The earthing decision comes before the cable size on any supply to a garage, shed or garden office. BS 7671:2018+A4:2026 has no general ban on exporting a PME (TN-C-S) earth to an outbuilding, and NICEIC's own worked example does exactly that with 6 mm² two-core SWA on a 32 A Type B breaker. What catches people out is the armour. It passes as a circuit protective conductor, but it is nowhere near big enough to act as main protective bonding on a PME supply. This guide covers the PME-or-TT decision, armour sizing, volt drop on long runs, burial, testing, notification and EICR coding.
- Exporting PME to an outbuilding is allowed. The design question is whether an open PEN fault could put a dangerous touch voltage on metalwork someone can touch while standing on true earth.
- 6 mm² two-core SWA armour (about 22 mm² of steel) passes as a cpc with k = 51, but it fails as main bonding where the PEN is 35 mm². Steel needs roughly eight times the copper area.
- If the outbuilding has extraneous-conductive-parts and is fed PME, the options are three-core SWA, a separate 10 mm² conductor, or TT.
- Volt drop, not current-carrying capacity, usually sets the size on garden runs. 6 mm² at 32 A over 40 m uses about 4 % of the 5 % budget on its own.
- BS 7671 gives no burial depth. Regulation 522.8.10 asks for "sufficient" depth and marking. 450 to 600 mm with tape is standard practice.
- In England a new submain or radial to an outbuilding is notifiable because it is a new circuit, not because it is outdoors.
Should the outbuilding be PME or TT?
Exporting the house's TN-C-S earth is the simple route. The earth is already there, Ze is low, so overcurrent devices disconnect fast, and there is no electrode to drive or re-test. For a timber garden office with a suspended floor and no metal in contact with the ground, it is the normal design.
The case against is the open PEN conductor. On PME the neutral and earth share one conductor back towards the substation. If that conductor breaks, every earthed metal part in the installation can rise towards line voltage relative to true earth. Indoors, main bonding keeps everything you can touch at roughly the same potential. Outside, standing on wet grass, you are at true earth. Touch an exported-earth metal door, tap or light post during an open PEN fault and you get the full difference. That is why Section 722 has its own open-PEN rules for EV charge points (see EV charger installation certificate: Section 722), and why special locations such as Section 705 agricultural buildings treat PME differently.
| Outbuilding situation | Usual earthing choice | Why |
|---|---|---|
| Timber garden office, no extraneous metalwork | Export PME | Low risk, low Ze, no electrode |
| Brick garage with metal water pipe or steel frame entering from the ground | Export PME with main bonding sized to Table 54.8, or TT | Bonding needs copper-equivalent size the armour can't give |
| Outdoor tap, metal light posts, metal fencing near the building | TT is often preferred | Metalwork touchable from true earth |
| Supplies an EV charge point | TT, or PME with the Section 722 open-PEN measures | Section 722 requirements |
| Livestock or horticultural building | Section 705 applies, TT common | Wet conditions and animals |
| Hot tub or pool | Section 702 applies | See hot tub and pool Section 702 |
For the theory behind each system, see earthing arrangements TN-S, TN-C-S and TT explained.
Can the SWA armour be the cpc, and can it be the bonding?
These are two separate checks, and mixing them up is the commonest design error on outbuilding supplies.
As a cpc (Regulation 543.1). NICEIC's worked example takes 6 mm² two-core 90 °C thermosetting SWA on a 32 A Type B breaker. It uses the 160 A disconnection current from Figure 3A4, the 5 s disconnection time, k = 51 for steel armour from Table 54.4 (on the 70 °C basis, because Regulation 512.1.5 limits the conductor to the temperature the connected accessories are rated for), and an armour area of 22 mm² from BS 5467. The armour passes both the adiabatic check and the Table 54.7 selection method. For the selection method, 6 × 143 / 51 gives about 16.8 mm² of steel needed, and 22 mm² is available.
