Garage Consumer Units: Powering a Garage, Shed or Garden Room

Proper power to a garage, shed or garden room usually means a dedicated cable from the house consumer unit, often steel wire armoured (SWA) cable run underground, feeding a small consumer unit in the outbuilding with its own breakers and 30 mA RCD protection. Because that is a new circuit, it is notifiable under Part P of the Building Regulations. It has to be installed and certified by a registered electrician who can self-certify it, or notified to building control before any work starts.

This is not a job to do yourself, and nothing here is a set of instructions. It explains what a proper installation involves, so you know what to ask for and what you are paying for.

Does a garage need its own consumer unit?

Not always, but anything beyond a light and a socket or two usually does. A small garage consumer unit lets the supply be split into separate circuits, so a fault on a power tool trips its socket circuit and leaves the lights on while you find out why. It also gives a local main switch, so the whole outbuilding can be isolated without going back to the house.

A garden room used as an office, with heating, lighting and sockets, is effectively a small building of its own and almost always gets its own board. A shed with one light and one socket may be served by a single circuit instead. Which one suits depends on the load, the distance and how the outbuilding will be used, and those are design decisions for the electrician.

The house end matters too. The cable to the outbuilding needs its own protective device in the main consumer unit. If your board is full, old or has no RCD protection, the outbuilding job may need a fuseboard upgrade first, and our guide to consumer unit replacement cost explains what that typically adds.

Why the cable is usually armoured

Armoured cable is the standard way to take power underground to an outbuilding. SWA cable has a layer of steel wires wrapped around the insulated cores, under a tough outer sheath. The IET notes that the SWA made today is almost all thermosetting (XLPE) insulated, to standards such as BS 5467.

The armour does two jobs. It protects the cable from a spade or fork, and when it is properly earthed it acts as a protective conductor, so if something does cut through to a live core, the fault is cleared quickly.

BS 7671 is specific about buried cables. The IET's guidance on regulation 522.8.10 says a cable buried in the ground generally needs an earthed armour or metallic sheath, or has to run inside a conduit or duct that gives equivalent protection. Its route has to be marked with cable covers or a suitable marker tape above it, so the next person to dig finds the warning before the cable.

How deep does armoured cable need to be buried?

BS 7671 does not give a single figure for a domestic garden. It says buried cables must be deep enough to avoid damage from any reasonably foreseeable disturbance of the ground, and leaves the depth to the designer's judgement. For ground that is not built to highway standards, the IET says its Guidance Note 1 recommends a minimum depth of 500 mm, to protect the cable from ground movement and frost heave. Flower beds, vegetable patches and anywhere a post might be driven are reasons to go deeper or choose another route.

Before anyone digs, the IET points to HSE guidance HSG47 on avoiding underground services. Gas, water and existing cables are often closer to the surface than people expect, and a record of the new cable's route and depth should be left with you when the job is finished.

RCD protection in a garage or shed

Outbuildings are exactly where RCD protection matters most. BS 7671 requires 30 mA RCD protection for socket outlets rated up to 32 A in homes, and for mobile equipment up to 32 A used outdoors. Electrical Safety First's coding guide treats a missing RCD on sockets likely to power outdoor equipment, such as a lawnmower or pressure washer, as C2 (potentially dangerous) on a condition report.

Where the 30 mA protection sits is a design choice. The IET's guidance notes there is no need to add a second 30 mA RCD in the outbuilding if the circuit is already protected by one at the house, and that doing so gives no guaranteed selectivity: in a fault, either one may trip first. Your electrician will choose an arrangement that protects the outbuilding without leaving you walking back to the house every time something trips.

Earthing is the other decision that needs care. How the outbuilding is earthed depends on the supply to the house and on what is in the outbuilding, and it is decided after testing, not assumed.

One practical warning. Electrical Safety First's product recall list has included a small garage consumer unit sold online whose RCBO could fail to cut out under fault conditions, creating a serious fire risk. A cheap unit of unknown origin is a poor saving on a part that is meant to protect you, and it is one reason a registered electrician will usually supply the board they are certifying.

Why a garage supply is notifiable work

Approved Document P is clear that Part P applies to electrical installations in outbuildings such as sheds, detached garages and domestic greenhouses. It also says that garages, sheds and similar buildings that share their electricity with a house are not exempt from Part P, even where other Building Regulations might not apply to them.

The notifiable part is the new circuit. Approved Document P gives a useful example: connecting an electric garage door to an existing isolator switch is not notifiable, but installing a new circuit from the consumer unit to that isolator is. The same logic applies to a garage, shed or garden room supply.

In practice that leaves two usual routes:

1. A registered electrician designs, installs and tests the work, issues an Electrical Installation Certificate, and self-certifies it. You should receive a Building Regulations compliance certificate within 30 days.

2. Building control is notified before work starts, and inspects and tests the work (usually for a fee) before issuing a completion certificate.

Work done without either route can surface later, when a buyer's solicitor asks for certificates or an insurer asks how the outbuilding was wired.

What about an extension lead to the shed?

For occasional use, an outdoor-rated extension lead from an RCD-protected socket is fine. Electrical Safety First advises using only leads and reels suitable for outdoor use, with the reel fully unwound and a thermal cut-out, and an RCD for all outdoor equipment. A lead left permanently run across a lawn or through a door is a different matter: it gets crushed, wet and chewed, and it is not a substitute for a fixed supply.

Whether you need an electrician in Newport, Shifnal or Telford, we install garage, shed and garden room supplies across Wolverhampton and the surrounding area. If you are planning one, talk to SM Electrical and we will look at the route, the load and your existing board before we quote.

Garage electrics questions, answered

Can a garage consumer unit supply an EV charger?

An EV charger normally needs its own dedicated circuit, and it has its own specific requirements for protection and earthing. If a charger is likely in future, say so at the design stage, because it changes the cable size and the board.

Can I dig the trench myself to save money?

Some electricians are happy for you to dig to their specification, but agree the route, depth and timing with them first, and follow HSG47 before breaking ground. The electrical work, including laying and connecting the cable, stays with the electrician.

Should an existing outbuilding supply be checked?

Yes, especially if it was already there when you bought the house. An EICR will test the cable, the earthing and the RCD protection, and tell you whether it is safe to keep using. Our guide to what a consumer unit is explains the parts the report will refer to.