---
title: "Extension Lead Overload Calculator | Watt Cottage"
description: "Can I plug a heater into an extension lead? Add up what's on one UK lead or US extension cord, check it against its rating, and see the heater warnings."
url: https://wattcottage.me/extension-lead-overload-calculator/
---

Extension lead overload calculator

# Can I plug a heater into an extension lead?

Best not. Electrical Safety First, the UK’s electrical safety charity, says to avoid powering a heater from an extension lead because leads can easily be overloaded, and the US Consumer Product Safety Commission says to always plug an electric portable heater into a wall outlet, never a power strip. A fan or radiator heater, typically 2,000 to 3,000 W on Electrical Safety First’s list, uses most or all of a 13 A lead’s 3,000 W by itself. For everything else, add up the watts on the lead and keep the total under its rating.

## How to check what a lead can take

An extension lead doesn’t add capacity: everything plugged into it shares the one wall socket it plugs into. So add up the watts of what’s plugged in, divide by the voltage (230 V in the UK, 120 V in the US), and compare the amps with the rating marked on the lead. Electrical Safety First says extension leads are rated at 13 A, apart from some older ones at 10 A, and the rating should be clearly marked. At 230 V, 13 A is 2,990 W, in line with the 3,000 W it gives as the maximum rating of a wall socket, which is the figure the calculator checks a 13 A lead against; a 10 A lead carries 2,300 W. In the US the rating is on the cord itself, in amps or watts. The calculator shows the total, the current and the headroom, and turns amber at 90% of the rating and red above it.

Where do the wattages come from? The UK presets are Electrical Safety First’s own table, which it says is indicative and uses the highest figures it could find, so it errs on the safe side. The US presets come from Honda’s and Champion’s generator sizing guides. Your appliance’s rating label beats both; type its watts in.

## Why heaters are different

A heater draws close to its full rating for hours at a time, and on Electrical Safety First’s list a radiator or fan heater is typically 2,000 to 3,000 W, a patio heater 1,200 to 3,000 W. On a 13 A lead, a 3,000 W heater alone leaves no headroom at all; add a TV and the lead is overloaded. So the advice on both sides of the Atlantic is the same: heater straight into the wall.

- **UK:** Electrical Safety First says to avoid powering a heater from an extension lead, never to leave a heater unattended while in use or while you are asleep, and to keep combustible materials at least one metre away. It recommends not using a portable heater overnight.
- **US:** the CPSC says to always use a wall outlet for an electric portable heater, never a power strip, never to run its cord under rugs or carpeting, and to keep all sides at least three feet from beds, clothes, curtains, papers and sofas. The CDC also says to keep space heaters 3 feet from drapes, furniture and bedding.

A heater on a shared lead (left) can overload it by itself. Give it its own wall socket (right), and keep the lead for small things.

## Leads, reels and adaptors

- **One lead per socket.** Electrical Safety First says not to plug one extension lead into another (daisy chaining), and to use only one socket extension device per wall socket.
- **Block adaptors count too.** Don’t overload an extension lead, cable reel or block adaptor by plugging in appliances that together exceed its maximum current rating.
- **Unwind cable reels.** A reel must be unwound to reach its rating, and running one wound up can make it overheat and catch fire. Electrical Safety First says a wound reel’s rating is typically as low as 3 or 4 A, and that a reel should have a thermal cut-out. Tick “still wound” and the calculator checks against 3 A.
- **US cords:** the CPSC says a cord’s wattage rating should exceed the total wattage of everything connected to it. The Electrical Safety Foundation International says to use extension cords only temporarily, rated for indoor or outdoor use as needed, not to run them through walls or ceilings, and not to pinch them in doors, windows or under heavy furniture.

## A worked example

The calculator opens with a UK 13 A lead holding a 3,000 W radiator or fan heater, a 55-inch TV (120 W) and an LED desk lamp (5 W). That is 3,125 W, 13.6 A at 230 V: 125 W over the 3,000 W that Electrical Safety First gives as a socket’s maximum, and a heater on a lead besides. Move the heater to its own wall socket and the lead carries 125 W, under 1 A. In the US version, a 1,500 W space heater on a 13 A cord draws 12.5 A at 120 V, 96% of the cord’s rating before anything else is added, and the CPSC says it belongs in a wall outlet anyway.

