Circuit for Electric Cookers

Small table cookers and other portable appliances having individual loadings not in excess of 3 kW are run off 13 A fused plugs and socket-outlets. They require no special wiring though extra socket-outlets may be needed in the kitchen when additional electrical cooking appliances are bought. These additional socket-outlets may be served from the ring circuit and spurs.

Family-size cookers, whether free-standing or built-in split-level, need an exclusive circuit from a fuseway of appropriate current rating.

Current ratings of circuits

Most electric cookers have a total loading of about 12 kW or less and are supplied by a 30 A circuit, wired in 6 mm² cable (or, sometimes 4 mm² cable) from a 30 A fuse- way. The cable terminates at a cooker control unit or switch of 50 A current rating.

Large-size family cookers, usually with loadings in excess of 12 kW are supplied from a 45 A circuit wired in 6 mm², or 10 mm² cable from a 45 A fuseway but terminated at the same 50 A cooker control unit or switch as the 30 A circuit cable. This is because the switchgear makers have designed the control units for the higher current demand.

Cooker control units

The conventional cooker control unit consists of a double-pole switch and a switched socket-outlet for an electric kettle. Some units have a neon indicator for the cooker switch and also for the kettle. The unit can be either flush- or surface-mounted.

An alternative to a combined unit is a cooker control double-pole switch (with or without neon indicator) but which does not incorporate a kettle socket-outlet.

Cooker current demand

If the total current of a 12 kW electrical appliance used on 240 V is calculated by dividing 12,000 by 240, then the result will be 50 A. If you add to this figure, 12 A for a 3 kW electric kettle plugged into the socket-outlet of a cooker control unit, you get a total of 62 A which is the total current rating of a 12 kW family size cooker and kettle.

A cooker of the above rating is, in fact, supplied from a 30 A circuit which at first sight does not make sense though complying with the regulations. The reason for the apparent discrepancy is that the regulations have assumed a current demand based on the known fact that not all boiling plates, grill and oven of a cooker, and also the kettle are in use at the same time.

Even on those rare occasions, such as cooking the Christmas dinner, when everything may be switched on for a time, the thermostat of the oven and the variable controls of boiling plates and the grill reduce the actual current demand. The regulations have therefore introduced a diversity factor for calculation assessed or assumed demand for domestic electric cookers.

Calculating cooker diversity

The assumed demand for an electric cooker is based on 100 % of the first 10 A, plus 30 % of the remaining current, plus 5 A for an electric kettle socket-outlet if incorporated in the control unit. For a 12 kW cooker plus kettle the assumed demand is as follows:

Total current of cooker: 50 A

100 % of first 10 A	10
30 % of remaining 40 A	12
Kettle socket	5
Total assumed current	27

The current rating for the circuit supplying the cooker is therefore 30 A.

Circuit cable route

The cable running from the circuit fuse to the cooker control unit takes the shortest possible path. This is usually under the floorboards and up the wall to the control unit.

If the floor is solid, as in many kitchens, the cable is taken up the wall through the ceiling into the void beneath the boards (or roof space if a bungalow) and down the wall to the control unit. The cable may be run on the surface or be buried in the plaster. If run on the surface, cable with white sheathing is usually chosen.

The cooker control unit is normally situated at a height of about 1.5 m (5 ft) from the floor and preferably to one side of the cooker. It must be within 2 m (6 ft) of the cooker it controls.

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