How to price a consumer-unit extension or new installation (electrician)

You get the job: the customer wants two new circuits for the workshop, a charging point in the carport and a bit more room in the board. Sounds like half a day's work. But the price has to cover more than cable and a few breakers if it is going to hold up both technically and financially. An underpriced extension eats your hours, and an overpriced one you lose to the competition. Here is the breakdown that lands right every time.

Work behind the board requires a licensed electrician. This guide is for you who holds the authorisation and needs to build a quote, not a DIY manual.

Start from the actual load, not the nominal rating

The most expensive mistake in a consumer-unit extension is sizing the cable to the breaker's rated current and forgetting what the circuit actually draws. An 11 kW charging point draws continuously, not in short peaks like a socket outlet. A workshop with a compressor and a welder has completely different diversity factors than a living room.

Cable cross-section must be sized to the real, continuous load, the temperature of the routing path and the grouping of circuits in the same conduit or tray. When several loaded cables lie close together, the current rating is reduced and you may need to go up a size. A 2.5 mm² cable that would handle the circuit on its own does not necessarily hold when it lies alongside four others in a warm routing path.

• Calculate for continuous load and diversity, not just the breaker rating
• Correct for temperature and number of cables in the same routing path
• Include voltage drop on long runs - a 16 A circuit over 35 metres may need 4 mm² instead of 2.5 mm²
• Document the choice so it can be verified

Choose the right protection: RCD type A, RCBO and residual current

Protection is where quotes most often end up too cheap. An ordinary type AC residual current device does not catch pulsating DC fault current, and that fault current arises in anything with electronics: frequency converters, washing machines, chargers, LED drivers and heat pumps. Circuits with that kind of equipment must have at least an RCD of type A, and chargers often require type B or built-in DC fault detection.

When a circuit needs individual protection, a combined breaker (RCBO) is often the clean choice: it combines overcurrent and residual current protection in one unit, so a fault on one circuit does not trip whole groups. It costs more in materials than a shared RCD, but gives a board that is easier to fault-find and more reliable - and that is an argument that sells to the customer.

• RCD type A as a minimum on circuits with electronics and frequency converters
• Chargers: type B or RCBO with DC fault detection
• RCBO for individual circuit protection - selectivity and easier fault-finding
• Cost the materials honestly: an RCBO per circuit adds up

Surge protection (SPD) belongs in the domestic board

Surge protection (SPD) has become standard in modern domestic boards, and for good reason: lightning strikes on the grid and transients from heavy loads destroy electronics that cost more to replace than the protection itself. If you extend a board without an SPD, it is an obvious recommendation to include - both technically correct and real added value for the customer.

Put it in as its own line in the quote with the SPD module, the required backup fuse and the labour time. Then the customer sees what they get, and you cover both materials and time instead of eating it as a "small extra".

Fire-stopping at cable penetrations is a requirement, not an option

If you route cables through a fire-separating building element - a floor separation, a fire wall between flats or between garage and dwelling - the penetration must be fire-stopped with an approved product so the fire resistance of the separation is preserved. It is a requirement in the building code, not a luxury, and it is the kind of thing that gets expensive to come back and redo.

Use a tested and approved sealing system - fire mortar, fire collar or intumescent compound depending on the penetration - and price both the product and the time. It is also a place where documentation pays off: a photo of the finished seal and the product specification is your cover.

• Approved, tested sealing system matching the building element's fire class
• Product and labour time as their own line - not hidden in "miscellaneous"
• Document with photo and product name

Build the quote in clear sections

A quote the customer can read and you can stand behind splits the work into sections rather than one big sum. It makes clear what is mandatory, what is a recommendation, and where the hours sit:

• Board: RCBO/RCD, breakers, SPD module, space and labelling, LK Fuga or equivalent gear
• Circuits: number of new circuits, sockets and outlets with materials per circuit
• Routing: running metres of cable per cross-section, conduit, tray and fixing
• Connection: labour hours for pulling, connecting and commissioning
• Documentation: measurements, board documentation, fire-stopping photo and handover report

Labour hours are where the margin is won or lost. A consumer-unit extension with five circuits, fire-stopping and an SPD is rarely under a day's work for one person, and long cable runs through an existing building take longer than new-build. Set the hours by the actual access and routing, not by a round number.

How Calco does it fast

Calco calculates the quantities for you: running metres of cable per cross-section, number of circuits and sockets, the gear from your wholesaler catalogue at your prices - and it knows the requirements, so RCD type A, RCBO for individual protection, SPD in the domestic board and fire-stopping at fire-separating penetrations come in as lines, not as something you have to remember on site. You get a quote with the right breakdown in minutes where you used to spend the evening on it.

See how an electrical quote is built from job to finished price at /tilbud-elektriker.