Which cable for an extension lead?

An extension lead for indoor use is built on a flexible sheathed cable — in our range that means OWY (H05VV-F) 300/500 V and OMY (H03VV-F) 300/300 V, both with stranded copper conductors, PVC insulation and PVC sheath. The question “which cable for an extension lead” has no single numerical answer: the number of cores and the cross-section are set by the power and the number of appliances working at the same time, the length of the cable, the operating conditions and whether the socket has an earth contact. An extension lead working permanently outdoors is a separate case — we return to it later in this text.

A flexible cable for an extension lead: OWY or OMY

People commonly say “extension cable” or “cable for an extension lead”, though formally this is a cord: a flexible sheathed product intended for mobile and portable appliances. A cable for an extension lead has to withstand repeated coiling and uncoiling, and therefore must have stranded, not solid, conductors.

Both types are round, have stranded copper conductors and PVC insulation and sheath, and are made to PN-E-90103 and PN-EN 50525-1. What differs is the rated voltage, the range of cross-sections and the number of cores, as well as the application described on the product pages.

• OWY (H05VV-F), 300/500 V — round, from 2 to 5 cores, cross-sections from 0.5 to 6 mm², in black or white (other colours by arrangement). The product page states the application: homes, kitchens and offices, household appliances — including those working in a damp environment under medium duty, such as washing machines and refrigerators — and kitchen appliances and heating equipment.
• OMY (H03VV-F), 300/300 V — round, stocked from 2 to 4 cores, cross-sections from 0.5 to 1.5 mm², black or white (other colours by arrangement). According to the product page this is a general-purpose supply cable for mobile and portable electrical equipment: small household appliances and light portable loads — radios, table lamps and floor lamps. Recommended where the cable is bent frequently.
• OMYp (H03VVH2-F) — flat and two-core only, and therefore without a protective conductor. It is not suitable for an extension lead with an earthed socket.

One distinction is crucial here: 300/500 V and 300/300 V are rated voltages of the cable itself — a property of the product, not the voltage of the circuit or the installation; in Polish installations that is 230/400 V. Which type and which cross-section suit a particular set of appliances is assessed by an electrician, not by a catalogue.

What cross-section: appliance power and length

The cross-section follows from the current drawn by the appliances and from the length of the cable — the longer the extension lead, the greater the voltage drop along the conductors. No single universal figure can be given, because the permissible current-carrying capacity depends on the operating conditions: the ambient temperature, the way the cable is laid, and whether it lies free or is coiled or run in a bunch.

The number of appliances working at the same time also matters: a strip with several sockets adds up the power of every appliance, and the lead's cable is shared by all of them. That is why the question of which cable for an extension lead starts with the list of appliances, not with the number of metres. Hence a practical note: a cable coiled on a drum dissipates heat worse than an uncoiled one, so under a heavier load the drum should be unwound completely.

We give no current tables or ready cross-sections in this text. The choice of cross-section for specific appliances, and the protection of the circuit the extension lead is plugged into, should be assessed on site by a qualified electrician.

On the range side it looks like this: three-core OWY 300/500 V is available in 0.75; 1; 1.5; 2.5 and 4 mm² — including OWY 3×1.5 300/500 V and OWY 3×2.5 300/500 V. The whole type, from 2 to 5 cores, covers cross-sections from 0.5 to 6 mm², with 6 mm² appearing in the four- and five-core versions. Questions about available cross-sections and core counts are taken through the form on the Contact page and by telephone on +48 669 055 666 (service in Polish, Mon–Fri 8:00–15:00).

How many cores: a three-core cable with a protective conductor

If the extension lead has an earthed socket (with a pin), a three-core cable is needed: line, neutral and protective. The conductors are identified to PN-HD 308 S2; for three-core OWY and OMY the product pages give the colours: line brown, neutral blue, protective green-and-yellow. In the four- and five-core versions the colour set is different — in five-core OWY it is black, blue, brown, green-and-yellow and grey — so the colours should be checked each time in the “Structure” block on the page of the given product.

The protective conductor must be connected along the whole path: from the plug, through the cable, to every socket in the strip. Broken or omitted — that is the most common mistake when assembling an extension lead oneself.

Two conductors are sufficient only where the appliances supplied have a plug without an earth contact (protection class II) and the lead's socket likewise has no pin. Four- and five-core OWY cables are in the range, but how they are used — for example with a three-phase supply — and the choice of accessories should be left to an electrician.

Workshop and garden extension leads — what to watch for

An extension lead works by definition as a mobile load, so a narrower temperature range applies to it: for OWY and OMY cables that is −10…+70 °C in mobile use (and −30…+70 °C when permanently laid). In frost the PVC sheath stiffens, and bending and uncoiling a frozen cable damages it easily.

The second matter is working outdoors. OWY and OMY cables have PVC insulation and sheath, and the applications given on their product pages concern interiors: homes, kitchens, offices and household appliances. A PVC sheath is not resistant to sunlight, so these cables are not suitable for permanent outdoor use — such conditions call for rubber-sheathed cables, a separate group of products. Whatever the cable, outdoors and in damp rooms the accessories too — plug and socket — must be intended for such conditions.

A few further rules, independent of the type of cable:

• An extension lead is a temporary solution: it is not laid permanently in a wall, under flooring or in the ground — laying in the ground calls for underground cable.
• The cable is not run across a door threshold or in places where it may be crushed or driven over.
• Before every use the sheath, plug and socket should be inspected; a cable with a worn or cut sheath is fit only for replacement.
• A long extension lead does not replace an additional circuit. If a socket is needed permanently in a given place, a fixed installation is the more durable and safer solution.

Work on an electrical installation should be carried out by a qualified electrician.

In summary

An extension lead used indoors needs a flexible sheathed cable — with a protective conductor if the socket has an earth contact. In our range that means OWY workshop cables (H05VV-F) 300/500 V and — for light household loads, in line with the application given on the product pages — OMY household cables (H03VV-F) 300/300 V. Both types are copper, flexible and sold in 100 m coils. Check the available cross-sections and core counts in the collections, and confirm the choice for your specific appliances and installation with a qualified electrician.