How much electrical cable do I need for a home? A calculation guide by cross-section

Calculating a home's cabling correctly before giving a quote avoids two frequent problems: buying too little and having to go back to the shop halfway through the job, or buying too much and losing margin on material left unused.

Why the cable cross-section matters

The cable cross-section isn't an aesthetic choice. Each circuit in a domestic installation has a minimum cross-section required by the Low Voltage Electrotechnical Regulation (REBT) according to its use: lighting, general-purpose sockets, kitchen, air conditioning, etc.

Using an insufficient cross-section doesn't just breach regulations: over time, an undersized cable heats up more than it should under continuous load.

Usual cross-sections by circuit

• 1.5 mm² — lighting
• 2.5 mm² — general-purpose sockets
• 4 mm² — kitchen and oven
• 6 mm² — air conditioning
• 10 mm² — service connection or individual feeder

How much margin to add over the measurement

Measuring the length in a straight line between two points and buying exactly that amount of cable is the most common mistake of those starting out. In practice, the real route of the cable isn't a straight line: it goes up and down chases, turns at corners and leaves slack in junction boxes for future joints.

A margin of 10% over the measurement in concealed installation (somewhat less, 5% if exposed) usually covers that deviation without falling short or buying too much. If the circuit runs grouped in a multi-core cable (several conductors in a single sheath, typical of service connections and consumer units), the standard margin is 10% regardless of the type of installation.

Worked example

For a circuit of 30 metres measured in a straight line, concealed in conduit:

1. Measured length: 30 m
2. 10% margin: 3 m
3. Total to buy: 33 m

This is exactly the same calculation the PresupuestoYa calculator applies automatically for the Electrician trade, including the margin and the line-by-line breakdown.

Don't forget the protective conduit

Cable almost never runs on its own: in concealed installation it needs corrugated conduit, and on the surface, rigid conduit or cable tray. Each has its own margin for cuts and changes of direction, so don't calculate the conduit "by eye" from the cable:

• Corrugated conduit for concealing: +10% over the measured length.
• Rigid surface conduit: +8%, because there is less handling when it isn't hidden in the wall.
• Cable tray or trunking: +5%, the tightest margin because it is cut in long lengths with few joints.

Frequently asked questions

What cable cross-section do I need for the sockets in a home? 2.5 mm² is the standard cross-section for general-purpose socket circuits in a home under the REBT.

How much waste should always be added to electrical cable? At least 5% on exposed installation and 10% on concealed, to cover joints in junction boxes and bends around frames.

Is the margin the same for conduit as for cable? Not always: concealed corrugated conduit takes 10%, but exposed rigid conduit only 8%, because it is handled less when not hidden in the wall.

Conclusion

A well-done materials calculation isn't just about avoiding running short on site — it's the basis of a quote the client can understand and that protects your margin. Automating this part of the quote removes the human error of "calculating by eye" and leaves an itemised, professional document ready to send.