The Outbuilding as a Second Installation: Planning Loads Before the Tools Move In

A detached garage or garden workshop often begins modestly: a light, a socket or two, perhaps a legacy supply run from the house years ago. Then the property changes. A larger compressor arrives, followed by a welder, a space heater, a hot tub, or an EV charger. The outbuilding, once an afterthought, becomes a second working property with its own demands. At that point, the electrical design needs to catch up with the way the space is actually used.

A spur is not a design

Many outbuildings are served by a cable that was adequate for occasional use and little more. It may have been installed for a freezer, a lawnmower charger and a single fluorescent fitting. That does not make it suitable for a workshop with high-demand equipment, nor for a garden room with heating and hot water.

The first question is not whether the existing consumer unit has a spare way. It is what the outbuilding is expected to do, how often, and with which equipment running at the same time. A small consumer unit in a shed can look reassuringly complete, but its rating, protective devices, cable size and earth arrangement all need to be checked against the intended load. A second enclosure is not automatically a second installation.

Start with a load schedule, not a shopping list

A useful load plan lists every intended item and its electrical characteristics. For domestic equipment, the rating on the label is a starting point, but the electrician will also consider start-up currents, duty cycles and whether items are continuous or intermittent. Welding equipment, compressors, pumps and some garden machinery can draw significantly more at start-up than their running rating suggests.

A practical schedule might include:

• lighting and general-purpose sockets;
• fixed heating or air conditioning;
• compressor, welder or other workshop machinery;
• hot tub, sauna or garden water feature;
• EV charge point, if one is planned;
• battery storage or solar equipment, where relevant;
• future items the owner reasonably expects to add.

The schedule should distinguish between loads that may run together and those that are unlikely to coincide. It should also record the supply characteristics at the origin and the proposed route and length of the submain. This is where diversity calculations begin.

Diversity is a judgement, not a discount

Diversity is sometimes misunderstood as a way to make a load fit. In reality, it is a structured assessment of how much current is likely to be drawn at once, allowing for the way a real property behaves. A workshop might run a heater and lighting for long periods, then use a welder intermittently. A hot tub may run its heater and circulation pump for hours, while a compressor starts only occasionally.

The assessment has to be honest about the occupants’ habits. If two people routinely use high-demand equipment at the same time, the design should reflect that. If an EV charger is programmed overnight while a heater runs in winter, the overnight load is not hypothetical. Diversity calculations are not a licence to undersize a submain; they are a way to size it sensibly against realistic use.

The submain is the backbone

Once the load is understood, the submain can be designed. Cable size is influenced by current-carrying capacity, voltage drop, installation method, grouping and the length of the run. A long garden run has different characteristics from a short cable clipped to a garage wall. Buried armoured cable, ducted routes and overhead spans each bring their own considerations.

The protective device at the origin must coordinate with the submain and with the distribution board in the outbuilding. Selectivity, fault protection and disconnection times all matter. So does the question of isolation: can the outbuilding be safely isolated for maintenance without disrupting essential supplies in the house?

A properly sized submain also leaves room for reasonable future demand. That does not mean designing for every conceivable appliance, but it does mean avoiding a cable that is already at its limit when the first significant tool is plugged in.

Earthing and bonding deserve early attention

Metallic outbuildings — steel-framed garages, metal-clad workshops and structures with substantial extraneous metalwork — require careful consideration of earthing and protective bonding. The arrangement depends on the supply type and the characteristics of the installation. An exported protective earth from the main building may be appropriate in some circumstances; in others, a local earth electrode or a different protective measure may be needed.

This is not a decision to make from a distance. The electrician needs to assess the supply arrangement, the presence of extraneous-conductive-parts, the likelihood of simultaneous contact and the bonding requirements. Metallic water pipes, structural steel and other services entering the outbuilding may need bonding, but only after the earthing system has been reviewed as a whole. Adding a random earth rod without understanding the supply arrangement can create as many problems as it solves.

Small consumer units can create false confidence

The consumer unit in an outbuilding should be selected for the installation it serves. It needs suitable protective devices, adequate ways for the planned circuits, and clear labelling. If the enclosure is in a damp or unheated space, its ingress protection and location matter. If it is in a workshop, it should be positioned where it remains accessible and not exposed to mechanical damage.

A small unit may be perfectly adequate for a modest garden room. It is not adequate simply because it is new. The rating of the unit, the type and rating of its devices, and the fault level at that point all form part of the design. The outbuilding should be treated as a small installation in its own right, with its own test schedule and records.

Plan for the equipment that has not arrived yet

Owners often buy equipment in stages. A welder is added after the workbench, a hot tub after the decking, an EV charger after the car. Each addition can be safe in isolation and still push a legacy supply beyond its intended duty.

Before the next significant purchase, it is worth asking a certified electrician to review the outbuilding supply. A load schedule, a submain calculation and an earthing assessment are not paperwork for their own sake. They are the difference between a workshop that grows safely and a supply that is quietly asked to do too much.

Custodian Electrical designs and inspects outbuilding supplies with the same care as the main property. Safety first, always.