What cable size do I need for a shed?

For a basic shed with lighting and a few sockets, 2.5mm² SWA on a 16A or 20A circuit is usually sufficient for runs up to 25-30m. If you plan to use power tools, heaters, or multiple appliances, upgrade to 4mm² or 6mm² SWA on a 32A circuit. Always check voltage drop for your specific length.

Can I run an extension lead to my shed?

Extension leads are not a safe or legal permanent solution for shed power. They create trip hazards, weather exposure risks, and fire hazards. A proper buried SWA cable installation is the only safe option. It also adds value to your property and ensures insurance compliance.

Do I need an electrician for shed electrics?

Yes, outdoor electrical work is notifiable under Part P Building Regulations. It must be done by a registered competent person (like NICEIC or NAPIT) or inspected by Building Control. DIY outdoor electrical work is illegal and dangerous.

How deep should I bury cable to a shed?

Minimum 450mm under lawns and flower beds, 600mm under driveways or areas with vehicle access. Use sand or sifted soil around the cable and lay cable warning tape 150mm above. Avoid routes near tree roots, drainage, or other services.

Does my shed need its own consumer unit?

Not always. A basic shed can be supplied from a dedicated circuit in the house consumer unit with an outdoor isolator at the shed. Larger summer houses with multiple circuits benefit from their own small consumer unit for convenience and easier isolation.

What RCD protection do I need for a shed?

30mA RCD protection is mandatory for all outdoor circuits. This can be provided by an RCBO in the house consumer unit, or an RCD in a shed consumer unit. Some installations use both for added protection.

Can I put a ring main in my summer house?

Yes, larger summer houses can have a ring final circuit for sockets, supplied from a local consumer unit. The supply to that consumer unit (submain) must be appropriately sized. For occasional use buildings, radial circuits are often simpler and adequate.

What about running power to a metal shed?

Metal sheds require proper earthing - the metal structure must be bonded to the electrical earth. SWA cable armour provides earth continuity. Your electrician must verify earthing arrangements, especially with PME supplies where additional earth electrodes may be needed.

How do I calculate the load for my shed?

Add up what you'll use: lighting (100-200W), power tools (1-2kW each), heater (1-3kW), fridge (100W). For a typical shed: 16A handles 3.6kW, 32A handles 7.2kW. If total exceeds this or you want expansion room, size up.

Is armoured cable necessary for a shed?

For buried runs, yes - SWA cable is required for mechanical protection. For short overhead runs (not recommended) or runs entirely in conduit, alternatives exist. But SWA direct burial is the most common and practical solution for garden buildings.

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Shed & Summer House Cable Sizing Calculator

Calculate the correct cable size for sheds, summer houses, and garden building power supplies

Shed & Summer House Cable Quick Facts

Most sheds need 16A-32A supply depending on use
SWA cable required for buried outdoor runs
RCD protection mandatory for outdoor circuits
Minimum burial depth: 450mm (600mm under driveways)
Outdoor isolator switch recommended at shed entry

Cable Size Calculator

BS 7671:2018+A2:2022 compliant

1. Load Current

2. Cable Length (meters)

3. Installation Method

Lighting circuit (3% voltage drop limit)

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Common Shed & Summer House Installations

Basic shed (lighting + sockets only)

16A circuit, 2.5mm² SWA for runs up to 30m

Summer house with heating

20A-32A circuit, 4mm² or 6mm² SWA depending on length

Shed with power tools

32A circuit recommended, 6mm² SWA for most runs

Summer house with kitchen appliances

32A circuit, 6mm² SWA, consider dedicated circuits

Long run to bottom of garden (40m+)

Upsize cable for voltage drop - likely 6mm² or 10mm²

Related Garden & Outbuilding Calculators

For larger garden buildings, see our garden office calculator with submain guidance. Planning workshop use? Check the garage and workshop guide. Add outdoor lighting to illuminate the path to your shed.

Shed & Summer House Cable Sizing

Garden building cable sizing determines the conductor size needed to safely supply power to sheds and summer houses. Unlike indoor circuits, outdoor runs require SWA (Steel Wire Armoured) cable for mechanical protection when buried. Cable must handle the expected load while keeping voltage drop acceptable over the outdoor distance.

Shed Electrical Installation Costs (2024)

Typical UK installation costs including SWA cable, groundwork, and labour.

Installation Scenario	Materials	Labour	Total
Basic shed supply (<15m)	£150-280	£200-350	£350-630
Standard shed supply (15-25m)	£250-400	£280-420	£530-820
Summer house with consumer unit	£350-550	£400-600	£750-1150
Long run supply (30m+)	£400-700	£450-650	£850-1350
Outdoor isolator switch	£40-80	£60-100	£100-180

Prices as of 2024. Costs vary by cable run length and ground conditions.

SWA Cable for Garden Buildings

Steel Wire Armoured (SWA) cable is the standard choice for buried outdoor runs to sheds and summer houses. The steel armour provides mechanical protection against accidental damage from garden tools. SWA can be direct buried at minimum 450mm depth with sand surround and warning tape, or run through ducting.

Voltage Drop for Garden Runs

Long cable runs to sheds cause voltage drop - power tools may run slowly, lights dim. BS 7671 allows 5% drop maximum (11.5V on 230V). At 16A, 2.5mm² cable drops about 18mV per metre. A 30m run at 16A drops 8.6V (3.7%) - acceptable. Our calculator checks this automatically.

Outdoor Circuit Protection

Garden circuits face unique hazards: moisture, wildlife damage, garden tool strikes. Protection includes: RCD for earth faults, MCB for overload, SWA for mechanical protection, IP-rated enclosures for connections. All outdoor electrical equipment must be suitable for the conditions.

Frequently Asked Questions

Related Electrical Calculators

Garden Office

Larger outbuilding supplies

Outdoor Lighting

Garden and security lights

Voltage Drop Calculator

Check long outdoor cable runs

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Voltage drop, conduit fill, earth fault loop impedance and more. All BS 7671 compliant, all free.

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