Topic 5 of 5

Final circuits

A final circuit feeds current-using equipment directly, such as sockets, lights or a cooker. The common designs below come from the guidance in BS 7671 Appendix 15 and the On-Site Guide.

Ring final circuit

• 30 A or 32 A protective device.
• 2.5 mm² twin and earth (1.5 mm² CPC), both ends connected to the same device.
• Serves a floor area up to 100 m².
• An unfused spur can feed one single or one twin socket-outlet, or one item of fixed equipment.
• The number of unfused spurs shouldn't exceed the number of sockets on the ring itself, and you can't spur off a spur.
• A fused spur through a fused connection unit (13 A maximum) can feed more.

Radial socket circuits

A radial runs out from the consumer unit and ends at the last outlet. It doesn't come back.

Lighting circuits

Usually a radial on a 6 A or 10 A device in 1.0 mm² or 1.5 mm² twin and earth. Every lighting point needs a CPC, even if the fitting is plastic, so a metal one can be fitted later.

Isolation and switching

• Isolation: cutting off the supply so work can be done safely, for example by an isolator that can be locked off.
• Functional switching: turning equipment on and off in normal use, such as a light switch.
• Emergency switching: stopping quickly to remove danger, such as a stop button on a machine.

Exam tip: these ready-made designs only apply in normal conditions. Long runs, grouping or insulation can mean a bigger cable. Check with the cable size calculator.

Try it: Cable size calculator