Garage Supply Calculator — Load Assessment, Submain Sizing, and Consumer Unit Selection

Feeding a detached garage or outbuilding properly means installing a submain: a suitably protected circuit from the house consumer unit (or a dedicated switch fuse) out to a small consumer unit in the garage, which then serves the garage's own final circuits — sockets, lighting, and whatever the space is really for.

The parts of the job:

A single socket on a spur will do for a lawnmower; anything resembling a workshop, gym, office, or EV charging point deserves the full treatment.

The wrong way to size a garage supply is to add up the ratings of every protective device in the garage board — a 32A socket circuit plus a 6A lighting circuit plus a 16A heater circuit does not mean a 54A demand. The right way is to assess what actually runs simultaneously:

• Sockets — assess by use: a hobby workshop might realistically see one big power tool plus a dust extractor and battery chargers at once, not a fully loaded circuit.
• Lighting — LED workshop lighting is a small, steady load; apply standard lighting diversity.
• Heating — the load most likely to run continuously; take fixed heating at full rating in cold months.
• Fixed machinery — compressors, welders, and saws by their duty: a welder's demand is intermittent but high, and motor starting matters for device selection. See the motor starting current calculator.
• EV charging — the exception to diversity: a single charger is a continuous full-current load. Take it at 100%.

The calculator above lets you add each load with its type, applies the appropriate diversity, and recommends the main device rating — the number the submain is then designed around.

A detached double garage used as a workshop: one socket circuit, LED lighting, and a 2kW fixed heater. The trench run from the house board is 25 metres:

Add a 32A EV charger to the same garage and the picture changes completely: 30.7 + 32 = 62.7A assessed, pushing the design to a 63A/80A submain in 16mm² or 25mm² SWA — which is why the future-proofing conversation belongs at the start of the job, not after the trench is backfilled.

The submain itself is almost always buried SWA. The full sizing detail lives on the SWA cable size calculator page; the submain-specific points:

• Size for voltage drop first — the submain should use only part of the 5% total allowance, because the garage's final circuits add their own drop after the garage board. Keeping the submain under about 2% is a sound working habit.
• Burial requirements — BS 7671 requires buried cables to incorporate an earthed armour or metal sheath (or equivalent duct protection), marked with cable covers or marker tape, at a depth sufficient to avoid foreseeable disturbance. Accepted practice is around 450mm in gardens, 600mm under drives.
• Duct it if you can — a duct with a draw cord turns the future EV-charger upgrade into a cable pull instead of a second trench.
• Protect it properly at the origin — the submain device in the house board protects the cable; check disconnection times over the full run with the disconnection time calculator.

At the garage end, a small consumer unit distributes to the final circuits. Design points:

• Main switch and ways — a compact two-to-six-way board covers most garages: sockets, lighting, heating, and a spare way or two for the future.
• RCD protection — socket outlets up to 32A require 30mA RCD additional protection (Regulation 411.3.3). RCBOs per circuit avoid a workshop freezer defrosting because a power tool tripped a shared RCD.
• Discrimination with the house board — think about what trips first: RCD protection at the garage board serving the garage circuits, with the submain arranged so a garage fault does not take out house circuits.
• The earthing arrangement — either export the house earthing system to the garage via the submain, or create a TT island with a local earth electrode at the outbuilding. On PME supplies, the decision needs particular care where the garage has outdoor equipment, water, or an EV charger — assess, decide deliberately, and record it on the certificate.

The whole-house picture matters too: the garage's assessed demand lands on the house supply, so check it against the installation's maximum demand before finalising the design.

The single most common regret on garage supplies installed in the last decade is sizing them before EV charging arrived. A 7.4kW charger adds a continuous 32A — usually more than the entire rest of the garage — and it cannot share capacity by diversity, because it genuinely draws full current for hours.

Practical future-proofing, in ascending order of cost:

• Duct the route — even if today's cable is modest, a duct makes the upgrade a pull-through.
• Oversize the submain cable — the labour of the trench dwarfs the cost difference between 10mm² and 16mm² SWA. Install the bigger cable now.
• Design the submain for the charger — a 63A submain feeding a garage board with a 32A EV way and the general circuits, with load management if the house supply is tight. The EV charger load calculator covers the supply-capacity side, and the EV charger cable size calculator the circuit itself.

Have the conversation with the customer before quoting — "will this garage ever charge a car?" is a one-line question that changes the whole design.