Sizing Sub-Mains for Garden Offices & Outbuildings

Installing power to a garden office, gym, or summerhouse requires installing a distribution circuit (sub-main) originating from the primary dwelling. Because garden buildings are often located at the extreme rear of a property, the physical distance of the cable run introduces a critical engineering constraint: Voltage Drop.

Why Voltage Drop Defines the Cable Size

Under BS 7671:2018+A4:2026, the maximum permissible voltage drop for power circuits is 5% (11.5V) and 3% (6.9V) for lighting. However, this limit applies to the entire installation—from the main electrical intake in the house, all the way to the final socket plugged into the wall of the garden shed.

If you allow the sub-main cable connecting the house to the shed to consume the entire 5% allowance, any wiring *inside* the shed will instantly fail compliance. Good engineering practice dictates that the distribution circuit (the SWA down the garden) should be strictly limited to a 3% voltage drop. This safely leaves a 2% allowance for the final ring main or lighting circuits operating inside the outbuilding.

Because of this strict 3% limit, a 30-metre run feeding a simple 32A consumer unit will almost always fail on a 4.0 mm² cable, strictly requiring an upgrade to a thicker 6.0 mm² or 10.0 mm² SWA just to overcome the electrical resistance.

Exporting PME vs. Creating a TT Island

The majority of UK homes utilise a TN-C-S (PME) earthing arrangement provided by the DNO. When supplying an outbuilding, the electrician must decide whether to "export" this earth down the armour of the SWA, or isolate it and create a separate "TT Island" using an earth electrode (rod).

• Exporting PME: Permitted and safe only if the outbuilding does not contain any "extraneous-conductive-parts" (e.g., a metal water pipe entering from the ground, or a structural steel frame touching the earth). If no extraneous parts exist, you can export the PME earth via the 3rd core or armour of the SWA.
• Creating a TT System: If the outbuilding has a water supply or is built on a metal chassis, exporting a PME earth introduces a severe touch-voltage shock risk if the DNO loses their neutral connection in the street. In this scenario, the SWA armour must be earthed at the house end only (not connected to the shed's consumer unit), and a local earth rod must be driven into the ground at the shed, protected by a dedicated RCD.