How much can I plug into an extension lead?

The one piece of arithmetic that decides whether an extension lead is safe, why the socket count is irrelevant, and the appliances that use up the whole budget on their own.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: warning

Short answer

Add the wattages of everything plugged in and keep the total under 3000W — that is the 13A limit of the plug and flex. Four sockets does not mean four appliances' worth of capacity. A kettle is 3000W on its own, a fan heater 2000W, an iron 2400W. A phone charger is 10W. The socket count is a convenience; the wattage total is the limit.

Every extension lead in Britain, however many sockets it has, ends in one 13A plug on one flex. That plug and that flex are the bottleneck, and the fuse in the plug is there to protect the flex. Get the arithmetic right and extension leads are entirely safe; get it wrong and the flex heats up under a carpet where nobody can see it.

Safety

The number of sockets on an extension lead tells you nothing about how much it can carry. A four-way lead has one 13A plug and one flex, so the total of everything plugged in must stay within that. Overloaded extension leads are a recognised UK domestic fire cause, and a coiled reel carries far less than its label unless it is fully unwound.

Extension lead load arithmetic, and why a coiled reel derates

Step by step

Find the lead's rating.Printed on the body or on a label. Most UK four-ways are rated 13A / 3000W (sometimes stated as 3120W). Some cheap leads and travel leads are rated much lower — 5A or 10A — and that figure governs, not the plug.
Read the wattage of each appliance.On the rating plate, usually on the back or underneath. If it gives amps rather than watts, multiply by 230 for an approximate wattage.
Add them up.Simply. A 3000W kettle plus a 900W microwave is 3900W, which is over the limit even though only two of the four sockets are used. A laptop at 65W, a monitor at 30W, a router at 12W and a lamp at 8W totals 115W, which is nothing at all.
Learn the big consumers by heart.Kettle 2500 to 3000W. Fan heater 2000 to 3000W. Iron 2000 to 2400W. Tumble dryer 2500 to 3000W. Washing machine up to 2200W while heating. Dishwasher up to 2200W while heating. Microwave 800 to 1500W input. Toaster 800 to 1500W. Hair dryer 1500 to 2200W. Any one of these can occupy most or all of a lead's capacity.
Learn the small ones too.Television 50 to 150W. Games console 100 to 200W. Laptop 45 to 100W. Desktop computer 150 to 400W. Router 10 to 15W. Phone charger 5 to 30W. Lamp with an LED bulb under 15W. A whole desk of these sits comfortably on one lead.
Halve the assumption for a coiled reel.An extension reel has two ratings on it, one coiled and one uncoiled — often 3000W uncoiled and 750W or 1000W coiled. Coiled cable cannot shed heat, so it must be fully unwound before carrying any real load.
Check the fuse in the lead's plug.A 13A lead has a 13A fuse. If someone has fitted a 3A fuse it will blow under moderate load; if the lead is a low-rated one with a 13A fuse fitted, the flex is under-protected. It should match the lead's rating.
Think about where the lead runs, not just what is on it.Under a carpet or a rug, a flex cannot lose heat and is also crushed underfoot. Behind a radiator, coiled behind a sofa, or looped tightly all have the same effect. Extension leads want to be visible and straight.

Tips

A rough conversion: watts divided by 230 gives amps. 3000W is 13A, 2000W is about 8.7A, 500W is about 2.2A.
Individually fused sockets on a lead protect each appliance's flex, which is a genuine feature worth paying for. They do not increase the lead's total capacity.
If a lead is warm anywhere along its length, it is carrying too much. Unplug something and check the total.

Common mistakes

Assuming four sockets means four appliances' capacity — Every socket is fed by the same 13A plug and the same flex. The socket count is about convenience, not about current.
Running a lead under a carpet or rug — It cannot lose heat, it is crushed by foot traffic, and any damage is invisible. It is one of the clearest recognised causes of extension lead fires.
Using a coiled reel at its uncoiled rating — The coiled rating is typically a quarter of the uncoiled one. A coil of loaded cable is a heater with insulation around it.

If it doesn't work

Lead's plug or body is warm

Cause: Total load too high, or a poor connection at the wall socket — Fix: Unplug everything, add the wattages, and reduce. If the total is well within limits, treat it as the hot-plug problem instead.

Lead's fuse blows regularly

Cause: Genuine overload, or a fault in something plugged in — Fix: Add the wattages first. If they are within limits, unplug items one at a time to find the fault. Never fit a larger fuse.

Flex is warm along its length

Cause: Undersized flex for the load, or a partially coiled reel — Fix: Uncoil fully, reduce the load, and replace the lead if the flex is thin relative to what it is being asked to carry.

Lead trips the RCD

Cause: A fault in the lead or in something plugged in — Fix: Unplug everything, plug the lead in alone. If it trips with nothing on it, the lead is faulty and goes in the bin.

Questions people ask

Is it safe to run a fridge or freezer on an extension lead?

It is not ideal. A fridge or freezer is a permanent load in a place people move things around, and an accidentally unplugged or damaged lead is an expensive discovery. Electrically the load is small; practically, put it in a wall socket.

Do surge-protected leads carry more?

No. Surge protection has nothing to do with current capacity. A surge-protected 13A lead is still a 13A lead.

Can I use several extension leads in different sockets on the same ring?

Yes, and that is far better than daisy-chaining. The ring circuit's own 32A protection covers the total, and with normal domestic loads you are nowhere near it. The risk is concentrating everything through one 13A plug.