Electrical First Fix: A Practical Guide for Electricians on New Build and Renovation Projects

First fix electrical work is the phase that happens before plastering and finishing — installing the infrastructure that everything else depends on. Get it right and second fix flows smoothly. Get it wrong and you're chasing cables through plaster, drilling through joists you shouldn't touch, and having difficult conversations with builders about making good.

This guide covers the key decisions and standards for first fix electrical work on domestic projects: consumer unit positioning, cable routing and containment, back box installation, circuit planning, and the specific requirements of the 18th Edition IET Wiring Regulations (BS 7671:2018+A2:2022).

What Happens at First Fix

First fix covers everything that goes into the structure before plastering or dry-lining:

• Consumer unit (fuseboard) position confirmed and back box or enclosure fitted
• Cable runs laid through the building structure — through joists, studs, wall chases, or containment
• Back boxes (socket boxes, switch boxes) fixed in position
• Cables pulled through to each back box and left with sufficient tail length for second fix connections
• Any in-wall or in-floor containment (conduit, trunking) installed
• Earth bonding conductors run to gas and water entry points
• Smoke and CO alarm positions confirmed and cables run

What doesn't happen at first fix: no connections are made (other than earth bonding at the meter), no accessories are fitted, and the consumer unit is not energised.

Circuit Planning Before You Start

Before drilling a single hole, the circuit schedule should be agreed and documented. A typical 3-bedroom house might have:

The 18th Edition mandates that all circuits in domestic premises be protected by 30mA RCD. The most common approach is to use RCBOs (individual RCD/MCB combinations) for each circuit in the consumer unit — this avoids nuisance tripping affecting multiple circuits. Dual RCD boards (split load) are still permitted but less common in new installations.

Consumer Unit Positioning

The consumer unit position affects the entire cable layout. Key considerations:

• Height: The 18th Edition requires that all protective devices (MCBs, RCBOs) in a consumer unit are between 1350mm and 1450mm above floor level in domestic premises (measured to the operating handle). This is to prevent young children from easily operating them.
• Access: The consumer unit must be accessible for operation and maintenance — not inside a fitted cupboard with a fixed shelf in front, and not behind a door that obstructs access. A meter cupboard with a proper door is acceptable.
• Fire resistance: Consumer units in domestic premises must be in a non-combustible enclosure (metal) — plastic consumer units are no longer acceptable for new installations per Amendment 3 to BS 7671 (effective 2016). Steel consumer units are standard.
• Meter position: The consumer unit is typically adjacent to the electricity meter, which is positioned by the DNO (Distribution Network Operator). Confirm meter position with the DNO before first fix — moving it later is expensive.
• Cable entry: Allow adequate space below the consumer unit for cable entry — cables arriving from multiple circuits need to be managed neatly. A 500mm clear zone below is typical.

Cable Routing: The Rules

Cable routing is where the 18th Edition has specific requirements that catch out less experienced electricians.

Safe Zones (Prescribed Zones)

Regulation 522.6.6 of BS 7671 specifies that cables concealed in walls or partitions must be installed in one of the following safe zones, or be mechanically protected:

• Within 150mm of the top of a wall (running horizontally)
• Within 150mm of an angle (corner) of two adjoining walls (running vertically)
• Horizontally or vertically to an accessory or consumer unit (the cable runs straight to the socket/switch without deviating)

If a cable runs outside these zones, it must be enclosed in earthed metallic conduit or have additional mechanical protection against nails and screws. This is not optional — it is a BS 7671 requirement.

In practice: plan your cable routes carefully. Vertical drops from a joist or ceiling level down to a socket are fine. Horizontal runs should follow the top of the wall within 150mm of the ceiling. Do not run diagonally across walls unless the cable is in conduit or armoured.

Notching and Drilling Through Joists

Running cables through joists requires following building structural requirements (not just electrical ones):

• Notches in joists: Permitted only in the top of the joist, within the zone 0.1–0.25 of the span from the support (i.e., near the ends, not the middle). Maximum notch depth is 0.125× the joist depth.
• Holes through joists: Maximum diameter 0.25× the joist depth. Must be drilled on the neutral axis (the centreline of the joist depth). Must be at least 3× hole diameter apart, and located 0.25–0.4 of the span from the support.
• Where cables pass through holes in joists, no bushing or grommet is required (unlike metalwork), but cables should not be strained tight against the joist edge.

Thermal Insulation

If cables are to be buried in thermal insulation (e.g., in a loft), they must be derated — thermal insulation prevents heat dissipation and a cable carrying its rated current in insulation will overheat. Either:

• Use a larger cable cross-section (derated per BS 7671 Appendix 4), or
• Route cables along joists where they will not be buried in insulation

Back Box Installation

Depths

• Single socket: 25mm or 35mm deep back box (25mm for slim sockets, 35mm preferred)
• Double socket: 35mm standard
• Single switch: 25mm or 35mm
• FCU (fused connection unit): 35mm or 47mm
• Consumer unit: surface mounted or flush in a suitable enclosure

In Masonry Walls

Chase the wall with an angle grinder or proprietary chasing machine. Cut the recess for the back box accurately — a poorly fitting back box will not be flush after plastering. Fix with drywall screws into plugged masonry. Ensure the box is level and at the correct height (standard socket height in UK domestic is typically 300–450mm finished floor level, or higher for worktop-level sockets in kitchens at 1000–1200mm).

