Shed Electrical Installation: What You Need to Know

Getting power to a shed turns it from a dark store into a workshop, a hobby room or a proper garden office you can use after dark. It is also one of the few shed jobs where I will tell you straight: this is not a DIY project. The digging and the donkey work, yes, you can crack on with that. But the live connection, the design and the certificate are a sparky's job, and there are good legal and safety reasons for that. This guide explains how a shed supply actually works so you know what you are paying for and can plan the job sensibly.

Part P and Why the Law Cares

In England and Wales, electrical work in and around the home is covered by Part P of the Building Regulations. Running a new circuit out to an outbuilding such as a shed is notifiable work. In plain terms that means the work has to be either carried out by an electrician who is registered with a competent person scheme (such as NICEIC, NAPIT or ELECSA) and can self-certify it, or notified to your local authority building control so they can inspect it. Scotland and Northern Ireland have their own equivalent rules, but the principle is the same: this work is regulated for a reason.

When the job is done properly you should be handed an Electrical Installation Certificate, along with a Building Regulations compliance certificate. Keep these safe. They are not just paperwork: they are proof the installation was designed and tested to the current wiring regulations (BS 7671). That matters in three real-world ways.

• Insurance - if there is a fire and the work was unsafe and uncertified, your insurer can refuse to pay out.
• Resale - a buyer's solicitor will ask for certificates for any outbuilding electrics. Missing paperwork holds up sales.
• Safety - testing confirms the earth, the protection and the connections are sound before anyone plugs anything in.

What You Can Realistically Have

For most sheds and garden rooms the wish list is modest and entirely sensible. A well-designed circuit will comfortably run:

• Lighting - LED ceiling lights and maybe an outside light by the door. Light loads, easy to provide for.
• Sockets - a handful of double sockets for tools, a laptop, a charger or a kettle.
• A small heater - a modest electric heater is often fine, but heaters draw a lot of current, so the load must be designed in from the start, not added as an afterthought.

What you should not assume is that a shed circuit will run heavy machinery, a hot tub, an electric car charger and a heater all at once. Each of those is a serious load on its own. If you have big plans, tell the electrician up front so the supply, cable size and protection are designed for it. It is far cheaper to size the circuit correctly now than to dig the trench up again in two years.

How the Supply Usually Runs

A proper shed supply is a dedicated circuit, run from a spare way in the house consumer unit (the fuse board), out to the shed, where it lands in its own small consumer unit. That little board in the shed gives you local isolation and circuit protection, so you can switch the shed off independently and so faults out there do not knock out the house.

The journey from house to shed looks like this:

1. A dedicated circuit at the house board - the electrician fits a suitably rated breaker and RCD protection at the origin, sized for the cable and the expected load.
2. Steel wire armoured cable between the buildings - SWA is run either buried in a trench across the garden, or overhead on a catenary wire. This is the bit that has to survive years outdoors.
3. A small consumer unit in the shed - the SWA terminates in a compact garden consumer unit with its own RCD and breakers for the lighting and socket circuits.
4. Final circuits inside the shed - from that little board, standard cable runs to the sockets and lights, all installed and tested to the regulations.

That structure, dedicated circuit, armoured run, local board, is the standard professional approach. It is robust, safe and certifiable, which is exactly why it is worth doing properly.

Cable, Trench and Catenary

Steel wire armoured cable is the workhorse here. The steel braid under the outer sheath protects the conductors from a spade strike, a fork, or a rat with an appetite. That armour is what lets the cable be buried or run overhead safely. Ordinary twin-and-earth house cable is not suitable for this and must never be buried in the garden.

Burying it in a trench

If the cable goes underground, depth matters. As a rule of thumb, SWA should sit at least 450mm deep under a lawn or border, and deeper, around 600mm, where it crosses a path or drive that might be dug or disturbed later. Good practice is to lay the cable on a bed of sand, cover it with more sand to protect it from sharp stones, then lay cable warning tape in the trench above it. That tape is a courtesy to whoever digs there in ten years' time: it warns them before the spade reaches the cable.

Running it overhead on a catenary

Where digging is not practical, the cable can be run overhead, supported along a tensioned steel catenary wire so its weight is not hanging on the conductors. Overhead runs must be kept at a sensible height so they are well clear of people, vehicles and machinery passing underneath. The electrician will set the height and support spacing.

