Wiring in an electric cooker

The average domestic electric cooker consists of an oven, a hob containing three or four rings and a grill. The conventional position for the grill is below the hob and usually built into it, although with some models it is positioned at eye level. Some models have a double oven.

The majority of cookers are compact, self-contained, free-standing units positioned against the wall and connected to a wall-mounted control switch by means of a trailing cable to allow the cooker to be moved forward when you want to clean behind it; a lockable bogie placed under the cooker will make it easier to move. Other models are built into the kitchen units; these usually have separate hob and oven sections and are termed split-level cookers, the oven being raised to a more practical height for cooking.

Circuit rating

Electric cookers are heavy current-consuming appliances, having loadings of up to 12kW (12,000 watts) for medium size cookers and even higher loadings for large, family size cookers including the double oven models. Cookers with loadings up to 12kW are usually supplied from circuits of 30amp current rating, while those in excess of 12kW are usually supplied from circuits of 45amp rating.

Although the maximum current demand of a 12kW cooker on a 240v supply is 50amps with everything switched on, allowance in rating the circuit has been made bearing in mind that rarely in the average home are all the rings, the grill and the oven in use at any one time. This means the circuit rating need only be 30amps. Even if everything is in use at one time, the current demand on the circuit at any one moment is still probably less than 50amps. This is because the rings, the grill and the oven are thermostatically controlled or include a simmering device which reduces the total current demand. In rating the circuit, allowance has also been made for an electric kettle plugged into the socket outlet of the cooker control unit and therefore taking current from the cooker circuit. Although a high-speed electric kettle takes up to 13amps from the circuit. Kettles are assessed at only 5amps.

There is an official formula for arriving at the circuit requirements of an electric cooker, based on the average use when cooking for a family. The first 10amps is estimated at 100 percent and the remaining current at 30 percent; 5amps is allowed for an electric kettle plugged in the cooker control unit. If the current demand of a 12kW cooker is 50amps, the first 10amps is included, the remaining 40amps is estimated at I 2amps and there is a 5amp allowance for a kettle. The total assessed load is 27amps and the circuit required is therefore 30amps.

Although the regulations permit a 12kW cooker to be supplied from a 30amp circuit, it is always worthwhile considering installing a 45amp circuit, provided the consumer unit will accept a 45amp fuse unit; most will not. If there is no spare fuseway for an electric cooker circuit in the existing consumer unit and a separate switch fuse unit has to be installed, it is worth fitting one with a 45amp rating. A 45amp circuit means a larger size cable, but the extra cost is comparatively small and the work involved is the same as for a 30amp circuit.

Cooker circuit

A circuit for an electric cooker consists of a two core and earth flat PVC-sheathed cable starting at a fuseway and terminating at a 45amp cooker control unit or cooker switch. From the control unit or switch the same size cable runs through a cable entry into a terminal block in the cooker. Where the oven and hob are separate, one cooker control unit and switch can be used for both provided neither is more than 2m (6 ft) from it.

Cooker control unit

This is a double pole switch and a switched 13amp socket outlet mounted on one panel. It is made with or without neon indicators and is available in either surface or flush-mounted versions.

Cooker switch

This is simply a 45amp double pole plate switch without a kettle socket outlet, with or without a neon indicator and available in surface or flush-mounted versions. The cooker switch is cheaper than a control unit since it has no socket outlet; it can be fitted when you do not want to operate the kettle from the control unit, which can often be a disadvantage and also potentially dangerous. If the control unit is fixed in the traditional position above the cooker, the kettle flex could trail over the rings; if these are switched on, the flex will burn and could start a fire or give an electric shock before the fuse blows.

With the introduction of the ring circuit, the traditional single utility plug and socket in the kitchen has largely been replaced by numerous socket outlets; this means a socket on the control unit is unnecessary, since the kettle can be used from one of these extra socket outlets.

Cable sizes

For a 30amp cooker circuit, you should use 6sq mm cable and I Osq mm cable for a 45amp circuit. Cable is available in grey or white sheathing; white is usually preferred, especially if part of the cable is fixed to the surface and if the trailing section between the control unit or switch and the cooker is visible.

Cable route

Having decided on the position for the control unit or switch, which must be within 2m (or 6ft) of the cooker so it can quickly be reached by someone using the cooker, choose the route for the cable. Where the consumer unit is on the same floor as the control unit in the kitchen (as in the conventional house), the simplest route is under the ground floor – assuming this is of timber construction. Run a cable down the wall below the consumer unit and pass it behind the skirting (if any) and into the void of the suspended floor. Feed it under the joists. Where it can lay unfixed on the sub-floor, and up through a hole drilled in the flooring immediately below the position of the control unit or switch.

If you have a solid ground floor (as in many modern houses), you will have to find an alternative route. This usually means running the cable up the wall above the consumer unit and into the void above the ceiling, under the upstairs floorboards and down through a hole in the ceiling above the control unit or switch. If you adopt this route, you will have to lift floorboards and probably drill through joists.

Where the bathroom – and particularly a combined bathroom/WC – is above the kitchen, you may have problems raising floorboards and the cable route will have to be diverted. Under these conditions it is best to prepare the route before buying the cable, so you can measure the exact length required and save buying too much of this relatively expensive material.

For a single-storey building with a solid floor, the cable can readily be run in the roof space. In a flat which has solid floors and where there is no access to the ceiling above, surface wiring will be necessary. Here the cable can be enclosed in conduits which can be in the form of hollow skirting.

