Which cable for sockets?

Household socket circuits are mostly wired with solid-conductor installation cables: most often white YDY or YDYp with a protective core, sold in 100 m coils. Which cable is right for a particular socket circuit depends on the power and nature of the loads, the length of the run, the method of installation, the ambient temperature and the protective device chosen. Cross-section and core count are not copied from a guide; they follow from calculations and from the installation design, and the decision on site is made by an electrician.

What the cross-section for a socket circuit depends on

The question “which cable for sockets” comes down in practice to the conductor cross-section. Socket circuits work under a larger and less predictable load than lighting circuits, because a vacuum cleaner, a heater or a kettle can be plugged in at any moment — and in a kitchen several such loads at once. The cross-section does not follow from the purpose of the circuit, though: it is calculated separately for every circuit leaving the distribution board.

The result of those calculations is driven above all by:

• the total power and the nature of the loads planned for the circuit;
• the length of the run, because the longer it is, the greater the voltage drop;
• the method of installation: in conduit under plaster, surface-mounted, bunched with other circuits, or in a layer of thermal insulation;
• the ambient temperature where the cable is laid;
• the type and rated current of the circuit's protective device;
• how the installation is divided into circuits: kitchen and bathroom sockets, and those feeding high-power appliances, are normally planned as separate circuits.

No current value can therefore be tied permanently to a cross-section: the same cable in conduit under plaster, bunched with other circuits, and in a layer of thermal insulation has a different current-carrying capacity in each case. The actual figures follow from the design calculations for the given circuit, and it is the electrician who matches them to the conditions on site. A cross-section taken “with a margin” replaces neither a correct division of the installation into circuits nor a protective device matched to the cable.

YDY or YDYp under plaster?

Both types are installation cables for permanent laying: they have solid copper conductors, PVC core insulation and a white or black PVC outer sheath. According to the product pages they are suitable for surface-mounted and concealed installation, in dry and damp environments, without additional mechanical protection, and the operating temperature range is −30…+70 °C. What differs is the shape: YDY is round and made to PN-E-90056:1987 and PN-EN 50525-1:2011, YDYp is flat and complies with PN-E-90060:1987 and PN-EN 50525-1:2011.

Choosing between them is mainly a question of how the run is routed. A round cable is easier to pull into conduit or flexible tubing, and such an installation can later be replaced without chasing the walls. A flat one is more convenient to lay in a shallow chase and under a layer of plaster, because it protrudes less beyond the face of the wall. Both are rigid cables intended for permanent connections, and in both cases the same rules for choosing the cross-section apply.

Before deciding which cables for sockets to order, it is worth comparing the product pages. The items YDY 3×2.5 and YDYp 3×2.5 show the full set of parameters: shape, number of cores, cross-section, rated voltage, standards, operating temperature range and core colours. The rated voltage — 300/500 V or 450/750 V, for example — is a parameter of the cable itself: it states the voltage its insulation was designed for, not the voltage present in the circuit. Installations in Poland work at 230/400 V. In our range every YDY item and the three-core YDYp are rated 450/750 V, while the two-core YDYp comes in two versions: 300/500 V and 450/750 V. The value for a particular item is given on its product page.

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

Sockets with an earth contact require a circuit with a protective conductor; a cable without one must not be used for them. In a single-phase circuit that means a three-core cable: a line, a neutral and a protective conductor. The number of cores is not a matter of habit carried over from the previous circuit. The cable must have as many cores as the installation design foresees line, neutral and protective conductors for that circuit.

The core colours are given on the YDY and YDYp product pages, identified to PN-HD 308 S2:2007. In the three-core version they are blue, green-and-yellow and brown. In the five-core version: black, blue, brown, green-and-yellow and grey. Four-core cables call for particular attention, because the product data lists two different colour arrangements for them: with a protective conductor (black, brown, green-and-yellow, grey) and without one (black, blue, brown, grey). Before ordering it is therefore necessary to check which version a particular item covers, because a cable without a protective conductor is not suitable for sockets with an earth contact.

That is why the answer to the question of which cables to order for sockets rarely comes down to a single item. Which cable for sockets ends up on the materials list for a detached house follows from the installation design: one core count applies to single-phase circuits, and another where a three-phase supply is brought in.

Circuit protection and the cable cross-section

An overcurrent protective device protects the cable first and the connected equipment only second. For that reason the cross-section and the protective device are not chosen separately: the rated current of the breaker must match the current-carrying capacity of the cable under the real conditions of its installation. Increasing the protection on an existing circuit without checking the cross-section is one of the most dangerous shortcuts in an installation.

Permissible currents, the type and the rated current of the overcurrent breaker cannot be quoted “from a catalogue”, because they follow from calculations for a specific circuit. Independently of them, socket circuits in flats and houses additionally require protection by a residual current device. Work on an electrical installation should be carried out by a qualified electrician.

The most common mistakes when choosing a cable

• Copying the cross-section from another circuit because “that is how it was done before”, instead of calculating it for the circuit being designed.
• Doing without a protective conductor on sockets with an earth contact.
• Using flexible cables in a fixed installation: they are made for mobile connections and extension leads, not for laying under plaster.
• Ignoring the installation conditions — bunching, thermal insulation and high ambient temperature — which reduce the cable's current-carrying capacity.
• Choosing the protective device in isolation from the cable cross-section.
• Ordering too short a length; it is worth calculating the run with a margin for the drops to boxes and to the distribution board.

In summary: the cable for a socket in a house or a flat is usually a three-core installation cable with a protective conductor, and the choice between the round and the flat version depends on how the run is routed. The cross-section and the number of cores remain the result of calculations and of the installation design. Round YDY installation cables and flat YDYp cables are available in 100 m coils, and the product pages in both collections give the full set of parameters. If you are unsure which item from the range to choose, our specialists can help: the form in the Contact tab, or by telephone on +48 669 055 666 (service in Polish) or +48 607 496 325 (Polish and Ukrainian), Mon–Fri 8:00–15:00.