As main protective bonding (Regulation 544.1.1). On a PME supply, bonding is sized against the supply PEN conductor using Table 54.8. A 35 mm² PEN needs at least 10 mm² copper-equivalent bonding. NICEIC uses resistivities of 17.24 mΩ/m for copper and 138 mΩ/m for steel, so steel needs roughly eight times the copper area. 22 mm² of steel is about 2.7 mm² copper-equivalent, which falls well short. To get there, you would need two-core 50 mm² SWA with 86 mm² of armour.
So if the outbuilding has extraneous-conductive-parts and you are exporting PME, you have three options:
- Use three-core SWA (10 mm² cores) and use a core as the protective and bonding conductor.
- Run a separate 10 mm² protective conductor alongside the SWA.
- Don't export PME. Make the outbuilding TT with its own electrode.
Options 1 and 2 get expensive on long runs, which is why TT is often the practical answer for a garage with a metal water pipe. If the outbuilding has no extraneous-conductive-parts, no main bonding is needed there. Record that on the certificate. More on sizing is in what is main protective bonding?.
If you go TT, keep the two earthing systems apart. Either earth the armour at the house end and terminate it at the outbuilding in a way that keeps it off the outbuilding earth bar (an insulated gland arrangement, or an all-insulated enclosure), or earth it only at the outbuilding. Make it a deliberate design, record it, and label it. The next inspector who finds armour earthed at both ends with a rod in the garden will rightly ask what the earthing arrangement is. A metal enclosure in a TT outbuilding needs RCD protection upstream of it. That usually means a 100 mA time-delayed (S-type) device at the house end or as the outbuilding main switch, with 30 mA devices downstream for selectivity.
What size SWA, and how much volt drop?
Current-carrying capacity rarely limits a domestic outbuilding submain. 10 mm² two-core SWA direct in the ground (Method D, 70 °C basis) is rated 60 A in Table 4D4A. Volt drop is what bites on a 30 to 50 m garden run.
Appendix 4 allows 3 % for lighting and 5 % for other uses from the origin on a public supply, which is 6.9 V and 11.5 V at 230 V. That budget covers the submain and the final circuits together. Using the 70 °C two-core values from Table 4D4B (6 mm² 7.3, 10 mm² 4.4, 16 mm² 2.8 mV/A/m) at a 32 A design current:
| Run length | 6 mm² drop | 10 mm² drop | 16 mm² drop |
|---|---|---|---|
| 20 m | 4.7 V (2.0 %) | 2.8 V (1.2 %) | 1.8 V (0.8 %) |
| 30 m | 7.0 V (3.0 %) | 4.2 V (1.8 %) | 2.7 V (1.2 %) |
| 40 m | 9.3 V (4.1 %) | 5.6 V (2.4 %) | 3.6 V (1.6 %) |
| 50 m | 11.7 V (5.1 %) | 7.0 V (3.0 %) | 4.5 V (1.9 %) |
Formula: mV/A/m × Ib × L ÷ 1,000. At 40 m, 6 mm² leaves only about 2 V for the lighting and socket circuits in the outbuilding, so 10 mm² is the sensible choice. Use the design current (Ib) after diversity, not the breaker rating, if you can justify it. For a garden office with a 3 kW heater and a few sockets, Ib is often well under 32 A. The full method is in cable sizing and volt drop basics.
How should the cable be buried and run?
Regulation 522.8.10 says a buried cable must have earthed armour or a metal sheath suitable as a protective conductor (or be insulated concentric), unless it is in a conduit or duct that gives equivalent protection. It must be marked by cable covers or marking tape and be buried deep enough to avoid damage from any reasonably foreseeable disturbance of the ground. There is no depth in the regulation. Common practice that scheme assessors accept:
- 450 to 600 mm in garden beds and lawns, deeper under drives or anywhere likely to be dug.
- Yellow warning tape roughly 150 mm above the cable along the whole run.
- A duct where the cable crosses a path or patio, so it can be replaced without lifting slabs.
- Sand bedding where the ground is stony.
- Mechanical protection up the wall to at least 2 m, and the sheath kept out of long-term direct sun where you can.
