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Basics
Electrical wire sizes and types explained
Choosing the right wire is about safety first: a cable that is too thin for its load or its breaker overheats. This guide explains how wire size, breakers and cable types fit together, and why the rules differ between countries.
1. Safety and local rules first
This guide is for understanding, not for carrying out electrical work. In many countries, new circuits and changes to fixed wiring may only be done or certified by a licensed electrician, and the rules, voltages and wire sizes differ from country to country. Always switch off and lock out the circuit, and test that it is dead before touching anything. When in doubt, ask a qualified electrician.
2. What wire size means
Most of the world gives the size of a conductor as its cross-section in mm² (1.5, 2.5, 4, 6 mm² and so on). North America uses AWG (American Wire Gauge), where a smaller number means a thicker wire: 12 AWG is thicker than 14 AWG.
A thicker conductor can carry more current without overheating and loses less voltage over distance. How much current a cable may carry also depends on how it is installed: in conduit, buried in insulation or bundled with other cables, it runs hotter and must carry less.
3. Wire and breaker go together
The circuit breaker (or fuse) protects the cable, so its rating must never exceed what the cable can carry. Typical pairings in homes, for copper conductors:
| Use | Europe (230 V) | North America (120 V) |
|---|---|---|
| Lighting | 1.5 mm², 10 A | 14 AWG, 15 A |
| Sockets / receptacles | 2.5 mm², 16 A | 12 AWG, 20 A |
| Larger appliances | 4–6 mm², 20–32 A | 10 AWG, 30 A |
These are common examples, not rules: the correct size depends on your national code, the installation method and the cable length. The Wire size calculator gives an estimate for both systems.
4. Long runs and voltage drop
Every cable has resistance, so some voltage is lost along it. On long runs, such as to a garden building or a workshop, the drop can become large enough to make motors struggle and lights dim. A common design limit is about 3% for final circuits. If the drop is too high, the next larger wire size is needed even when the current would allow a thinner one. The Voltage drop calculator works out the drop for a given length, current and size.
5. Common cable types
- Sheathed installation cable with two or three cores in one outer sheath: NYM in Germany and much of Europe, twin and earth in the UK, NM-B (often called Romex) in the US. Used for fixed wiring inside walls.
- Single-core wires such as H07V-U/R (NYA) or THHN, pulled through conduit.
- Flexible cords such as H05VV-F or H07RN-F, for appliances and extension leads, not for fixed wiring.
- Outdoor and buried cables such as NYY or UF-B, rated for moisture, sunlight or direct burial.
6. Color codes
| Conductor | Europe (IEC) | North America |
|---|---|---|
| Live / hot | Brown (also black, grey) | Black, red |
| Neutral | Blue | White |
| Protective earth | Green-yellow | Green or bare copper |
Older installations may use different colors. Never rely on color alone; test with a suitable voltage tester.
Common mistakes
- Fitting a larger breaker so it stops tripping, which leaves the cable unprotected.
- Choosing wire size by current only and ignoring voltage drop on long runs.
- Using flexible cord or extension leads as permanent fixed wiring.
- Assuming wire colors are correct in an old installation without testing.
- Working on a circuit without switching it off and proving it dead.
Frequently asked questions
What wire size do I need for a 16 A circuit?
In many European installations 2.5 mm² copper is typical for 16 A socket circuits, but the correct size depends on the installation method, length and national rules.
Is a lower AWG number thicker?
Yes. In the American Wire Gauge system, 12 AWG is thicker than 14 AWG.
Can I do my own electrical work?
It depends on your country. Many places allow only minor work by homeowners or require inspection; new circuits usually need a licensed electrician.
Why does a long cable need a bigger size?
Because voltage drops along the cable. A thicker conductor has less resistance and keeps the drop within limits.
Which color is the earth wire?
Green-yellow in Europe and most IEC countries; green or bare copper in North America.