What cable size do I need for a garden office?

Typical garden offices need 32A supply with 4mm² or 6mm² SWA cable. For runs over 25m, 6mm² is usually required to keep voltage drop under 5%. If you plan to add EV charging or significant loads, consider 10mm² submain to a mini consumer unit.

Do I have to use SWA cable for garden buildings?

For buried cables, SWA is the standard choice - it provides mechanical protection required for direct burial. Alternatives include running cable through buried conduit, but this is more labour-intensive. For short overhead runs, standard cable in conduit may be acceptable.

How deep should I bury SWA cable?

Minimum burial depths per BS 7671: 450mm under garden/grass, 600mm under driveways or areas with vehicular access. Cable should be laid on sand bedding and covered with warning tape. Always avoid running near other services (gas, water, drainage).

Why is voltage drop such an issue for garden buildings?

Long cable runs mean more resistance and greater voltage drop. A 30m run at 32A can drop 7-8% with 4mm² cable - exceeding the 5% limit. This causes equipment underperformance, LED flickering, and motor problems. Larger cable reduces resistance and voltage drop.

Do I need a separate consumer unit in the garden building?

Recommended for larger installations. A mini consumer unit (garage unit) provides local RCD protection, circuit separation, and easier fault finding. For simple shed lighting, a fused connection from the house may suffice. Garden offices typically benefit from dedicated consumer units.

What RCD protection is required for garden circuits?

30mA RCD protection is mandatory for all outdoor circuits under BS 7671. This can be at the house consumer unit or in a local unit at the outbuilding. Time-delayed RCDs at source prevent nuisance tripping when using RCDs at both ends.

Can I extend my ring main to the garden building?

Not recommended. Garden buildings should have dedicated radial circuits from the main consumer unit, or a submain to a local consumer unit. Ring mains are not suitable for outdoor extensions. The long cable run and potential load requires properly sized dedicated supply.

How do I calculate cable size for long runs?

Two factors: current capacity and voltage drop. Even if 2.5mm² handles your current, a 40m run at 20A drops 7.2V (3.1%) - add your existing drop from the supply. Our calculator checks both and recommends appropriate cable size.

Is Part P notification required for garden building electrics?

Yes, new outdoor circuits and garden building electrical installations are notifiable under Part P. Work must be done by a registered competent person or inspected by Building Control. This ensures safety compliance and is required for insurance validity.

What about 3-phase supply to workshops?

Large workshops with welders, compressors, or heavy machinery may benefit from 3-phase supply. This requires DNO application and 5-core SWA cable. Most domestic properties dont have 3-phase available without significant upgrade costs.

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Garden Office & Outbuilding Cable Sizing Calculator

Calculate the correct cable size for garden offices, sheds, workshops and outbuildings

Garden Building Cable Quick Facts

SWA (Steel Wire Armoured) cable required for buried runs
Minimum burial depth: 450mm under paths, 600mm under driveways
Long runs (20m+) often need 6mm² or 10mm² for voltage drop
RCD protection mandatory for outdoor circuits
Separate consumer unit recommended for garden buildings

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 Garden Building Installations

Small shed - lighting + sockets only

16A or 20A circuit, 2.5mm² SWA for runs under 20m

Garden office - full power needs

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

Workshop with power tools

32A or 40A circuit, 6mm² or 10mm² SWA for voltage drop

Outbuilding with EV charger

Separate 32A EV circuit + general circuit, may need 16mm² submain

Long run over 40m

Consider 10mm² or 16mm² SWA, check voltage drop carefully

Complete Garden & Outbuilding Electrical

Garden offices often benefit from solar PV installations for off-grid or reduced running costs. Add an EV charger to your outbuilding supply. For workshop spaces, see our garage and workshop guide. Property security? Check CCTV and security systems.

Garden Office Cable Sizing

Garden office cable sizing determines the conductor cross-sectional area (mm²) needed to safely supply power to detached garden buildings. Long cable runs create significant voltage drop, often requiring larger cables than the load alone would suggest. SWA (Steel Wire Armoured) cable is standard for buried outdoor runs, providing mechanical protection and earth continuity.

Garden Building Installation Costs (2024)

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

Installation Scenario	Materials	Labour	Total
Shed supply, short run (<15m)	£150-250	£200-350	£350-600
Garden office, medium run (15-30m)	£300-500	£350-500	£650-1000
Workshop, long run (30-50m)	£500-800	£450-650	£950-1450
Outbuilding with mini consumer unit	£200-350	£250-400	£450-750 (additional)
Groundwork/trenching (per metre)	£5-15	£20-40	£25-55/m

Prices as of 2024. Groundwork costs vary significantly by ground conditions and access.

SWA Cable for Outdoor Runs

Steel Wire Armoured (SWA) cable has steel wire armouring for mechanical protection and is suitable for direct burial. It provides its own earth via the armouring. Standard sizes are 3-core (L/N/E) for single circuits or 5-core for submains. SWA must be terminated with proper glands to maintain protection rating and earth continuity.

Voltage Drop in Long Cable Runs

Voltage drop is proportional to cable length, current, and inversely proportional to cable size. Formula: VD = (2 × L × I × mV/A/m) ÷ 1000. For a 30m run at 32A with 4mm² cable: VD = (2 × 30 × 32 × 11) ÷ 1000 = 21V (9.1%) - far exceeding the 5% limit. Using 6mm² reduces this to 14V (6.1%), still marginal. 10mm² gives 8.4V (3.7%) - compliant.

Submain vs Final Circuit

A submain supplies a distribution board (consumer unit) rather than directly supplying equipment. For garden buildings, a submain to a mini consumer unit allows multiple final circuits (lighting, sockets, dedicated appliances) with local protection. This is more flexible than running multiple final circuits from the house.

Frequently Asked Questions

Related Electrical Calculators

Voltage Drop Calculator

Critical for long garden building runs

Cable Sizing Calculator

Full BS 7671 cable sizing for all circuits

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

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