Getting electricity to a shed or outbuilding

Helpful video reference. John Ward of Flameport Electrical, Dorset, explains outbuilding earthing in his video "Earthing Systems for Outbuildings". Earthing is the part of an outbuilding supply that most people do not think about — but getting it wrong means that in a fault condition, metalwork in the shed could become live. John explains the difference between TN-S, TN-C-S and TT systems and why the length of the cable run changes the earthing requirement. Worth watching if you want to understand why an electrician asks about your existing earthing arrangement before quoting.

1. Work out what load you actually need

Before anything else, think about what you intend to run from the supply. The answer changes everything about how the installation is designed:

• Lights only: a 6A or 10A radial circuit is sufficient. A single armoured cable with a small consumer unit at the shed end is all you need.
• Sockets for tools and charging: a 20A or 32A radial, or a small ring main, depending on the load. A two-way consumer unit in the shed handles this neatly.
• Workshop with large power tools: 63A supply, possibly three-phase if you have industrial machines. A full consumer unit in the outbuilding with individual circuits for each heavy tool.
• Garden room or home office: heating adds significant demand. A 40A or 63A supply with separate circuits for heating, lighting and sockets is the norm.

Getting this assessment right at the start avoids the expense of upgrading the supply later.

2. Plan the cable route

There are two options for routing cable from the house to an outbuilding: overhead and underground. Overhead using a catenary wire is technically possible but rarely used for domestic supplies — it needs careful clearance calculations and looks untidy. Underground armoured cable (SWA) is the standard approach.

The cable needs to run at a minimum depth of 500mm under lawns and gardens, and 600mm under driveways and areas where vehicles drive. Where it passes under the foundation of any structure it needs additional protection. Mark the route on a sketch before any digging starts, so you can produce a record drawing after the work is complete — essential if you or a future owner ever wants to excavate in that area.

3. Choose the right cable

Steel wire armoured (SWA) cable is the standard for underground outbuilding supplies. The armour protects the cores from accidental damage and carries the fault current safely in a fault condition. The size depends on the load and the length of the run:

• 2.5mm² SWA suits low-power supplies (lighting, charging).
• 6mm² SWA is the most common choice for general domestic outbuilding supplies.
• 10mm² or 16mm² SWA is needed for high-load garden rooms or workshops.

Volt drop matters on longer runs — a run of more than 20 metres may need a larger cable than the load alone would suggest, to keep the voltage at the shed end within BS 7671 limits.

4. The earthing arrangement: the technical bit

This is the part that catches people out. The earthing of an outbuilding supply is not simply extending the house earth conductor into the shed. The correct earthing arrangement depends on:

• The earthing system of the house (TN-C-S, TN-S or TT).
• The length of the cable run.
• Whether the outbuilding has any metallic connections back to the house (water pipes, gas pipes, structural metalwork).

On a TN-C-S (PME) supply — the most common in UK properties — using the house earth for a separate outbuilding is generally not permitted unless specific earthing tests confirm it is safe. The reason is that a fault on the street network could raise the "earth" voltage at the outbuilding relative to local earth, putting metalwork in the shed at a dangerous potential. In these situations, an earth electrode (earth rod) is driven at the outbuilding and the supply is protected by a 30mA RCD. John Ward's video above explains this clearly.

5. RCD protection at the outbuilding

BS 7671 requires that the supply to a separate outbuilding has RCD protection at the origin of the outbuilding circuits. The standard approach is a small consumer unit in the outbuilding — a two-way, four-way or six-way unit depending on how many circuits you need — with either a main RCD plus MCBs, or individual RCBOs per circuit. An RCBO per circuit is preferable because a fault on one circuit does not take everything else off.

The consumer unit in the outbuilding must be in a suitable enclosure: IP-rated if the shed is damp, and mounted at a sensible height where the main switch is accessible but safe.

6. The connection at the house

The supply for the outbuilding comes from the house consumer unit — a new circuit protected by an MCB or RCBO sized for the cable. The SWA cable needs to be terminated correctly at both ends: the armour must be bonded to earth at the house end, and the whole installation needs testing and certification before use.

This work at the consumer unit is part of what makes the job notifiable under Part P. The electrician who does the work should provide you with an Electrical Installation Certificate (EIC) on completion — this is your proof the work was done correctly and is essential if you ever sell the property.

When to call us

This is the kind of job Richard particularly enjoys — it requires proper technical assessment, good cable installation skills, and a tidy finish. A supply to a detached garage or garden room in Sandwich or east Kent typically takes one day for a straightforward run, with the EIC provided on completion. Contact us for a fixed price before anything starts.