Understanding The Outside Electricity Supply

An electricity supply to a detached garage, outdoor workshop or greenhouse requires a separate circuit of 20A or 30A current rating. The circuit can be from a fuseway in the existing consumer unit with a double-pole 30A switch fixed near the consumer unit. If there is no spare fuseway it is necessary to install a main switch and fuse unit. This can be fitted with either a cartridge fuse or an MCB.

The cable running to the outbuilding can be twin and earth PVC sheathed (2.5mm2 for a 20A supply, 4.0mm2 for a 30A supply). The outdoor section of the cable can be run overhead fixed at a height of at least 3.5m above ground level, or it can be buried underground where it must be enclosed in high impact, rigid plastic conduit buried at least 500mm below ground level.

Overhead method

In addition to the cable, materials required are: a length of galvanised steel catenary

wire (preferably stranded), a length of 2.5mm’ green/yellow PVC insulated earth cable and a cable connector; a length of 100 x 50mm timber for fixing to the outbuilding to provide the fixing height for the overhead cable fixed to the catenary wire; two eye bolts or vine eyes and an adjuster; a main switch and fuse unit to be fixed in the outbuilding in addition to the one in the house.

Installing the cable

Run the cable out of the house through a hole drilled in the wall at the specified height. Fix one eye bolt and the adjuster to the house wall, the other to the timber at the

outbuilding. Splice the catenary wire between the eye bolts and tension it, and attach the PVC sheathed cable to the catenary using cable ties. Join the earth wire to the catenary at the outbuilding end using the cable connector. Fix the PVC sheathed cable and the earth cable to the upright timber and pass the ends into the outbuilding through a hole drilled in the wall and connect the red and black wires to the L and N mains terminals of the switch and fuse unit fixed near the door.

Underground method

Dig the trench to the required depth and place a 50mm layer of builders’ sand in the bottom of the trench. Install the 25mm conduit in the trench using wide bends at the corners and pass it through a hole drilled into the house wall above the damp proof course and at the other end into the outbuilding.

Dismantle the conduit and thread in the cable. Smear adhesive bought with the conduit over the ends before fitting the elbows and coat the joints with bituminous paint.

Fill in the trench, removing any sharp stones or flints. Connect up the cable as described above. In the outbuilding, socket outlets and a fused connection unit for the fixed lighting can be supplied from a radial circuit.