Domestic Final Circuit Arrangements
The IEE Regulations, specifically the On Site Guide, takes a lot of work out of the design of a domestic installation for a typical installer. This combined with the Building Regulations gives us a quick and easy platform to determine the requirements.
Ring and Radial socket outlet circuits
The On Site Guide gives us 3 options for the installation of socket outlets. These can be found in Appendix 8 of the On Site Guide and can be simplified to the following:
Designation |
Type |
Protective Device |
Maximum floor area served |
|
|
|
|
A1 |
Ring |
30 or 32A |
100m2 |
A2 |
Radial |
30 or 32A |
75m2 |
A3 |
Radial |
20A |
50m2 |
As domestic installations continue to demand more power, so more circuits are introduced. Whereas 30 years ago it was not uncommon to have an installation with 3 or 4 circuits, today it is easy to double that. With socket outlet circuits, it is not uncommon to have 3 circuits in a new installation, one for downstairs, one for upstairs and one for the kitchen.
The On Site Guide also gives us guidance on the amount of outlets expected in a room, again based on modern usage. These can also be found in appendix 8.
Lighting circuits
Although no guidance is given to the amount of circuits, it is reasonable to expect 2 or more within a domestic installation, usually for convenience ensuring if a circuit was to break (which can happen with sensitive circuit breakers and filament lamps) there is still illumination within the property.
It is reasonable to assume that a domestic lighting circuit will be protected with either 6A or 10A circuits however if there is a doubt then the current demand should be calculated (using the On Site Guide guidance of 100W minimum per lighting point).
Power Circuits
Showers
Often forming part of modern installations is the electric shower. With sizes now reaching 11kw, it is vitally important that we calculate the correct cable size. Remember the power formula? An 11 kW shower can draw currents up to 48A!
Cookers
Ovens and cooking devices
An electric cooker can often have enough parts (top oven, bottom oven, grill etc.) to produce a large amount of current. Although we can apply diversity from the On Site Guide, modern ovens can have a large amount of features that must be taken into account when designing the circuit.
Water heaters
As a general rule, any water heater over 15 litres should be fed by an independent circuit. Again, calculations should be made regarding the power consumption of the heating element although in most instances, it will be fed by a 16A protective device and 2.5mm2 cable.
Other Circuits
Fire alarm
Introduced into the Building Regulations, specifically, Part B: Fire Safety, was the provision of a separate circuit for a fire alarm. It is thought that a separate circuit will not be isolated for any period of time hence the need to remove it from circuits such as lighting or socket outlet circuits.