Load, Cable Size and Circuit Protection

Cable size is not a guess, it is a calculation. The electrician works out the total load the shed needs, factors in the length of the run (longer runs lose more voltage), and picks a cable size that can carry that current safely without overheating or dropping too much voltage. For a modest shed with lights, a few sockets and a small heater, something like 2.5mm or 4mm SWA is common, but the right answer depends on your specific load and distance. That is the electrician's sum to do, not yours.

Protection is layered. There is a breaker at the house end sized to the cable, RCD protection to cut the power fast if there is an earth fault (the kind of fault that can give you a serious shock), and a small consumer unit in the shed with its own RCD and circuit breakers. RCD protection on shed and garden circuits is not optional: anything feeding sockets that might run outdoor tools needs it, because outdoors is exactly where shocks turn fatal.

Earthing and TT Systems for Outbuildings

This is one of the genuinely technical bits, and it is exactly why outbuilding electrics need a qualified person. Many modern homes have what is called a PME (or TN-C-S) earthing arrangement on the incoming supply. For various safety reasons, that house earth cannot always be safely exported to a detached outbuilding, because if the supply neutral were lost, metalwork in the shed could become live.

To get round this, the electrician will often set the shed up as a TT system: the outbuilding gets its own earth electrode (a rod driven into the ground) and relies on RCD protection to disconnect quickly under fault conditions. Whether that is necessary, and how the earth is arranged, is a design and testing decision. The electrician measures the earth, checks the disconnection times and confirms the safe arrangement for your particular property. There is no rule of thumb that replaces that testing, which is the heart of why this work is regulated.

What You Can Do vs What the Sparky Must Do

Here is where a willing homeowner can genuinely save money without touching anything dangerous. The split is simple: you can do the labour that does not involve live electrics or the design, and the electrician does everything that requires competence, testing and certification.

Always agree this split with your electrician before you start. Some prefer to dig and lay the cable themselves so they can vouch for the depth, the bed and that nothing was nicked during the work. Trenches dug to the wrong depth, or backfilled before the sparky has seen the cable in, just create arguments. Coordinate first, then crack on with the spade.

Why You Do Not Just Run an Extension Lead

It is tempting. A long extension lead from the house socket reaches the shed, the kettle boils, job done. As a permanent solution it is genuinely dangerous, and here is why.

• The cable is not rated for it - ordinary flex is not built for burial or sustained outdoor exposure. It degrades, the insulation cracks, and water gets in.
• It is easy to damage - lying across the garden it gets caught by mowers, gates, feet and frost. Damaged flex outdoors is a shock and fire risk.
• No proper protection at the shed - there is no local board, no fault protection designed for that run, and often the lead is overloaded by a heater it was never meant to carry.

An extension lead is fine to charge a drill for an afternoon. It is not a substitute for a designed, tested, certified supply. If you are using the shed enough to want power out there, you are using it enough to do it properly.

When to Call an Electrician

To be blunt, for mains electrics the answer is always, and from the start. But specifically, get a registered electrician involved when:

• You want any permanent power, lighting or sockets in the shed.
• You are planning a heater, workshop machinery or anything that draws real current.
• You are running a cable between buildings, buried or overhead.
• You are turning the shed into a habitable room or garden office people will spend hours in.
• You inherited existing shed electrics and have no certificate or no idea who did them.

Bring them in at the planning stage, not after you have dug the trench in the wrong place. A good sparky will tell you what depth to dig, where to land the cable and what they need from you, so the labour you do actually helps rather than creating problems. While you are planning, it is also worth checking whether your overall project needs planning permission, as that can affect where and what you build.

Sensible Planning Checklist

Run through this before you commit to anything, so the conversation with your electrician is quick and the job goes smoothly.

• List your loads - lights, sockets, heater, any machinery, now and likely future, so the circuit is sized once and properly.
• Use a registered electrician - check they are on a competent person scheme and can self-certify Part P work.
• Confirm the certificate - agree you will receive an Electrical Installation Certificate and Building Regulations compliance.
• Plan the route - decide buried or overhead, and check for existing gas, water and electrical services along the way.
• Agree the split of work - who digs, who lays, who backfills, and at what depth, signed off before the spade moves.
• Protect the shed first - a dry, weathertight shed keeps the new electrics safe, so sort the building before the wiring.
• Keep your paperwork - file the certificates with your house documents for insurance and future resale.