How many watts can an extension lead safely carry?

Six outlets still share one incoming plug and cable. Before adding another appliance, check the rating of the complete extension lead and the load already connected to it.

There is no universal wattage limit for extension leads. Use the manufacturer’s total current and power ratings, including any restrictions for cable reels, continuous operation or the installation environment. A thick cable, a surge-protection badge or a large number of sockets does not establish the capacity of the finished product.

Read the complete product rating first

A plug marked 16 A may be fitted to an assembly rated at a lower total wattage. Switches, connections and other components form part of that assembly. If its instructions specify 3500 W maximum, calculating 230 × 16 = 3680 does not authorise an extra 180 W.

Also distinguish a component’s voltage marking from the actual supply voltage. A “250 V” marking does not mean the wall outlet supplies 250 V. The relationship between volts, amperes and watts and kilowatts is useful only when the correct values are used.

For a load with a power factor of 1, power is voltage multiplied by current. At 230 V, that gives 2300 W at 10 A, 2990 W at 13 A and 3680 W at 16 A. These are calculated equivalents, not ratings to assign to an unidentified lead. Different countries use different plug systems, so a rating familiar from one country must not be assumed elsewhere.

Make one load calculation for everything connected

Consider a workbench with equipment drawing 900 W and 1500 W at the same time. The combined input is 2400 W. Even at unity power factor, that requires approximately 10.4 A at 230 V, exceeding a 10 A lead. Two occupied sockets can therefore overload a product with several empty sockets remaining.

Use the equipment’s electrical input figures. A motor’s shaft output, a microwave’s cooking power and an adapter’s DC output are not interchangeable with mains input power. Where a suitable input-current rating is available, it is usually the clearer starting point.

For single-phase AC, P = U × I × PF. A 1000 W input load operating at a power factor of 0.70 draws approximately 6.2 A at 230 V. Simply dividing watts by volts would underestimate that current. See VA versus watts and the guide to converting watts to amperes.

The supply has its own constraints. Checking wall-socket capacity includes considering the circuit behind it and any other connected loads.

A reel can have two very different ratings

A cable reel’s available capacity changes when heat cannot escape from the wound cable. Read both the wound and unwound ratings before using it.

For example, Brennenstuhl specifies its BAT ST 260, 40 m H05VV-F 3G1.5 reel at 1000 W wound and 3500 W unwound, at 230 V. Its plug and socket outlets are marked 16 A. The complete product therefore provides a clear example of why plug markings alone do not settle the question.

Those figures belong to that particular model. Other reels have different limits. The explanation of why cable reels should be fully unwound covers the thermal issue. A thermal cut-out operating repeatedly is a reason to investigate the load and conditions, not a normal operating cycle.

Choose cable size for the whole job

A cross-section marking describes the conductor, not the complete lead’s approved load. 3G1.5 means three cores, including a protective conductor, each with a cross-section of 1.5 mm². It does not mean a combined 4.5 mm² supply conductor.

Start with the manufacturer’s instructions for both the lead and the equipment. Then consider cable cross-section selection and current-carrying capacity. Neither an outside diameter nor a “heavy duty” description replaces that information.

The guide to common metric cable sizes explains typical applications. For other markings, use the correct AWG and mm² conversion. Equal dimensions also do not make copper and aluminium conductors electrically equivalent.

Length matters even when the wattage looks acceptable

A tool can be within a lead’s nominal load rating and still receive inadequate voltage through a long cable. Starting a motor adds another demand that an average power reading may miss.

The longer route adds resistance. Under load, this causes voltage drop along the cable. Use a suitable lead no longer than necessary, and follow any equipment instructions specifying a minimum conductor size for a given distance.

Connecting another extension lead does not create additional capacity. The first lead carries the downstream load as well, with extra cable and contacts added to the route. Use a single suitable lead rather than chaining extensions together.

A load calculation does not approve every appliance

Portable electric space heaters should connect directly to a suitable, sound wall outlet. Do not treat spare wattage on a power strip as permission to use it for a heater. Follow the appliance’s connection instructions.

Other high-power appliances may also specify direct connection or particular supply arrangements. Read those requirements before relying on the extension’s headline rating. A short successful trial does not demonstrate suitability for hours of operation.

Watch the contacts as well as the cable

A poor plug contact can overheat while total current remains below the lead’s rating. A loose socket, damaged termination or worn contact may concentrate heat in a small area.

The location of the heat helps distinguish an overheating extension lead from a hot wall socket. Neither condition should be dismissed because the appliance still works.

Stop using an assembly showing blackening or melted plastic. A burning smell or crackling sound also calls for investigation. Tape does not restore a damaged connection.

Understand what each protection feature actually does

Surge protection addresses certain transient overvoltages. It does not increase the amount of current the cable or contacts can carry. Check the product specification to establish whether separate overload protection is included. The comparison of surge protection devices, voltage relays and protected sockets explains these distinctions.

An RCCB does not provide overload protection by itself. An RCBO combines residual-current and overcurrent protection. See MCB, RCD and RCBO functions. Likewise, protective earthing has a safety role; it adds no wattage allowance.

If protection operates, investigate the cause. Installing a higher-rated circuit breaker does not upgrade the extension lead and may leave wiring inadequately protected.

Match the construction to the surroundings

A lead for a dry office and one for a demanding outdoor job need more than matching wattage ratings. Cable flexibility, sheath material, mechanical exposure and the complete connector assembly all matter. The overview of cable construction and types provides useful background.

Using an outdoor-suitable cable does not weatherproof an indoor power strip attached to it. Check the suitability of the entire assembly.

Common rating questions

Technical references: Brennenstuhl: ratings for the reel used in the example · Electrical Safety First: extension leads and cable reels · CPSC: portable electric heater guidance · Schneider Electric: RCCB protection functions