that’s 3,150 W. At 230 V, the current is about 13.7 A. A 3.5 mm² wire on a 20 A breaker can handle that in a typical installation, but a 2.0 mm² lighting wire on a 15 A breaker would be pushed to its limit, and with any extra load the breaker would trip. That’s why kitchens often have more than one outlet circuit, and why you shouldn’t tap outlets off lighting wires.
Should you DIY wiring?
Changing a light bulb or plugging in an appliance is fine for anyone. But adding circuits, replacing wires or working inside the panel should be left to a licensed electrician. Mistakes may not show up immediately: a loose connection or undersized wire can run warm for months before failing. A professional will also make sure the work follows your local electrical code and passes inspection, which matters for safety and sometimes for insurance.
When to upgrade your home wiring
Older homes were wired for fewer appliances. If you’ve added aircons, a water heater or an induction cooker, the original wiring may be overloaded. Signs include warm outlets, flickering lights and frequent trips. An electrician can assess whether you need new circuits, a bigger panel or a service upgrade.