Why Does an Extension Lead Overheat? Causes, Risks and Safe Fixes

An extension lead overheats when the current passing through it produces more heat than the cable can shed. That usually comes down to four things: the lead is still coiled on its reel, the total load is above what the lead is rated for, a connection somewhere has gone loose or corroded, or the cable itself is thinner than the plug rating suggests. All four are fixable, and all four are worth taking seriously.

The physics in one paragraph

Copper has resistance. Push current through it and some of that energy turns into heat rather than reaching the appliance. Crucially, the heating effect rises with the square of the current, so doubling the load roughly quadruples the heat. A lead running a 500W lamp sits at room temperature all day. The same lead running a 3kW heater generates around thirty six times more heat. That is why a lead can behave perfectly for years and then get uncomfortably hot the first time you plug in something heavy.

Cause 1: the lead is still on the reel

This is the single most common reason, and the one most people have never been told about. A cable reel wound tight is a coil of hot copper wrapped in plastic insulation, with the inner turns unable to lose heat to the air. The outer turns stay cool and feel fine. The core of the drum can climb far higher.

Look at the label on almost any UK cable reel and you will see two ratings, something like 13A fully unwound and 5A or 6A wound. That second figure is not marketing caution, it is the real limit. Running a 2kW pressure washer or a heater off a coiled reel is a genuine fire risk, and melted reels are a recurring cause of garage and workshop fires.

Cause 2: the total load is too high

A standard UK 13A socket supplies around 3kW. That is the ceiling for everything plugged into a single lead combined, not per socket. A four gang lead does not give you four times the capacity, it just gives you four places to exceed it from.

Adding up your load is easy. Look at the wattage on each appliance rating plate, total them, and keep the result under 3000W. As a rough guide:

• Kettle: 2200W to 3000W
• Fan heater: 2000W to 3000W
• Pressure washer: 1400W to 2200W
• Workshop vacuum: 1000W to 1600W
• Laptop, router, lamp or charger: under 100W each

Two heat producing appliances on one lead will nearly always take you over. High resistance heating elements are the usual culprits. Kettles, heaters, irons and hair dryers deserve their own wall socket.

Cause 3: a loose or corroded connection

Every joint in the chain is a potential hot spot. A terminal screw that has worked loose, a plug pin that no longer grips the socket firmly, or a spot of corrosion all create extra resistance in a very small area, and the heat concentrates there rather than spreading along the cable.

The tell tale signs are browning around the pins, a plug that feels hot while the cable stays cool, a yellowed socket face, or a faint smell of hot plastic. Sockets plugged and unplugged thousands of times lose spring tension in their contacts, which is why old workshop leads often run warmer than new ones on the same load.

Cause 4: daisy chaining and cheap adaptors

Plugging one extension lead into another adds two more connections that can fail while keeping the same 13A limit. Block adaptors are worse again, because their pins are often lighter than a moulded plug and they hang under their own weight in the socket, loosening the contact. If one lead does not reach, use a single longer lead rather than two joined together.

Cause 5: undersized cable

Not every lead sold with a 13A plug has cable capable of carrying 13A comfortably over its full length. Budget leads sometimes use 0.75mm conductors where 1.5mm would be appropriate. Length compounds it, so a 25m lead runs warmer than a 5m one on the same load. Check the printing on the outer sheath for something like 3G1.5, meaning three cores of 1.5mm. For workshop use, 1.5mm is the sensible minimum on anything over about 10m.

What to do if a lead is hot right now

1. Switch off at the wall and unplug the appliances first, then the lead.
2. Let it cool before handling it properly. Hot plastic can be deceptive.
3. Inspect the plug pins, socket faces and cable for browning, distortion or a melted smell.
4. If anything is discoloured, softened or smells burnt, retire the lead. It is not repairable in any way worth trusting.
5. If it looks clean, work out what the actual load was and whether the reel was wound. Fix that before using it again.

Keeping leads cool in normal use

• Fully unwind reels, every time.
• Keep one lead to one heavy appliance.
• Do not run leads under rugs, through doorways or behind radiators where heat cannot escape.
• Choose leads with a thermal cut out for workshop and outdoor use.
• Give leads a quick visual check before each use and a closer look every few months.

Warm is not automatically dangerous. A lead carrying a genuine 3kW will feel noticeably warm, and that is just physics. Hot enough that you want to let go, or any smell, discolouration or softening, means stop using it.