SWA armoured cable specification for garden office power
The standard specification for underground power supply to a garden office in London is steel wire armoured (SWA) cable — also known as armoured cable. SWA cable has a steel wire armour layer between the inner insulation and the outer PVC sheath, providing mechanical protection against accidental damage from digging, garden forks, or ground movement. Two cable sizes are commonly used: 6mm² four-core SWA, suitable for circuits up to 40A (supporting a standard consumer unit with heating, lighting and sockets in a small to medium garden office); and 10mm² four-core SWA, suitable for circuits up to 63A (supporting high-draw loads such as electric vehicle chargers, large electric radiators, or underfloor heating in a larger office). The choice of cable size depends on the total connected load calculated by the electrician using the design current and diversity factors specified in BS 7671 (IET Wiring Regulations 18th Edition). Under-specifying cable size at installation is a common and costly mistake — increasing cable size retrospectively requires excavating the entire cable run. Builderr always calculates the connected load for the full specification before the cable is ordered, with a minimum 25% headroom for future load growth.
Burial depth, route and groundworks
UK wiring regulations (BS 7671) and HSG47 (Avoiding Danger from Underground Services) specify a minimum burial depth of 450mm for SWA cable under gardens, rising to 600mm under driveways and areas subject to vehicular traffic. At 450mm depth, a 300mm-wide warning tile or marker tape in yellow should be laid above the cable at approximately 150mm depth to alert future excavators. The cable route from the main house consumer unit to the garden office is the primary cost driver — a longer route across a large garden or one involving digging under a path, patio or tree root protection zone costs more. Typical cable runs in London gardens range from 5m to 30m. The cost per metre of trenching (hand-dig in London Clay): approximately £40–£80/m for manual excavation, £20–£40/m for machine excavation where accessible. Routing under a patio requires lifting and relaying paving: add £80–£150/m². Routing under a path using a mole plough or directional boring: £150–£300/m. Builderr typically routes cable runs along fence lines or structures to minimise risk of future accidental damage and avoid tree root protection zones — the route is agreed with the client and marked on an as-installed drawing for the property records.
Consumer unit, RCD protection and Part P compliance
A garden office must have its own sub-consumer unit (distribution board) with RCD protection — it cannot simply be run as an extension of a ring main from the house without overcurrent and fault protection. The standard specification is a two-way or four-way consumer unit with a 63A double-pole main switch, a 30mA RCD protecting all circuits, and individual MCBs for each circuit (lighting, sockets, heating). All sockets in a garden office must be RCBO or RCD protected under BS 7671. The sub-consumer unit in the office is supplied from a dedicated circuit in the main house consumer unit — a new 40A or 63A MCB is added to the main board to protect the SWA cable. Part P of the Building Regulations requires that all electrical work in dwellings and associated buildings (including outbuildings) is either carried out by a Part P registered competent person (NICEIC, ELECSA, or NAPIT scheme members) who self-certifies the work, or is inspected and tested by building control. An Electrical Installation Certificate (EIC) is issued by the installing electrician on completion and should be retained with the property documents. The EIC is specifically requested by solicitors and mortgage lenders on sale of the property.
Cat6 data, smart meter implications and DNO notification
Running Cat6 (or Cat6A for gigabit performance over longer runs) data cable in the same trench as the SWA power cable costs very little incremental expense compared with digging a separate cable trench — typically £200–£500 additional for supply and connection. The Cat6 cable should be separated from the SWA armoured cable by a minimum of 50mm (using a conduit or physical separator) to avoid electromagnetic interference from the power cable affecting data signal quality. The data cable should be run in 20mm conduit throughout to protect it and allow future replacement without excavation. For domestic supplies under 100A (most London residential properties), connection of an outbuilding sub-board does not require notification to the Distribution Network Operator (DNO — UK Power Networks in most of London). However, where the garden office has high-load equipment (EV charger, ASHP, large heating elements) that could push total property demand above 100A, or where the DNO tariff is single-phase and the total demand on the single-phase supply exceeds acceptable limits, DNO notification may be required. Smart meter implications: the smart meter communicates with the utility supplier based on total property demand — a garden office consumer unit wired downstream of the smart meter is included in the total consumption reading, which is correct.
