Supplying an outbuilding with electricity requires the right cable route (SWA underground or overhead at minimum 3.5m — 6m where vehicles pass), a consumer unit in the outbuilding, correct TT earthing, and RCD protection at both ends. This guide covers every aspect of outbuilding electrical supply.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Supplying a detached outbuilding usually means a buried steel-wire-armoured (SWA) cable from the house, its own small consumer unit in the outbuilding, RCD protection, and careful attention to earthing — a TT arrangement with a local earth electrode is often used rather than exporting a PME earth to a building with extraneous-conductive-parts. The circuit is sized for the outbuilding load, and the work is designed, tested and certified to BS 7671.
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Supplying electricity to a detached garage, shed, summerhouse, home office, or workshop is one of the most common domestic electrical jobs. The installation must comply with BS 7671 for the cable routing, earthing, and protection arrangements, and is notifiable under Part P of the Building Regulations.
The two primary routes for the supply cable from the dwelling to the outbuilding are underground (buried armoured cable) and overhead (catenary-supported cable). The choice depends on the distance, ground conditions, obstacles, and aesthetic preferences. Underground is the preferred option for most domestic installations because it is hidden, protected from UV degradation and storm damage, and does not require maintaining clearance heights.
In addition to the supply cable, the installation requires a consumer unit in the outbuilding for local protection and control, appropriate earthing arrangements, and RCD protection at both ends of the supply. Getting these right from the outset saves the customer problems later — particularly as homeowners increasingly use outbuildings as home offices and workshops with significant electrical loading.
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Steel Wire Armoured (SWA) cable buried underground is the standard and preferred method for supplying outbuildings. The armour provides mechanical protection against accidental damage and, when correctly terminated, provides the earth continuity path.
2-core SWA (live, neutral, with armour as earth) or 3-core SWA (live, neutral, dedicated CPC plus armour). 3-core is preferred because it provides a dedicated earth conductor — the armour is then bonded at both ends as an additional earth path but the 3rd core provides the primary CPC. Conductor size depends on the maximum demand of the outbuilding and the cable run length.
Minimum 0.5m in gardens and open ground; 0.45m under paved areas; 0.6m under driveways. Cable tiles (protective tiles placed above the cable) and warning tape at approximately 150mm above the cable are required. Record the cable route with accurate dimensions from fixed reference points on an as-installed drawing.
The cable enters the outbuilding via a suitable weatherproof entry point — typically a conduit sleeve through the wall or floor, sealed against moisture ingress. The cable should rise inside the outbuilding to the consumer unit via conduit or trunking on the wall to protect it from mechanical damage.
Where underground cable is not practical — for example, where the garden is paved and digging would be very disruptive, or where the outbuilding is in a position that makes underground routing difficult — an overhead cable route can be used.
For areas accessible to pedestrians only. The cable must be at a height where a person cannot reach it and where tall garden equipment (ladder, long-handled tools) is unlikely to contact it.
Where vehicles can pass under the cable — including vans, lorries, and delivery vehicles — BS 7671 Regulation 709.521.1.8 requires a minimum height of not less than 6m above ground. If existing support heights cannot achieve this clearance, supports must be relocated or the conductor raised to comply.
For spans over approximately 3m, the cable must be supported by a catenary wire (a separate steel wire tensioned between the two buildings) to prevent the cable sagging below the minimum height under its own weight and under wind and ice loading. Self-supporting aerial cables (with a built-in supporting messenger wire) are an alternative that simplifies installation.
Overhead cables are more susceptible to UV degradation and storm damage than buried cables. XLPE-insulated SWA cable or purpose-made aerial bundled cable is suitable for overhead use; PVC-insulated cable degrades faster in UV exposure. Overhead cable routes should be inspected periodically for sag, damage, and vegetation contact.
Every outbuilding with a permanent electrical supply should have its own consumer unit. This provides local isolation, overcurrent protection for the outbuilding's circuits, and a convenient point for switching off all power to the outbuilding.
Earthing is one of the most critical aspects of an outbuilding supply. The earthing arrangement for the outbuilding depends on whether there is a continuous metallic earth connection from the main building.
If the supply cable has a metallic armour (SWA) that is bonded to earth at both ends, and the armour resistance is adequate, the outbuilding can be treated as a TN system. The armour provides the earth fault return path.
If the outbuilding earth is not connected to the main building earth via a metallic path (overhead cable without separate earth, or where armour earth path is not relied upon), the outbuilding is a TT installation.
In practice, it is good practice to install an earth electrode at every outbuilding regardless of the earthing system type. This provides an additional earth reference, reduces earth fault loop impedance, and ensures the outbuilding has a reliable earth connection even if the main supply cable earth path develops a fault.
For TT installations, compliance of the earth electrode is confirmed by calculation under BS 7671 Regulation 411.5.3: the electrode resistance Ra (in ohms) multiplied by the RCD rated residual operating current IΔn must not exceed 50V. For a 30mA RCD this gives a maximum permissible Ra of 1,667Ω — confirming that compliance depends on the RCD rating selected, not a fixed resistance target. Record the measured Ra on the EIC schedule of test results.
Elec-Mate's EIC Certificate app includes earth electrode resistance recording in the test results schedule.
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Start free trialRCD protection is required at two levels for outbuilding supplies:
The outbuilding supply circuit at the main consumer unit should be connected to an RCD-protected way — either an RCBO in the main consumer unit, or an RCD section with the outbuilding circuit MCB on the protected side. This protects the supply cable from the main building to the outbuilding.
All circuits within the outbuilding must be individually RCD protected at 30mA. For TT system installations, this is a mandatory requirement under BS 7671 Regulation 411.5. For TN system installations, 30mA RCD protection is required for all socket-outlet circuits (rated ≤32A) under Regulation 411.3.3 — there is no documented-risk-assessment exception where the outbuilding is ancillary to a dwelling. Regulation 411.3.4 (A4:2026) separately mandates 30mA RCD additional protection for AC lighting circuits in domestic premises, so outbuilding lighting circuits also require 30mA RCD protection where the outbuilding serves a domestic dwelling. Using RCBOs for each outbuilding circuit is best practice — it provides selective protection so that a fault on one circuit does not de-energise the whole outbuilding.
An outbuilding supply is notifiable under Part P of the Building Regulations in England because it involves a new circuit from the dwelling consumer unit. This applies to:
Register with an approved competent person scheme to self-certify and issue the Electrical Installation Certificate on site. The certificate should cover the supply circuit from the main consumer unit to the outbuilding consumer unit, with a separate schedule for the outbuilding internal circuits. Include the earth electrode resistance test result if a TT earth system is used.
Outbuilding supply jobs are excellent revenue opportunities. A properly specified outbuilding supply — correct cable sizing, TT earthing, RCBOs per circuit, metal consumer unit, and professional certification — demonstrates expertise and commands a professional price.
Long cable runs to remote outbuildings cause voltage drop. Use the cable sizing calculator to confirm the SWA cable size meets both current-carrying capacity and voltage drop requirements for the run length.
Issue your Electrical Installation Certificate for the outbuilding supply on site before you leave. Include all test results — continuity, IR, polarity, Zs, RCD operation, and earth electrode resistance.
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