Why Does an Extension Lead Get Hot? Causes, Safe Limits and When to Stop Using It

An extension lead gets hot because current is meeting more resistance than the cable or socket was designed to handle, and that resistance turns into heat. The cause is almost always one of three things: too much load plugged in, a lead still wound on its drum, or a loose connection at a plug or socket. Slightly warm at the plug is normal. Too hot to hold is a fault.

Heat is just wasted electricity

Every metre of copper in a lead has a small amount of resistance, so a little energy is always lost as heat. That is harmless when the cable is sized for the load. The problem starts when the heat produced exceeds what the cable can shed into the surrounding air: temperature climbs, insulation softens, resistance rises further, and you get a slow feedback loop that ends in melted plastic. The real question is never "is it warm" but "why is it warmer than it should be, and is anything trapping that heat".

Cause 1: the lead is still wound on the drum

This is the most common reason a garage lead overheats, and the one people are most surprised by. A coiled cable cannot lose heat properly because every turn is insulated by the turns around it. Look at the rating label on any cable reel and you will usually see two figures, something like 13A fully unwound and 3.1A wound. That second number is not a suggestion. A fan heater drawing 8 to 10 amps on a wound reel will cook the inner turns within twenty minutes, and the outer turns will still feel cool while it happens.

• Unwind the reel completely for anything above roughly 700W.
• If you only need two metres, use a short lead rather than a reel with 23 metres still on the drum.
• Never leave a wound reel running a heater, compressor or dehumidifier unattended.

Cause 2: too much plugged in

A standard UK extension lead is rated at 13A total, roughly 3000W across all its sockets combined, not per socket. Four outlets do not mean four times the capacity, and it is easy to exceed without noticing.

Rough figures worth knowing: a kettle is 2000 to 3000W, a fan heater 2000W, a workshop dehumidifier 300 to 700W, a table saw 1500 to 2000W on startup surge, a laptop charger 65W. Two of the first three on one lead puts you at or over the limit; chargers and lights are irrelevant by comparison. Check the cable rating too, not just the plug: some cheap leads use 0.75mm² flex rated at 10A or less despite carrying a 13A fuse.

Cause 3: a poor connection somewhere

If the heat is concentrated at one spot rather than along the whole cable, you are almost certainly looking at a connection fault. Resistance at a loose joint is far higher than in the cable, so heat appears in a small area very quickly.

• The plug pins. Discolouration, pitting or a brown scorch mark means arcing. Replace the lead.
• The wall socket. Worn contacts grip loosely. Try the lead elsewhere and see whether the heat follows it or stays with the socket.
• Inside the plug top. On rewireable plugs, screw terminals work loose and fuse clips lose tension. A hot plug with a cool cable usually means one of those.
• The lead's own sockets. Repeatedly plugging heavy appliances in and out wears the internal contacts until they stop clamping the pins tightly.

Damage inside the cable counts here too. A lead that has been run over, trapped under a door, kinked repeatedly at one point or pulled out by the flex can have broken strands, and fewer strands carrying the same current means a hot spot you cannot see.

How warm is too warm

Use touch as the test. If you can hold the plug or cable comfortably in your hand indefinitely, it is fine. If you can touch it but want to let go after a few seconds, reduce the load or unwind the reel. If it is too hot to hold, if you can smell hot plastic, if the casing has deformed, or if there is browning around a pin, take it out of service.

Conditions matter too. A lead in direct sun, under a dust sheet or coiled behind a running dehumidifier has nowhere to dump its heat. Give leads air.

Practical habits that keep leads cool

1. Do not daisy chain leads. Each joint adds resistance, and the whole load funnels through the first plug in the chain.
2. Unwind reels fully before use, every time, not just for heavy loads.
3. Match the lead to the job. Buy a 1.5mm² heavy duty lead for tools and heaters, keep thin leads for lamps and chargers.
4. Give the lead a quick look before each use: pins, casing, and the first 30cm of flex either side, where damage concentrates.
5. Coil loosely in figure eights for storage rather than tight wraps around a hook, and pull the plug, never the cable.

None of this is complicated, but a hot lead is a common cause of domestic electrical fires precisely because the warning sign feels so minor. Warm plastic is easy to ignore. Treat it as information instead.