Is it safe to plug a heater into an extension lead?

As the evenings turn cold, a portable heater is often the quickest way to warm a room, and the nearest wall socket is often already taken. So is it safe to plug a heater into an extension lead? No. A heater should always go straight into a wall socket, never into an extension lead or adaptor. One heater can use most or all of what a lead can safely carry, so the lead, plug or socket can overheat and start a fire, often without anything tripping first.

This post explains why heaters and extension leads don't mix, how to spot a lead or socket that is getting too hot, and what to do when you simply don't have enough sockets. Every check here can be made safely from the outside of a plug or lead. Never open up a socket or try to wire in a new one yourself.

Why a heater is different from most appliances

Electricity flowing through a cable makes a little heat. Normally the cable is thick enough that you never notice. The more power an appliance uses, the more current flows and the warmer every cable and connection gets.

A typical fan heater or oil-filled radiator uses 2,000 to 3,000 watts. Most extension leads in UK homes are rated at 13 amps, which works out at about 3,000 watts in total, shared across every socket on the lead. So a single heater can use most or all of the lead's capacity on its own.

A kettle runs for a few minutes. A heater runs for hours. That long, steady load is what slowly heats up a lead, a worn plug or a loose connection inside a socket, until the plastic softens, scorches or melts. Electrical Safety First, the UK electrical safety charity, puts it simply: never power a heater from an extension lead.

Why the fuse box won't always protect you

An overloaded lead won't always trip anything.

The MCB (miniature circuit breaker, the switch in your consumer unit that protects each circuit) is sized to protect the cables in your walls, not the lead on your floor. The lead is protected only by the 13 amp fuse in its plug, and a fuse can take a long time to blow when it is only a little overloaded. In that time the lead can get hot enough to damage itself and whatever it is lying on.

An RCD (residual current device, which cuts the power if current leaks to earth) doesn't help here either. Overheating isn't a leak, so an RCD won't see it.

How to tell if a lead or socket is overheating

Check the plugs and sockets you use for heaters and other high-power appliances. Switch off and unplug first, then look for:

• a plug, lead or socket that feels warm or hot to the touch

• brown or black scorch marks around the pins or on the socket faceplate

• a burning or fishy smell, which can be a sign of hot plastic

• crackling, buzzing or sparking when something is plugged in or switched on

• plugs that are loose in the socket or fall out easily

If you find any of these on a wall socket, stop using it and have it checked. If it is sparking or smells of burning, switch that circuit off at the consumer unit and treat it as urgent. Our emergency electrician service is there for exactly that.

Using a heater safely

• Plug the heater straight into a wall socket, and don't run another high-power appliance from the same double socket at the same time.

• Keep it well clear of curtains, bedding, furniture and clothes, and never dry washing on it or cover it.

• Switch it off when you go out or go to bed.

• Check the heater's own cable and plug. If either is damaged, stop using it until it has been repaired or replaced.

Using extension leads safely for everything else

Extension leads are fine for low-power items such as lamps, TVs, phone chargers and computers. A few rules keep them that way:

1. Add up the watts. Every appliance has its power rating on a label or in the manual. Keep the total on one lead under 3,000 watts, and never put more than one high-power item, such as a kettle, toaster, iron or heater, on a lead at all.

2. Don't daisy-chain. Never plug one extension lead or adaptor into another.

3. Unwind cable reels fully. A cable left coiled on its drum can't lose heat and can overheat at a much lower load.

4. Keep leads in the open. Under a rug or carpet, damage and heat go unnoticed.

5. Replace damaged leads rather than taping them up.

If an extension lead keeps tripping the power, our post on why your electricity keeps tripping explains the safe checks to try.

When the answer is more sockets

If you rely on extension leads every day, or have to unplug one thing to use another, your home needs more sockets. Adding sockets is a job for a qualified electrician. Some of it, such as running a new circuit, is notifiable work under Part P of the Building Regulations in England, which means it must be done by a registered electrician or approved by Building Control.

Before we add sockets, we check that the circuit can take the extra load and that the existing wiring is in good condition, because a new socket is only as good as the cable feeding it. Where a room is heated with electric heaters, that check tells us whether extra sockets on the existing circuit will do or whether a new circuit is the safer choice. We explain what we find before any work starts.

When a plug or socket has already burnt out, the job is to find out why, not just swap the faceplate. One customer wrote that we "quickly analysed why the plug had burnt out and how to correct and repair the fault". As NICEIC Approved Contractors, we test our work when it is finished and give you the right certificate for it.

If you're short of sockets, or a plug or socket in your home is showing signs of heat, JM Electric can check it and fit the sockets you need safely. Find out more about our domestic electrician service and book a visit.