## Danger signs and when to call an electrician

Electrical Safety First lists the signs to act on: a smell of hot plastic or burning near an appliance or socket, sparks or smoke from a plug, scorch marks around a socket or plug, damaged or frayed cables, melted plastic, and fuses that blow or breakers that trip for no obvious reason. Unplug and get it checked. If you rely on extension leads every day, it suggests having more wall sockets fitted by a registered electrician instead. This calculator is general information, not electrical advice.

Choosing a heater for the room? Warm Window’s [heat loss calculator](https://warmwindow.homes/heat-loss-calculator/) works out the size. For converting a single appliance’s amps and watts, or checking a plug fuse, use the [watts, amps and volts calculator](https://wattcottage.me/watts-amps-volts-calculator/); for cords run from a generator, the [generator size calculator](https://wattcottage.me/) explains the outdoor-cord rules.

## Sources

All checked on 5 November 2026.

- Electrical Safety First: [Portable heaters](https://www.electricalsafetyfirst.org.uk/safety-advice/products-and-appliances/heating-cooling/portable-heaters/), [Extension leads, cable reels and block adaptors](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/extension-leads-cable-reels-and-block-adaptors/) (last updated 30 September 2026), [Overloading sockets](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/overloading-sockets/) (last updated 28 June 2026), [Plugs and fuses](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/plugs-and-fuses/) and [Ratings of electrical appliances](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/ratings-of-electrical-appliances/).
- US Consumer Product Safety Commission: [Winter weather safety tips](https://www.cpsc.gov/Newsroom/News-Releases/2026/CPSC-Issues-Winter-Weather-Safety-Tips-to-Prevent-Fires-and-Carbon-Monoxide-Poisoning) (16 March 2026) and [Portable generator hazards](https://www.cpsc.gov/s3fs-public/Portable_Generator_Safety_Alert_2017_5123.pdf) (revised September 2017).
- CDC: [Safety guidelines during and after a winter storm](https://www.cdc.gov/winter-weather/safety/stay-safe-during-after-a-winter-storm-safety.html) (reviewed 25 August 2026).
- Electrical Safety Foundation International (ESFI): [Understanding your home electrical system](https://www.esfi.org/understanding-your-home-electrical-system/) (extension cord safety tips).
- Honda Power Equipment: [Generator wattage estimation guide](https://powerequipment.honda.com/generators/generator-wattage-estimation-guide) and Champion Power Equipment: [Generator wattage chart](https://www.championpowerequipment.com/generator-wattage-chart/) (US presets).

## Questions

**Can I plug a heater into an extension lead?**

Best not. Electrical Safety First says to avoid powering a heater from an extension lead, as leads can easily be overloaded, and the US CPSC says to always use a wall outlet for an electric portable heater, never a power strip. Plug it straight into a wall socket of its own.

**How many watts can an extension lead take?**

A UK lead rated 13 A takes about 3,000 W in total (13 × 230 = 2,990 W); an older 10 A lead about 2,300 W. That is for everything plugged into it together, however many sockets it has. A US cord’s limit is the rating marked on it.

**Can I plug an extension lead into another extension lead?**

No. Electrical Safety First says not to plug an extension lead into another one, which it calls daisy chaining, and to use only one socket extension device per wall socket. Everything still runs through the one wall socket and its plug.

**Why does a cable reel need to be fully unwound?**

A reel only reaches its rating when unwound. Wound up, Electrical Safety First says its rating is typically as low as 3 or 4 A and it can overheat and catch fire. Unwind it fully, and use one with a thermal cut-out.

**How far should a space heater be from furniture?**

The US CPSC and CDC say at least three feet from beds, curtains, furniture and bedding; Electrical Safety First says at least one metre from combustible materials such as paper, furniture or curtains. Put it on a level surface where it won’t be knocked over.