The first question on a supply to a detached garage, shed or garden office is not the cable size but the earth. BS 7671 does not prohibit exporting a PME (TN-C-S) earth to an outbuilding, and for a dry garden room with no metalwork in contact with the ground it is the usual and reasonable design. Where the outbuilding has extraneous metalwork, an outdoor tap, a hot tub or an EV charger, the exposure to a broken PEN conductor changes the arithmetic and a local TT earth with its own electrode is often the better answer. This guide covers that decision, the SWA cable choice and burial, RCDs, bonding and what goes on the certificate.
- Exporting a PME earth to an outbuilding is permitted; the design question is whether an open PEN fault could put a dangerous touch voltage on metalwork someone could contact while standing on the ground.
- Never connect a TT earth and a PME earth together at the outbuilding; if you go TT, the SWA armour must be isolated from the outbuilding earth or bonded to only one system.
- BS 7671 gives no burial depth; Regulation 522.8.10 requires enough depth to avoid foreseeable damage, and 450 to 600 mm with marker tape is standard practice.
- SWA armour can serve as the cpc if it satisfies Regulation 543.1, and for typical 2-core and 3-core sizes it does.
- All socket-outlets up to 32 A need 30 mA RCD protection under 411.3.3, and a TT outbuilding needs RCD protection for fault protection on every circuit.
- A new submain to an outbuilding is notifiable in England and needs an EIC; outdoor cable installation is on the Schedule 4 list in its own right.
Should the outbuilding be PME or TT?
The case for exporting the TN-C-S earth is simple: it is already there, it gives a low Ze so overcurrent devices disconnect quickly, and it needs no electrode, no annual worry about ground conditions and no extra RCD hierarchy. For a garden office with a wooden floor, plasterboard walls and nothing metal touching the earth, this is the right design and the majority of installations are done this way.
The case against is the open PEN conductor. On a PME supply the neutral and earth are combined back to the substation, and if that combined conductor breaks, every earthed metal part in the property rises toward line voltage relative to true earth. Inside a house you are standing on a floor at roughly the same potential and the main bonding equalises the metalwork. Outside, standing on wet grass, you are at true earth potential, so touching an exported-earth metal enclosure during an open PEN fault can deliver a full shock. Regulation 411.3.1.2 and the notes on PME in Section 542 are the starting points, and the specific treatment for EV charging in Section 722 exists because the car body is exactly this hazard; see EV Charger Installation Certificate: Section 722 and the EIC.
A short decision table:
| Outbuilding situation | Usual earthing choice | Reason |
|---|---|---|
| Timber garden office, no external metalwork | Export PME | Low risk, simple, low Ze |
| Brick garage with metal water pipe and metal roller door | Export PME with main bonding to the extraneous parts | Bonding equalises potentials inside the building |
| Outbuilding with outdoor tap, external lighting on metal posts, or hot tub | TT at outbuilding | Metalwork touchable from true earth during open PEN |
| Outbuilding supplying an EV charge point | TT, or PME with open-PEN detection device per 722.411.4.1 | Section 722 requirements |
| Agricultural or horticultural building | TT is common; Section 705 applies | Livestock and wet conditions |
If you go TT, the whole outbuilding earthing system must be separated from the PME earth. That means the SWA armour, if it is connected to the main earthing terminal at the house, cannot be connected to the earth bar in the outbuilding board. Either use a non-metallic gland or an insulated gland shroud arrangement at the outbuilding end so the armour is earthed at the supply end only, or run the submain so the armour is earthed at the outbuilding only and terminated in an insulating enclosure at the house. Whichever way, the design must be deliberate and recorded on the certificate, because the next inspector who finds an armour earthed at both ends will rightly ask what the earthing arrangement is. A metal consumer unit in a TT outbuilding needs an upfront RCD, typically a 100 mA S-type main switch, so the enclosure is protected before the tails reach the outgoing RCBOs.
Which SWA cable and how should it be installed?
Two-core SWA with the armour as cpc is standard for a single-phase submain. Three-core SWA gives a copper cpc and leaves the armour as an additional parallel earth path, which some designers prefer for TT installations because the copper core carries the fault current and the armour can be earthed at one end only. Sizes from Table 4D4A for two-core 70 °C thermoplastic armoured cable:
| Cable | Clipped direct (Method C) | Direct in ground (Method D) | Volt drop (mV/A/m) |
|---|---|---|---|
| 4 mm² | 41 A | 41 A | 11 |
| 6 mm² | 53 A | 51 A | 7.3 |
| 10 mm² | 72 A | 68 A | 4.4 |
| 16 mm² | 97 A | 87 A | 2.8 |
Check the current edition of Table 4D4A before relying on these; the direct-in-ground figures assume 2.5 K·m/W soil resistivity and 20 °C ground temperature, and a cable in a duct or under thermal insulation is derated further.