In Stud/Dry-Lining Walls

Use dry-lining (plasterboard) back boxes designed for cavity fixing — these have lugs or wings that expand behind the plasterboard. Cut the opening accurately with a padsaw or hole saw. Pull cables through before fitting the box.

In Timber Stud Walls

Where possible, position back boxes so they fix into the timber stud rather than relying on plasterboard fixings. A box screwed directly into timber is far more secure than a cavity fixing.

Earth Bonding at First Fix

Main protective equipotential bonding (main bonding) must be connected at first fix — this is the bonding of gas and water services to the main earth terminal of the consumer unit. Requirements:

• Gas service: Bond within 600mm of the gas meter outlet using 10mm² green/yellow cable (or smaller if the main earthing conductor is smaller — see BS 7671 Table 54.8)
• Water service: Bond within 600mm of where the pipe enters the building. If the service pipe enters as plastic, bond to the first metal fitting inside the building.
• Oil supply: Bond the oil tank and pipework similarly
• Main bonding clamps must be marked "Safety Electrical Connection — Do Not Remove"

Supplementary bonding (in bathrooms) is generally not required in new installations where all circuits are RCD protected and the earthing system meets requirements. However, it may still be required in certain situations — refer to BS 7671 Section 701.

Smoke and CO Alarm Wiring

The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England) require:

• At least one smoke alarm on every storey used as living accommodation
• A CO alarm in every room with a fixed combustion appliance (boiler, gas fire, log burner — but not gas cookers)
• Alarms to be in working order at the start of each new tenancy

For new build and full rewires, hard-wired interlinked smoke alarms are standard (and required by Building Regulations Part B for new build). The wiring circuit should:

• Be on a dedicated circuit or a lighting circuit — but the alarm supply must not be lost if an RCD trips. Using a non-RCD protected circuit for alarms, or using an RCBO so only that MCB trips, is the typical approach.
• Have all alarms interconnected via an additional interconnect wire (typically 3-core T&E used — live, neutral, and the interconnect wire on the earth)
• Provide a local battery backup in each alarm in case the mains supply fails

Documentation During First Fix

Before plastering:

• Photograph all cable runs — both the overall routing and close-ups of wall chases with cables in position. These photos are invaluable for second fix and for any future work in the property.
• Note the actual positions of any back boxes that deviate from the plan
• Confirm any amendments to the circuit schedule with the client/building contractor
• Update your as-installed drawing if you are producing one (required for Building Regulations compliance via Part P notification)

Part P Building Regulations Notification

Most electrical work in domestic premises in England and Wales requires either:

• Self-certification by a registered competent person scheme (NIC EIC, NAPIT, ELECSA, etc.), or
• Notification to and inspection by the Local Authority Building Control

Full rewires, new circuits, and consumer unit replacements all require notification. Register the work with your competent person scheme as you go — don't leave it until the end of the project.

Products from APM

APM Electrics Plumbing stocks a range of electrical accessories for first fix including back boxes, wiring accessories, cable management products, and electrical sundries. Browse our electrical collection for sockets, switches, back boxes, and cable management.

Summary

• Plan the circuit schedule before starting — it determines your cable sizes, routing, and consumer unit capacity
• Consumer units must be metal (non-combustible) in new domestic installations; MCB/RCBO operating handles at 1350–1450mm
• All circuits in domestic premises require 30mA RCD protection (18th Edition); RCBOs per circuit prevent nuisance tripping
• Cable routes in walls must follow prescribed safe zones (150mm from top of wall, or vertical/horizontal to accessory) or be mechanically protected
• Notches in joists: top only, 0.1–0.25 of span from support, max depth 0.125× joist depth
• Main bonding of gas and water services at first fix — 10mm² green/yellow to main earth terminal, within 600mm of meter/entry
• Smoke alarm circuits: hard-wired interlinked, supply protected so RCD trip doesn't de-energise alarms
• Photograph all cable routes before plastering — this is essential for future reference
• Notify Part P competent person scheme as you go, not as an afterthought at project end

Shop Cable at APM Electricals — Trade Counter, Acton W3

APM Electricals stocks a full range of cable, consumer units, and electrical installation materials for trade professionals. Same-day collection from our Acton trade counter.

• Axiom Consumer Unit 12 Way 100A
• 6243Y 2.5mm 3 Core & Earth Cable Grey 100m Drum

Browse our full Cable range at apmi.uk. Visit us at 24 Western Avenue, Acton, London W3 7TZ or call 020 8702 8080.