Outbuilding Electrical Supply: Garage, Shed, and Workshop

A supply to a detached garage, shed, summerhouse or workshop is a new distribution circuit from the dwelling consumer unit, feeding a small consumer unit in the outbuilding. Here is the whole job in one table, with the regulation each figure actually comes from.

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 cable routing, earthing and protection, and it is notifiable under Part P of the Building Regulations.

There are two routes for the supply cable: underground, using buried armoured cable, and overhead, using a self-supporting or catenary-supported cable. The choice depends on distance, ground conditions, obstacles and appearance. Underground is preferred for most domestic work because it is hidden, protected from UV and storm damage, and does not require a clearance height to be maintained for the life of the installation.

Beyond the cable itself, the job needs a consumer unit in the outbuilding for local protection and isolation, a decided earthing arrangement, and RCD protection. Getting those three right at design stage is what separates a supply that lasts from one that gets coded on the next EICR — particularly now that outbuildings are routinely used as home offices and workshops with real load on them.

Steel wire armoured cable buried underground is the standard method. The armour gives mechanical protection and, correctly terminated at both ends, can serve as the protective conductor.

The regulation, in full

Regulation 522.8.10 is the one that governs this. It requires that, except where installed in a conduit or duct providing equivalent protection against mechanical damage, a cable buried in the ground incorporates an earthed armour or metal sheath (or both) suitable for use as a protective conductor; that the location of buried cables is marked by cable covers or a suitable marker tape; that buried conduits and ducts are suitably identified; and that buried cables, conduits and ducts are at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground. The note treats metallic or non-metallic underground conduits classified N750 to BS EN 50626-1 or BS EN 61386-24 as providing equivalent mechanical protection.

Note what is not there: no depth in millimetres, and covers or tape, not both. The depth figures quoted around the trade are borrowed from the special-location sections, where they appear as notes rather than as absolute limits.

Cable selection

Two-core SWA uses the armour as the protective conductor; three-core SWA gives a dedicated CPC with the armour earthed at both ends as an additional path. Three-core is the better choice where the run is long or the armour's continuity is hard to guarantee, because the earth fault loop impedance no longer depends on the quality of every gland. Conductor size follows the maximum demand of the outbuilding and the run length — on a long run it is usually voltage drop, not current-carrying capacity, that decides.

Entry into the outbuilding

Bring the cable in through a weatherproof entry — a conduit sleeve through the wall or floor, sealed against moisture. Run it up to the consumer unit in conduit or trunking so it is protected from mechanical damage inside the building, and record the buried route with dimensions taken from fixed reference points so the next person to dig knows where it is.

Where trenching is impractical — a fully paved garden, mature planting, a route that cannot be dug — an overhead run is the alternative.

BS 7671 sets no clearance height for overhead conductors at ordinary domestic premises. The figures everyone quotes come from the special-location sections, and they are consistent across all of them: Regulation 708.521.7.3 for caravan parks and Regulation 709.521.1.8 for marinas both require that every overhead conductor is insulated, that poles and other supports are located or protected so they are unlikely to be damaged by any foreseeable movement of vehicles, and that conductors are at a height above ground of not less than 6m in all areas subject to vehicle movement and 3.5m in all other areas. In the absence of a domestic figure, those are the numbers to design to.

Support and cable type

Every overhead conductor must be insulated. Use a self-supporting aerial cable with an integral messenger wire, or support an armoured cable on a separate tensioned catenary wire; manufacturers state a maximum unsupported span, and in practice anything beyond a few metres wants a catenary so the cable cannot sag below its clearance under its own weight, wind or ice. Do not run unsupported twin-and-earth in the open air. Where support heights cannot achieve 6m over a vehicle route, move the supports or raise the conductor — do not accept the shortfall.

Overhead runs age faster than buried ones. XLPE-insulated SWA or purpose-made aerial bundled cable stands up to UV better than PVC-insulated cable, and the run should be inspected periodically for sag, damage and vegetation contact.

Every outbuilding on a permanent supply should have its own consumer unit, giving local isolation, overcurrent protection for its circuits, and a single point to kill all power without walking back to the house.

Main switch and position

Regulation 462.1.201 requires a main linked switch or linked circuit-breaker as near as practicable to the origin of every installation, as a means of switching the supply on load and as a means of isolation. Where that switch is intended for operation by ordinary persons — which it is, in a household outbuilding — it must interrupt both live conductors of a single-phase supply, so a double-pole main switch. “As near as practicable to the origin” is the test for where the board goes: keep the length of unprotected tail between the cable entry and the board short, and site the unit somewhere dry, accessible for maintenance and clear of mechanical damage or flooding.

Enclosure

Regulation 421.1.201 requires that, within domestic (household) premises, consumer units and similar switchgear assemblies comply with BS EN 61439-3 and either have an enclosure manufactured from non-combustible material, or be enclosed in a cabinet or enclosure of non-combustible material complying with Regulation 132.12. The note gives ferrous metal, for example steel, as an example of a non-combustible material. Because BS 7671 defines “premises” as covering the land and all facilities including buildings belonging to it, the garage or workshop of a house sits within the domestic premises — treat 421.1.201 as applying and fit a metal board. This has been a requirement since Amendment 3 to the 17th Edition took effect on 1 January 2016, not a change introduced by A4:2026.