Volt drop is the usual constraint on a long garden run, not current. A 40 m run in 6 mm² at 32 A drops 32 × 40 × 7.3 = 9.3 V, or 4 % of 230 V, before the outbuilding circuits add their own share, and Appendix 12 allows 5 % for power and 3 % for lighting from the origin. For a 40 m garden office submain, 10 mm² is normally the right answer even though 6 mm² carries the current; see Cable Sizing and Volt Drop Basics: Ib, In, Iz and Table 4D5 for the method.
On burial, Regulation 522.8.10 says a buried cable must be marked by covers or marking tape and be at a depth sufficient to avoid damage by any reasonably foreseeable disturbance of the ground. There is no number in the regulation. The practice most of us and most scheme assessors accept is 450 mm in a garden bed, 600 mm under a drive or anywhere that will be dug, in a duct where it passes under a path so the cable can be replaced, with yellow warning tape at about 150 mm above the cable. Where the cable runs up the outside wall to the outbuilding, protect it from mechanical damage to at least 2 m above ground level with steel conduit or channel and keep it out of direct sunlight where possible because UV degrades the sheath over years.
Regulation 543.1 governs whether the armour can be the cpc. The adiabatic check uses k = 51 for steel armour against 143 for copper, and the manufacturer's data sheet gives the armour cross-section. For two-core 6 mm² SWA the armour is about 24 mm² of steel, which is equivalent to roughly 8.6 mm² of copper, comfortably above the 6 mm² line conductor, so the armour meets Table 54.7 without calculation. Two-core 16 mm² gets tighter and is worth checking against the actual fault current. Glands must be the proper brass SWA type with an earth tag and a banjo; a plastic stuffing gland with the armour twisted round a screw is a C2 waiting to happen.
What protection and bonding does the outbuilding need?
Every socket-outlet up to 32 A needs 30 mA RCD protection under Regulation 411.3.3, and Regulation 411.3.4 extends 30 mA RCD protection to lighting circuits in domestic premises. Outdoor lighting under Section 714 and any outdoor socket on the building follow the same rule. If the whole outbuilding is TT, RCDs provide the fault protection for every circuit and the submain itself should have an upfront RCD, so the arrangement is usually a time-delayed 100 mA RCD as the outbuilding main switch with 30 mA RCBOs downstream, which gives selectivity so a fault in the garden office does not trip the house.
A single radial to a shed for a light and a socket can be fed from a 16 A or 20 A RCBO in the house consumer unit with a local double-pole isolator in the shed. Once the outbuilding has two or more circuits, it needs its own consumer unit with a main switch, and the submain from the house is a distribution circuit protected by a suitably rated device, typically 32 A to 63 A.
Main protective bonding in the outbuilding applies to extraneous-conductive-parts in that building: metal water pipes entering from the ground, structural steel, a metal-framed building. A 10 mm² bonding conductor is required for TN-C-S supplies, and the same is normal practice at the outbuilding board. If there are no extraneous parts, none is needed, but say so on the certificate. See What is main protective bonding? Explained.
What tests and paperwork are needed?
In England a new submain to an outbuilding is a new circuit and is notifiable under Schedule 4 of the Building Regulations 2010; the installation of a cable outdoors is listed separately as notifiable too. The certificate is an EIC, not a Minor Works certificate, and the outbuilding board is recorded as a distribution board fed from the origin. Wales has its own version of Part P with broader notifiable scope, and Scotland uses the Building (Scotland) Regulations. See Part P notification: when and how electricians notify building work.
For the schedule of test results, record the submain as a circuit in its own right: cable type, csa of live conductors and cpc (armour or copper core), reference method (usually D for the buried section), protective device, R1+R2, insulation resistance, polarity and Zs at the outbuilding board. Then record each outbuilding circuit with Zs at the furthest point. For TT, record the electrode resistance RA on the supply characteristics section and test the RCD at the outbuilding; the minimum earth electrode resistance target is RA × IΔn ≤ 50 V, which is 1667 Ω for a 30 mA device, but the IET's practical advice is to aim for 200 Ω or less so the value is stable in dry weather. See Earth Electrode Testing for TT Systems: Methods and Limits for the measurement method.
On an EICR, the observations that recur on outbuilding supplies are: SWA armour not earthed at either end (C2 if the armour is relied upon as cpc, otherwise C3 with the armour treated as an exposed part), TT and PME earths connected together at the outbuilding board (C2), no RCD protection for outdoor socket-outlets (C2), buried cable at shallow depth with no marker tape (C3 unless exposed), and flex or twin and earth run outdoors without protection (C2). Coding logic is covered in EICR observation codes explained: C1, C2, C3 and FI.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.