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How to choose an extension cord or power strip for home and office
Cable cross-section for the load, length, number of sockets, earthing, a switch, surge protection and outdoor use: what to look at when buying an extension cord.
Cable cross-section and load
The key parameter is the cable cross-section, shown in the marking: 3G1.5, for example, means three cores of 1.5 mm². A general guide: 0.75 mm² for up to about 6 A (1.3 kW), 1.0 mm² for about 10 A (2.2 kW), 1.5 mm² for 16 A (3.5 kW), 2.5 mm² for heavy tools and long runs. The maximum load is also printed on the strip itself: 10 A/2,200 W or 16 A/3,500 W.
Add up the power of the appliances that run at the same time: a kettle and a heater draw about 2 kW each, so they cannot share one thin extension cord. A computer, TV, router and chargers together draw a few hundred watts, so 0.75–1.0 mm² is enough for them. If the cord or plug gets warm, the load is too high.
Length and cable reels
The longer the cable, the greater the voltage drop, so a long extension needs a thicker cross-section: 1.5 mm² normally works fine up to about twenty metres at 16 A, while a 25–50 m site reel for a compressor or welder needs 2.5 mm². For a room 1.5–3 m is enough, for a desk 3–5 m; spare cable just tangles on the floor.
Unwind a reel fully under heavy load: coiled cable cools poorly and heats up, which is why some reels have thermal protection. Do not daisy-chain extension cords; each connection adds resistance and a hot spot.
How many sockets and how they are laid out
Choose the number of sockets by the number of devices plus one or two spare. Look at the spacing between outlets: bulky adapters block the neighbouring socket; some models have angled or widely spaced outlets. USB ports for a phone and small gadgets save power adapters. For an office desk, mounting holes or double-sided tape on the underside are handy.
A switch with an indicator lets you turn everything off at once and avoid standby consumption; some models have a separate switch for each socket.
Earthing and child protection
A computer, fridge, washing machine, microwave and any tool with a metal body need an earthed (Schuko-type) extension cord with three cores and earth contacts in the outlets. Earthing only works when the wall socket is earthed too. A two-core cord with a flat Europlug is for lamps, chargers and a TV. For a child's room choose a model with safety shutters on the outlets.
Surge protection and voltage spikes
A surge-protected power strip differs from a plain extension by a varistor and a fuse: it absorbs short voltage spikes and protects equipment. The box states the absorbed energy in joules; the higher the better. An indicator shows that the protection still works, and after several strong surges the strip is replaced. It is worth having for a computer, TV, router and audio equipment.
A surge protector does not help against low voltage, prolonged overvoltage or a direct lightning strike: for those a voltage relay or stabiliser is fitted in the distribution board, and a server or point-of-sale equipment gets an uninterruptible power supply (UPS).
Extension cords for outdoor use
For the yard and a building site take a cord rated IP44 or higher, with covers on the outlets and a rubber or frost-resistant sheath. Indoor PVC cable (H05VV-F type) stiffens in frost and cracks in the sun. The cable must not lie in a puddle, be buried or be pinched by a door or window. ASB Trade stocks extension cords, surge protectors, reels and Unal cable, as well as sockets and plugs if you prefer to assemble your own.
Frequently asked questions
Can a heater be plugged into an extension cord?
Yes, if the cord is rated 16 A/3,500 W, the cable is at least 1.5 mm² and no other high-power appliances share it. A thin 0.75 mm² cord gets hot under a 2 kW heater.
How does a surge protector differ from a plain extension cord?
A surge protector has a varistor and a fuse that absorb short voltage spikes; a plain extension with a switch just extends the mains. Neither protects against low voltage or prolonged overvoltage.
Which reel should I take for construction tools?
On site a 25–50 m reel with 2.5 mm² cable and IP44 protection is the usual choice. Under heavy load the cable must be unwound fully so it does not overheat.