Circuit protection

Individual MCBs or RCBOs per circuit — lighting, socket ring or radial, EV charger if fitted, fixed workshop equipment. RCBOs are the better choice here because a fault on one circuit does not put the whole outbuilding in the dark.

On AFDDs, be precise about what Regulation 421.1.7 says. AFDDs to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets rated up to 32A in high rise residential buildings, houses in multiple occupation, purpose-built student accommodation and care homes. For all other premises — which includes an ordinary house and its garage or workshop — the regulation recommends them for the same circuits. Where used, an AFDD is placed at the origin of the circuit it protects, and using one does not remove the need for the other protective measures in BS 7671. A combined AFDD/RCBO covers arc-fault and residual current in one device and is worth quoting on a workshop full of extension leads and power tools.

Earthing is the decision that most often goes wrong on outbuilding supplies. It turns on one question: is there a continuous, reliable metallic earth path back to the main building?

Do not fit an electrode reflexively

It is widely repeated that an earth electrode at every outbuilding is good practice regardless of system type. BS 7671 says the opposite. Regulation 411.4.2 recommends that an additional connection to Earth, by means of an earth electrode to Chapter 54, is made to the main earthing terminal — and then states that this recommendation does not apply to outbuildings of dwellings served by the installation. On a TN outbuilding, an unnecessary electrode adds a parallel path, a test to record and a component to maintain, for no gain the standard recognises. Fit an electrode because the installation is TT, not out of habit.

Reading Ra correctly

For a TT outbuilding, Regulation 411.5.3 requires the disconnection time of Regulation 411.3.2.2 or 411.3.2.4 to be met and Ra × I_Δn ≤ 50V, where Ra is the sum of the resistance of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts. The requirement is met if the earth fault loop impedance of the RCD-protected circuit meets Table 41.5, which gives 1,667Ω for a 30mA device.

That 1,667Ω is a compliance ceiling, not a target. The note to Table 41.5 states that the resistance of the installation earth electrode should be as low as practicable and that a value exceeding 200Ω may not be stable, cross-referring to Regulation 542.2.4 — which requires the type and embedded depth of the electrode to be such that soil drying and freezing will not increase its resistance above the required value. An electrode reading 800Ω in August passes the arithmetic and will still be a problem in a dry spell. Record the measured Ra on the schedule of test results.

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At the main consumer unit

Run the outbuilding circuit from an RCD-protected way at the house — an RCBO, or an MCB on the protected side of an RCD section. That protects the buried or overhead supply cable itself. Where two RCDs end up in series, the upstream device needs to be time-delayed S type or otherwise selective, or a fault in the outbuilding will take the house out with it.

At the outbuilding consumer unit

Regulation 411.3.3 requires 30mA additional protection in AC systems for socket-outlets rated up to 32A in locations where they are liable to be used by persons of capability BA1 or by children (BA2), for socket-outlets up to 32A in other locations, and for mobile equipment up to 32A for use outdoors. The regulation does contain a documented risk-assessment exception, but it applies only to the second of those three indents — never to socket-outlets liable to be used by ordinary persons or children, and never to mobile equipment for use outdoors. A garage or workshop socket used by the householder, and the extension lead that gets dragged out of it into the garden, sit squarely in the indents the exception cannot reach. Where the exception is relied on at all, the risk assessment must be undertaken with the involvement of a skilled person (electrically) and provided with the certificate.

Regulation 411.3.4 separately requires, within domestic (household) premises, 30mA additional protection for AC final circuits supplying luminaires — so the outbuilding lighting circuit needs it too.

Where the outbuilding is TT, the RCD is doing more than additional protection: under Regulation 411.5.2 it is the preferred device for fault protection, and Regulation 411.5.3 sets the Ra condition it has to satisfy. Individual RCBOs per way remain the best answer, for selectivity as much as for compliance.

A supply to an outbuilding is notifiable under Part P of the Building Regulations in England because it is the installation of a new circuit from the dwelling consumer unit. That covers a new supply to a detached garage, to a garden shed or workshop, to a summerhouse or garden office, and the upgrading or extension of an existing outbuilding supply circuit where a new circuit results.

A scheme-registered electrician notifies the work through their competent person scheme, which issues a Building Regulations compliance certificate to the customer. Alongside it, issue the Electrical Installation Certificate covering the supply circuit from the main consumer unit and the circuits within the outbuilding, with the earth electrode resistance recorded where the outbuilding is TT.

Outbuilding supplies are good work. A properly specified one — cable sized on calculation rather than habit, the earthing arrangement decided deliberately, RCBOs per circuit, a metal board and a certificate handed over on the day — is straightforward to price honestly and hard to argue with afterwards.