First Fix Electrical: Getting the Hidden Infrastructure Right

First fix electrical is the stage of an electrical installation that takes place before the walls are plastered and the floors are finished. It is the hidden infrastructure — the cables, containment, back boxes, and mounting points that will be concealed behind the finished surfaces. Everything installed at first fix must be correct before it is covered up, because accessing it afterwards means damaging the finished building.

On a typical domestic new build or full rewire, first fix accounts for roughly 60-70% of the total installation time. It involves running all cables from the consumer unit position to every switch, socket, light, and fixed appliance position in the property. It also includes installing containment (conduit, trunking, or cable tray), fitting back boxes into walls, and pulling cables for smoke detectors, data points, and any specialist systems.

The quality of first fix directly determines the quality of the finished installation. Cables routed outside BS 7671 safe zones risk damage from future fixings. Back boxes fitted at the wrong height or in the wrong position mean awkward accessory placements that the customer will notice. Containment that is too small for the number of cables creates overheating risks and makes pulling cables difficult.

Cable routing is the most critical aspect of first fix. Every cable must follow a route that minimises the risk of damage during and after construction, complies with BS 7671 safe zone requirements, and allows the cable to carry its design current without overheating.

In practice, planning cable routes before starting drilling and chasing saves significant time. Mark out accessory positions on the walls, plan the cable runs from each position back to the consumer unit, and identify any crossings or shared routes where containment will be needed.

For timber-framed properties, cables are typically run through drilled holes in joists and studs rather than chased into masonry. Holes in joists must be positioned in the centre third of the joist depth, drilled on the neutral axis, and spaced at least three diameters apart. Notches are generally avoided in modern construction — drilled holes are stronger.

Back boxes (also called pattress boxes) are installed at first fix to provide a mounting point for the accessories that will be fitted at second fix. Getting back box selection and positioning right at first fix avoids problems later.

Allow sufficient cable at each back box position for the second fix electrician to work comfortably. Leave at least 200mm of cable protruding from each box. This gives enough length to strip, terminate, and push back into the box without straining the connections.

Containment protects cables from mechanical damage, simplifies cable management, and provides a professional finish in exposed or surface-mounted installations. The choice of containment depends on the installation environment, the number of cables, and whether the containment will be concealed or visible.

When calculating containment fill rates, use Elec-Mate's conduit fill calculator to verify the cables fit within the maximum permitted fill percentage. Overfilled containment makes cable pulling difficult, can damage cable insulation, and reduces current-carrying capacity due to inadequate heat dissipation.

First fix electrical rarely happens in isolation. On any building project — whether a new build, extension, or rewire — the electrician must coordinate with plumbers, plasterers, carpenters, and the main contractor. Poor coordination is one of the most common causes of delays and rework.

Use Elec-Mate's quoting app to produce a clear first fix specification and quote that details every accessory position, circuit allocation, and containment requirement. Sharing this with the builder and other trades at the start of the project prevents misunderstandings and keeps the programme on track.

1st and 2nd Fix Electrical: Stage Guide 2026

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Part P of the Building Regulations (England and Wales) makes certain domestic electrical work notifiable. This means the work must either be carried out by a registered competent person (who can self-certify) or notified to Building Control before it starts.

First fix work that is notifiable under Part P includes:

If you are registered with NICEIC, NAPIT, ELECSA, or another competent person scheme, you can self-certify the work. You complete the installation, carry out initial verification testing, issue the Electrical Installation Certificate, and notify Building Control through your scheme. The scheme issues a Building Regulations Compliance Certificate to the customer.

If you are not registered with a competent person scheme, you must notify Building Control before starting the work. They will inspect at first fix stage (before plastering) and at completion. Building Control charges a fee for this inspection — the amount varies by local authority.

First fix mistakes are expensive because they are hidden behind finished surfaces. By the time a mistake is discovered, it often requires opening up walls or ceilings to fix. These are the most common first fix errors:

Before signing off first fix and allowing plastering to proceed, work through this checklist to ensure nothing has been missed:

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