What size cable for lighting?
Home lighting circuits usually use 1.0 mm² or 1.5 mm² twin and earth on a 6 A breaker (sometimes 10 A). The limit that matters is voltage drop: just 3 % for lighting, against 5 % for other circuits.
- V – Voltage
- I – Current
- P – Power
Current rating
Even the smallest cable is rated well above a 6 A breaker. Twin and earth 1.0 mm² is rated 16 A clipped direct and 8 A in the worst insulated-wall method, so current rating is rarely the problem. Use 1.5 mm² for a 10 A circuit through insulation.
Voltage drop: the real limit
Lighting is allowed only 3 % drop (6.9 V on 230 V). The longest run depends on the current the lights actually draw:
| Cable | Current | Longest run |
|---|---|---|
| 1 mm² | 6 A (breaker fully loaded) | about 26 m |
| 1.5 mm² | 6 A (breaker fully loaded) | about 39 m |
| 1 mm² | 2 A | about 78 m |
| 1.5 mm² | 2 A | about 118 m |
Worked example: 20 LED downlights at 8 W each is 160 W, or 160 ÷ 230 = 0.70 A. That's a tiny current, so 1.0 mm² easily covers a whole house. Old halogen or filament lamps draw far more, which is where 1.5 mm² earns its place.
Other things to check
- Every lighting point needs a CPC (earth), even if the fitting is plastic.
- Two-way switching uses 3 core and earth (6243Y) between the switches.
- Lighting circuits in rooms with a bath or shower need 30 mA RCD protection.
Assumes 70 °C flat twin and earth (6242Y), 230 V, an MCB or RCBO, 30 °C ambient, the circuit not bunched with others, and no thermal insulation round a clipped cable. Voltage drop limit 3 % (6.9 V) for lighting, BS 7671 Table 4D5 values. Grouping, heat or insulation can need a bigger cable: check with the cable size calculator. For study and estimating only; the installing electrician must design and certify the circuit.