Glands should be proper brass SWA glands with an earth tag (banjo) bonded to the enclosure earth. A plastic stuffing gland with the armour wires twisted under a screw is an observation waiting to be written.
What protection does the outbuilding need?
Socket-outlets rated up to 32 A need 30 mA RCD protection under Regulation 411.3.3. Final circuits supplying luminaires in domestic premises need it under 411.3.4. Outdoor sockets on the building follow the same rule; see garden and outdoor sockets regulations.
A single radial for a light and a socket in a shed can come from a 16 A or 20 A RCBO in the house board, with a local double-pole isolator. Once there are two or more circuits, put a consumer unit with a main switch in the outbuilding, and treat the cable from the house as a distribution circuit. On a TT outbuilding, RCDs provide fault protection for every circuit. A 100 mA S-type upfront and 30 mA RCBOs downstream keeps a fault in the shed from tripping the house.
What do you test and record?
Treat the submain as a circuit in its own right on the schedule: cable type, live conductor and cpc csa (state "armour" or the copper core), reference method (D for the buried section), protective device, R1+R2, insulation resistance, polarity and Zs at the outbuilding board. The current model forms record Zdb and Ipf at each distribution board, which is where the outbuilding board's figures go.
A worked PME check, using NICEIC's resistivity figures: Ze 0.35 Ω, 30 m of 6 mm² two-core SWA with 22 mm² armour. R1 is about 30 × 3.08 mΩ = 0.09 Ω. R2 through the armour is about 30 × 138 / 22 mΩ = 0.19 Ω. R1+R2 is about 0.28 Ω, so Zs at the outbuilding is about 0.63 Ω. Against the 80 % check value of 1.10 Ω for a 32 A Type B (Table 41.3 maximum 1.37 Ω), it passes comfortably. Measure it, though. Gland resistance and real armour data vary. See maximum Zs values.
For TT, record RA in the supply characteristics. Regulation 411.5.3 requires RA × IΔn ≤ 50 V: 1,667 Ω at 30 mA and 500 Ω at 100 mA. Those are limits. Readings above 200 Ω may not be stable, so test in dry weather if you can and design with margin. Method is in earth electrode testing for TT systems. Then test every RCD at its rated residual current; see RCD testing explained.
Is it notifiable, and which certificate?
In England, Part P notifiable work since the 2013 changes is limited to a new circuit, a consumer unit replacement, and additions or alterations in a special location (a room with a bath or shower, or a pool or sauna). The IET's Part P FAQ confirms that Part P applies to outbuildings and garden installations, but being outdoors doesn't make work notifiable on its own. A new submain or radial to an outbuilding is a new circuit, so it is notifiable and needs an EIC. Extending an existing circuit to a garden socket usually isn't notifiable, and a Minor Works certificate covers it. Wales has made its own Building Regulations since 2014, so check the Welsh Approved Document P there. Scotland works under its own building standards. See Part P notification.
What gets coded on an EICR?
Code on the actual risk, following Electrical Safety First Best Practice Guide 4 and EICR observation codes explained. Typical findings on outbuilding supplies:
| Finding | Typical code | Note |
|---|---|---|
| Armour relied on as cpc but not terminated or earthed at either end | C2 | No effective fault path |
| No 30 mA RCD protection for sockets in the outbuilding or outside | C2 or C3 | Depends on use and whether they could supply outdoor equipment |
| Twin and earth buried directly or run on a fence unprotected | C2 | Fails 522.8.10 and is liable to damage |
| PME exported, extraneous metalwork present, no adequate bonding | C2 | Armour alone is not adequate bonding |
| PME and TT parts simultaneously accessible, or earthing arrangement unclear | C2 or FI | Establish the arrangement before coding |
| Buried cable shallow or without marker tape, not otherwise at risk | C3 | Improvement recommended |
| TT electrode reading high but within 411.5.3 | C3 or no code | Note seasonal stability |
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 £4.99 a month, charged when you sign up.