Before the Electrician Arrives: Planning Power in a Shed or Workshop

Adding power to a shed sounds like a small job. A socket, a light, maybe a little heater for the winter. In practice it is one of those projects where the decisions made in the first hour determine whether you end up with a genuinely useful workshop or a trip hazard on an extension lead that you re-plug every time it rains.

The work itself should be done by a competent electrician. The thinking, though, is yours. Here is what to work out before you pick up the phone.

Build a load list, not a shopping list

Before anything else, write down everything that might ever be plugged in at the same time. Not everything you own — everything that could realistically run together.

Do the maths in amps, because that is what protective devices care about. Divide watts by 230.

• A 2kW convector heater: about 8.7A
• A 3kW kettle: 13A
• A 1.5kW table saw: around 6.5A, with a higher surge on start-up
• A 300W battery charger: 1.3A
• LED bench lighting: negligible, well under 1A
• A small welder or compressor: read the plate, and read it twice

That heater alone uses two-thirds of a 13A circuit. Add a kettle and you have exceeded it before the saw is switched on. This is the single most common reason a shed supply feels inadequate six months after installation: the heater was never in the plan.

Two routes, one decision

There are broadly two ways to power an outbuilding, and the gap in cost between them is smaller than most people expect if the run is short.

Option one: an outdoor socket on the house. An RCD-protected double socket on the external wall, fed from an existing circuit, with a suitably rated outdoor extension lead to the shed when needed. Fine for occasional use, a light, a mower charger. Not a permanent supply, and not something to run a heater from through a rolled-up lead — coiled cable under load gets hot, and that is how shed fires start.

Option two: a dedicated supply. A cable taken from a spare way in the house consumer unit, underground to the shed, terminating in a small consumer unit with its own RCD or RCBOs, feeding sockets and lighting on separate circuits. This is the version that behaves like a proper workshop. It is also the version worth planning now even if you only fit one socket initially — running the right cable once is far cheaper than digging twice.

Cable route: the bit everyone regrets skimping on

Armoured cable (SWA) is the standard choice for an underground run, and it needs to go deep enough not to be found by a spade. Around 600mm is the usual guidance for a domestic garden run, laid on a bed of sand, with warning tape above it and the route recorded — photograph it before you backfill, because in three years you will have forgotten.

Ducting under a patio or path saves a great deal of grief later. So does keeping the route away from drains, soakaways and the roots of anything you would rather not sever.

Then there is voltage drop. A long run at a modest cable size will leave you with dim lights and a heater that never quite gets going. As a rough guide, a 20m run at 13A is comfortable in 2.5mm²; a 40m run feeding a workshop wants 6mm² or more. Your electrician will calculate this properly, but knowing the distance from the consumer unit to the shed before the conversation starts is genuinely useful.

RCD protection and the earthing question

Every socket that might reasonably supply equipment outdoors needs 30mA RCD protection — either a single RCD protecting the shed circuits or, better, individual RCBOs so one fault does not kill the lights as well.

Earthing is less obvious and worth raising explicitly. If the house supply is PME (TN-C-S), exporting the earth to a detached outbuilding is not automatically appropriate. Many electricians will instead install a local earth rod at the shed and create a TT system for that building. It is not a fudge; it is the correct approach in a lot of garden situations. Ask the question, and be suspicious of anyone who does not have an answer.

Metal sheds and metal-clad buildings may also need equipotential bonding of the structure itself. Mention what the shed is made of.

Part P, notification and paperwork

In England and Wales, electrical work in a shed within the curtilage of a dwelling is covered by Part P of the Building Regulations. Installing a new circuit or replacing a consumer unit is notifiable. A registered competent person can self-certify the work and issue the certificate; otherwise you go through local building control, which costs money and time.

Replacing a like-for-like accessory is generally not notifiable, but this is not a grey area worth gambling on.

Scotland works differently again, with a building warrant system, and Northern Ireland has its own arrangements. The practical advice is the same everywhere: use someone who can produce a certificate, keep the certificate, and if you are in any doubt, ring your local building control before the work starts rather than after.

Uncertified electrical work is a real problem at sale, and insurers take a dim view of a shed fire behind an untraceable circuit.

Ventilation and condensation: the expensive invisible mistake

You can do everything above correctly and still end up with rusty tools and peeling paint, because a shed is not a house.

A well-sealed shed with a 2kW heater running through a damp November is a condensation machine. Warm moist air meets cold tools and cold roof felt, and the water goes exactly where you do not want it. Blocking the vents to keep the heat in makes it worse, not better.

Keep low-level and high-level ventilation open. Do not fill the eaves gaps with expanding foam. If you are heating the space regularly, insulate the roof and walls properly and accept that you need a background warmth rather than a blast. A 60W to 120W tubular heater running continuously does more for tool condition than a 2kW fan heater switched on for twenty minutes.

Avoid paraffin and unflued gas heaters entirely in a small timber building. The moisture output is substantial and the carbon monoxide risk is not theoretical.

Lighting and bench power

For a working shed, an IP-rated LED batten at around 4000K gives you honest light for close work without the harshness of a cold white tube. Two shorter fittings, one over the bench and one general, beat a single long one.

Put bench sockets at about 1.1m above floor level — above the sawdust line and reachable without stooping. Keep battery chargers on a shelf rather than the floor, and resist the urge to daisy-chain four-way adaptors off one socket.

The mistakes that cost the most

1. Treating an extension lead as a permanent supply.
2. Under-sizing the cable for the run, then adding a heater.
3. Skipping the RCD, or never testing the one you have.
4. Sealing the shed to keep the heat in, then wondering why everything rusts.
5. Unnotified work, discovered at survey stage.
6. Running the heater, the kettle and the saw from one circuit.

What to have ready for the electrician

Your load list, the distance from the consumer unit to the shed, the shed's construction, whether the route is grass, gravel or paving, and a rough idea of where the bench will stand. That is a fifteen-minute conversation that will save you a great deal of money and a great deal of regret.