Which extension lead should I use for a heavy load?
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.
A coiled cable reel is a genuinely under-appreciated hazard. Coiled cable cannot lose the heat generated by its own resistance, and the coil also acts as an inductor, so reels carry two ratings — a much lower one wound and the full one unwound. Running a 3 kW heater from a partially unwound reel can soften and then melt the insulation inside the drum where nothing is visible, and reel fires of exactly this kind are common. Beyond that, extension leads should not be daisy-chained, because each connection adds resistance and each socket block has its own limit, and the multi-way block is often the first thing to overheat since several loads share one fuse and one set of contacts.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Fully unwound cable reel | Reaches its rated current because the heat can escape. | Uncoiling and recoiling 25 metres is a nuisance, which is exactly why people do not. |
| Heavy-duty lead with a large conductor | The right answer for heaters, welders, kilns and site tools. | Heavy, stiff and expensive. |
| Light-duty extension lead | Fine for a lamp, a laptop or a television. | A thin conductor gets hot under a 2-3 kW load, and the socket block heats faster than the cable. |
| A properly installed socket | The correct fix for a permanently needed supply. | Requires an electrician and a certificate. |
Technique that changes the outcome
- Unwind a cable reel completely before drawing any significant load
- Match the lead to the load, not to what is nearest
- Feel the plug and the socket block after ten minutes under load; warm is a warning
- Use an RCD adaptor for anything outdoors or with a mains-powered garden tool
What goes wrong
- Running heaters, kettles or power tools from a coiled reel
- Daisy-chaining extension leads and multi-way blocks
- Covering a lead with a rug or running it under a door, where heat cannot escape and the sheath is crushed
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 |
|---|---|
| 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. |
| 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. |
| 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 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.
- What should I avoid?
- Running heaters, kettles or power tools from a coiled reel Daisy-chaining extension leads and multi-way blocks Covering a lead with a rug or running it under a door, where heat cannot escape and the sheath is crushed
Related pages
- 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 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.
- 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