Outbuilding Electrical Supply
Supplying outbuildings, garages, and garden offices — cable selection, earthing, and RCD protection
SWA Cable Selection
Steel Wire Armoured (SWA) cable is the standard choice for underground supply to detached outbuildings.
Common SWA cable sizes for outbuilding supplies
BS 7671 Table 4D4A, Column 7| Cable Size (mm²) | Rating (A) Direct Buried | Typical Use |
|---|---|---|
| 4 | 37 | Lighting-only shed or small load |
| 6 | 47 | Garden office, workshop up to 10 kW |
| 10 | 64 | Workshop with power tools, large office |
| 16 | 85 | High-load workshop, hot tub supply |
| 25 | 110 | Large annex, multiple circuits |
Ratings for 3-core SWA direct buried. Apply derating factors for grouping, soil thermal resistivity, and depth.
VD = (mV/A/m x Ib x L) / 1000- mV/A/m
- = Millivolt drop per amp per metre (from BS 7671 Table 4D4B)
- Ib
- = Design current in amps
- L
- = Cable run length in metres
BS 7671 Appendix 4, Regulation 525.1
Earthing Arrangements
The earthing arrangement for a detached outbuilding is a critical design decision that affects safety.
Where the main building has a PME (TN-C-S) supply, BS 7671 Regulation 411.4.2 restricts extending the PME earth to a detached outbuilding because a broken PEN conductor could place the full neutral voltage on the outbuilding's metalwork. The preferred solution is a TT earthing arrangement with a local earth electrode (typically a 1.2 m copper-clad earth rod driven at the outbuilding).
PME Restrictions
Earth Electrode Resistance
Installation Requirements
Key requirements checklist
BS 7671 18th Edition| Requirement | Detail | Reference |
|---|---|---|
| Isolation at origin | Switch or isolator in main building | Reg 537.2 |
| Cable burial depth | 500 mm (hard surface) or 600 mm (open ground) | Reg 522.8.10 |
| Cable route protection | Cable tiles above, marker tape 150 mm higher | Best practice |
| RCD protection | 30 mA RCD for sockets and outdoor circuits | Reg 411.3.3 |
| Local consumer unit | Separate CU in outbuilding with main switch | Reg 514.12 |
| Part P notification | Notifiable as new circuit to detached building | Building Regs Part P |
Voltage Drop Considerations
Outbuilding supplies often involve cable runs of 20-50 m or more. BS 7671 permits a maximum voltage drop of 5% (11.5 V at 230 V). Limit the sub-main drop to 3% (6.9 V) to leave headroom for final circuits.
Total Voltage Drop