What cable cross-section does an extension cord need?

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The conductor cross-section of an extension cord determines how much power you can safely draw through it and how far the voltage drops at the connected device. It matters more than cable length or the number of sockets — and it is the specification buyers overlook most often.

What 3 × 1.5 mm² actually means

Every cable jacket carries a marking such as 3 × 1.5 mm². The first number is the count of conductors: line, neutral and protective earth. The second is the cross-sectional area of a single conductor. The larger the cross-section, the lower the resistance — and therefore the lower the losses and the operating temperature.

Matching cross-section to load and length

Treat the following as a starting point for a standard 230 V installation protected by a 16 A breaker:

  • 1.0 mm² – light loads up to roughly 2300 W, runs up to 10 m. Suitable for lighting, electronics and small tools.
  • 1.5 mm² – the most common choice, up to roughly 3600 W on runs up to 25 m. Enough for most workshop tools and household equipment.
  • 2.5 mm² – the same power, but on runs up to 50 m. The choice for construction sites, gardens and anywhere the cable is long or the load continuous.

Voltage drop, or why length matters

Resistance rises in proportion to cable length. On a 50 m run of 1.5 mm² under full load, the voltage drop can exceed what sensitive equipment tolerates. Electric motors react most strongly: at reduced voltage they draw more current, run hotter and lose torque. The usual symptom is a tool that “has no power” despite being in perfect working order.

A practical rule: if you are torn between two cross-sections, take the larger one. The price difference is small and the headroom stays with you for years.

What is easy to forget

The power ratings of connected devices add up. A cord with four sockets does not permit four times the load — a single limit applies to the whole cable, stated on the rating plate. A heater and a kettle sharing one 1.0 mm² cord is the single most common cause of overheating.

The values above are indicative. The rating plate of the specific product and the parameters of your installation are always authoritative.