Question 1

What size SWA cable do I need for a garden office?

Accepted Answer

A typical garden office with lighting, a few sockets, a small heater, and computer equipment draws around 3-5kW after diversity. For a 25m cable run, 4mm² 2-core SWA is usually sufficient (rated 40A buried direct). If you add a shower, cooker, or high-power heating, you will likely need 6mm² or 10mm² SWA. Always calculate the diversified demand and check voltage drop for your specific cable length.

Question 2

Do I need an earth rod for a garden office or outbuilding?

Accepted Answer

If your supply is PME (TN-C-S) — the most common UK earthing arrangement — you should install an earth rod for the outbuilding. Regulation 411.4.2 restricts the use of the PME earth for outdoor installations because a lost PEN conductor could make the outbuilding metalwork live. If you have a TN-S supply, the main earth can be extended via the SWA armour without an additional earth rod.

Question 3

Is a garden office supply notifiable under Part P?

Accepted Answer

Yes. Installing a new sub-main to an outbuilding is notifiable work under Part P of the Building Regulations (England and Wales). You must either be registered with a Competent Person Scheme (such as NICEIC, NAPIT, or ELECSA) or apply to your Local Authority Building Control (LABC) before starting work. This applies regardless of whether the outbuilding is used as a dwelling or workspace.

Question 4

How deep should SWA cable be buried for an outbuilding?

Accepted Answer

BS 7671 Regulation 522.8.10 requires buried cables to be at sufficient depth to avoid damage. In practice, a minimum of 500mm (0.5m) is recommended for garden installations. Use 600mm under driveways or areas subject to vehicle traffic. The cable should be laid on a bed of sand, covered with warning tape at 150mm above the cable, and cable tiles should be placed above the cable for additional protection.

Question 5

Can I use T&E cable instead of SWA for an outbuilding supply?

Accepted Answer

No. T&E cable must not be used for underground cable runs or exposed outdoor installations. SWA (Steel Wire Armoured) cable is required because the steel wire armour provides mechanical protection against damage from gardening tools, subsidence, and rodents. The armour also serves as the circuit protective conductor (CPC). T&E may only be used for the indoor section within the main building.

Question 6

What sub-board do I need in a garden office?

Accepted Answer

The outbuilding should have its own consumer unit (sub-board) with a main switch and individual MCBs or RCBOs for each circuit. A 30mA Type A RCD is required for socket and general circuits. The main switch rating should match or exceed the MCB rating at the main consumer unit. A typical garden office sub-board might contain a 30mA RCD, 6A lighting MCB, 32A socket ring MCB, and any dedicated circuit MCBs needed.

Load Type	Diversity Rule	Example
Lighting	66% of total connected	500W → 330W
Sockets	First 10A at 100% + 40% of remainder	4kW → 2,980W
Cooker	First 10A at 100% + 30% remainder + 5A socket	8kW → 5,810W
Shower	100% (no diversity)	8.5kW → 8,500W
EV Charger	100% (no diversity)	7.4kW → 7,400W
Heating	100% largest + 40% remainder	3kW + 2kW → 3,800W

Cable Size	Current Rating (A)	mV/A/m	Typical Use
4mm²	40	9.5	Small shed, basic garden office
6mm²	51	6.4	Garden office with heater
10mm²	69	3.8	Workshop or studio
16mm²	91	2.4	Annexe with shower/cooker
25mm²	116	1.55	Full annexe/dwelling

Garden Office & Outbuilding Supply Calculator

How Does the Outbuilding Supply Calculator Work?

What Diversity Factors Are Applied?

What SWA Cable Sizes Are Available?

Why Is PME Earthing a Concern for Outbuildings?

Installation Best Practice for Buried SWA

Frequently Asked Questions