Fitting control unit

Surface-mounted control units are available in two versions. One is all-plastic, with an enclosed back and thin plastic sections which are knocked out to provide entry holes for the cable; it is fixed to the wall with a couple of screws. The second type consists of a square plate, usually metal, which is mounted on a metal box; both are finished in white and the box is fixed to the wall with screws. The box has a selection of knock-out holes fitted with blanks, two of which are knocked out for the cables and fitted with PVC grommets to protect the cable sheathing. A range of smaller, oblong plastic control units is also available; these are mounted on plastic surface boxes or metal flush boxes.

The standard flush-mounted control unit is also of square plate design. Mounted on a matching metal box sunk into the wall so it sits flush with the plaster. This box also has knock-out holes for the cable and these must be fitted with PVC grommets. This type is suitable where the cable is buried in the wall; where it is not possible to bury all the cable such as with a tiled wall you will have to cut short channels above and below the box to feed in the cable.

A cable trailing down a wall from the control unit to the cooker is not only unsightly, but can also be an obstruction. It can be fixed to the wall with cable clips for most of its length provided the final loop is left free in case you want to pull out the cooker from the wall. However undue strain is likely to occur on the bottom clip and to overcome this you can fit a connector unit.

Connector unit

This consists of a terminal block fitted into a metal flush box and a moulded plastic cover plate with an entry hole for the cable. The connector unit is fitted about 1200mm (or 21ft) above the floor or lower if necessary; the cable running down from the control unit is connected to the terminals, the cable preferably being buried in the wall. The trailing length of cable running from the cooker is also connected to the terminals on the connector unit and the sheathing is clamped to prevent any strain being exerted on the terminals when the cooker is moved out from the wall. If the cooker is changed or temporarily removed, you can disconnect it easily by releasing the cooker cable from these terminals.

Cable outlet unit

An alternative arrangement is a cable outlet unit. Here the cable between the control unit and the cooker is not cut, but merely passes through, and is clamped in, the outlet box. Which should be positioned behind the cooker.

Connecting control unit

Having fixed the box to the wall with about 200mm (or 8in) of each cable within the box, strip the sheathing off the end of each cable, leaving about 25mm (1in) within the box; strip about 8mm (3/8 in) of insulation from the ends of the four current carrying conductors. Slip green/yellow PVC sleeving over the bare earth wires, leaving about 8mm (in) exposed. Connect the red circuit conductor to the mains terminal marked L and the black to the mains terminal marked N. Connect the red conductor of the cooker cable to the load terminal marked L and the black to the load terminal marked N. Connect the two earth conductors to the earth terminal of the control unit. Arrange the wires neatly in the box. Fix the switch to the box and screw the cover and front plates to the switch assembly. Separate cables are not required for the socket outlet since this is connected internally to the cooker terminals.

The method of connection is the same for a cooker switch. With a unit made entirely of plastic, the cables are threaded into the unit with the cover removed and the unit fixed to the wall. The cable connections are then made and the cover replaced.

Connecting to consumer unit

With the mains switched to OFF, remove the consumer unit cover, run the cable into the unit and prepare the end of the conductors as before. The red wire is connected to the fuseway terminal, the black to the neutral terminal bank and the green yellow PVC-sleeved earth wire to the earth terminal bank. Insert and fix the fuse unit, replace the cover and put the main switch back to ON.

Switch fuse unit

Where there is no spare fuseway you will have to install a separate switch fuse unit consisting of a double pole 60amp mains switch and a fuse unit of 30 or 45amp current rating. Fit the unit near the consumer unit and connect two 3m (or 10ft) lengths of 10sq mm PVC-sheathed cable a red insulation cable to the L terminal and a black insulation cable to the N terminal. Also connect 6sy mm green/yellow insulated earth cable to the F terminal. The cooker circuit cable is wired to load or circuit terminals as for the consumer unit.

The mains leads are connected to the mains by the electricity company. You may also have to fit a two-way service Connector box for the two pairs of meter leads you will now have.

Connecting split-level cooker

The same circuit cable from the 30 or 45amp fuse-way to the cooker control unit or switch is required for a split-level cooker; the one control will serve both sections provided each is within 2m (or 6ft) of the control unit. If the control unit is fixed midway between the two units, they can be spaced up to 4m (or 12ft) apart, which is adequate for most kitchen layouts. Otherwise a second control unit is required, one being linked to the other using the same size cable as for the circuit.

Where, as in most cases, the one control unit is to serve both sections of a split-level cooker, you can either run two cables from the load side of the control unit one to each section – or, depending on the relative positions, you can run one cable to the nearer of the two sections and then run a cable from the terminals of the nearer one to the other section.

Every cable must be of the same size as the main circuit cable, even though one or both may carry less than the total current. The reason for this is that, with no intervening fuse, the cable rating is determined by the rating of the circuit fuse.

The cables being run direct to the sections of a split-level cooker can be fixed to the surface or buried in the wall to suit individual requirements.

Connecting small cookers

Microwave cookers, which are becoming increasingly popular in the home, have loadings around 500 watts and are fitted with flex to be run off a – 13amp fused plug and socket outlet. These, therefore, need no special circuit. The same applies to baby cookers, which have a maximum loading of 3kW.

23. May 2011 by admin
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