Which cable size is used for a lighting circuit?
Which cable size is used for a lighting circuit?
Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
Cable size is not chosen from the load alone. It has to satisfy four things at once: it must carry the design current, it must be protected by the device in front of it, it must keep voltage drop within limits over its length, and it must withstand the fault current for long enough for the protective device to operate. The installation method then derates it — a cable clipped to a joist carries far more than the same cable buried in loft insulation or bunched with others in a conduit. That is why the same 1.5 mm² cable is fine in one house and undersized in another, and why an electrician calculates rather than looks up a single number.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| 1.0 mm² twin and earth (UK) | Standard for a straightforward domestic lighting circuit on a 6 A protective device. | Marginal on long runs or where the cable is buried in insulation, both of which derate it. |
| 1.5 mm² twin and earth (UK) | The common default: more headroom for long circuits and for cables surrounded by insulation. | Stiffer and harder to fit into a shallow pattress or a small ceiling rose. |
| 14 AWG (US) | The standard conductor for a 15 A lighting and receptacle branch circuit under the NEC. | Cannot be protected at 20 A; the breaker size and the conductor must match. |
| Low-voltage LED wiring | Extra-low-voltage LED strip and downlight drivers use small flexible cable on the secondary side. | Voltage drop dominates at 12 or 24 V, so runs must be far shorter than the current alone suggests. |
Technique that changes the outcome
- Size for the protective device and the installation method, not just the load
- Check voltage drop on long runs; the limit is a percentage of nominal voltage
- Derate cable that is buried in thermal insulation or bunched with others
- Keep cables in the recognised safe zones so that later drilling does not find them
What goes wrong
- Treating any published figure, including this one, as a substitute for the wiring rules
- Working on a live circuit
- Notifiable work done without the required certification and inspection
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other cable and wire questions
| For | Use |
|---|---|
| Extension leads | One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating. |
| A socket circuit | Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method. |
| 12 V DC and automotive | Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest. |
| Outdoor and buried runs | Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable. |
| An electric shower | Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions. |
| Speakers | 16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium. |
Common questions
- Which cable size is used for a lighting circuit?
- Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
- What should I avoid?
- Treating any published figure, including this one, as a substitute for the wiring rules Working on a live circuit Notifiable work done without the required certification and inspection
Related pages
- Which extension lead should I use for a heavy load? One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating.
- Which cable size is used for socket circuits? Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method.
- Which wire size should I use for a 12 V circuit? Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest.
- Which cable should I use outdoors or underground? Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable.
- Which cable size does an electric shower need? Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions.
- Which speaker cable should I use? 16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium.
Sources
- Typical range compiled from manufacturer specifications — At One Place