Understanding domestic wiring circuits

Most homes in the UK are equipped with one or other of the two main types of 30 amp electiical circuit. Modern houses have a 30 amp ring circuit—a multi-outlet circuit supplying numerous 13 amp sockets and fixed appliances via fused outlets. Some older houses have a 30 amp multi-outlet radial circuit which supplies a limited number of 13 amp socket outlets and/or fixed appliances via fused outlets. In all cases, the outlets have a maximum capacity of 13 amps.

But some appliances, notably electric cookers and electric shower units, require their own separate 30 amp or 45 amp circuit because of their high power rating.

Note that you should not confuse a 30 amp radial circuit with the older, 15 amp, 5 amp and 2 amp radial circuits installed until about 1947. If you have one of these, your house is almost certainly long overdue for complete rewiring.

Canadian wiring practice is different, and uses more such single outlet circuits. That for a range will usually be on a 240 volt supply and the current rating and cable used may vary. The detailed information given here applies only to UK systems.

Electric cooker circuit

Essentially, electric cooker circuits consist of a cable running from a circuit fuseway in the consumer unit to a control switch, and then on to the cooker itself.

The current rating of the fuse, or MCB , and of the cable in the circuit is deter- mined by the current demand of the cooker. The majority of domestic electric cookers have a loading of between 10,000 watts and 13,000 watts. On the 240 volts electricity supply standard in the UK, the maximum possible current demand of a 10,000 watt cooker is 42 amps, and of a 13,000 watt cooker, 54 amps.

However, because of the oven thermostat and variable controls of the boiling rings, there will never be more than a momentary maximum demand. UK regulations provide a simple formula for assessing the current demand of a domestic cooker, in which the first 10 amps are rated at 100 percent, and the remaining current at 30 percent. A socket connected to the control unit is rated at 5 amps.

This means that a socket and a 10,000 watt cooker drawing a theoretical 42 amps have an assessed demand of 25 amps. And a 13,000 watt cooker with a theoretical 54 amp demand, plus socket, is assessed as 28 amps. Consequently, most cookers fall well within a current rating of 30 amps. Only very large ones and those with double ovens require a 45 amp circuit.

Consumer unit to control switch

Hopefully your existing consumer unit will have a spare fuseway from which to run the cable.

However, where there is no spare fuseway—and usually where you have a large cooker requiring a 45 amp circuit—you must install a separate mams switch and fuse unit, called a switchfuse unit.

The cable

The only difference between a 45 amp and a 30 amp circuit is the size of the cable. If your calculations have shown you that a 30 amp fuse is adequate for your cooker, then you need cable with a current rating of at least 30 amps— 6mm2 twin and earth PVC sheathed will usually do. With a 45 amp fuse, 10mm2 cable is needed.

The cable should obviously take the shortest possible route from the switchfuse unit to the cooker control unit. Where there are no solid floors it is an easy task to run the cable straight down from the switchfuse unit, under the floorboards, to emerge once again immediately below the planned position of the control unit.

However, should you need to run a cable across the direction of the joists, these must be drilled—not notched— at least 38mm below the top edge of the joist.

But, where, as in many kitchens, there is a solid floor, it is advisable to take the cable up from the switchfuse unit, into the ceiling void, across and between the joists, and then down the kitchen wall to the control switch.

Positioning the control unit

The control unit is normally fixed at about 1.5m above floor level, and to one side of the cooker. UK regulations require that the switch should not be more than 2m from the cooker, to allow for rapid access in emergencies.

Control units come in various styles, either surface or flush mounting, with or without neon ‘power’ indicators, and with or without a kettle socket outlet. The current rating is always 50 amps. If it is necessary to fix the switch above the cooker, one without a socket is recommended to avoid the risk of a flex trailing across a hot ring.

Control unit to cooker

To allow a free-standing cooker to be moved for cleaning or servicing, you must run the last section of cable in the form of a trailing loop. The first few feet can either be fixed to or buried in the wall immediately below and behind the unit.

If you opt for burial, you must install an outlet box. You can either use a through box, simply anchoring the uncut cable with a clamp, or a terminal box, which would make it possible to remove the cooker altogether.

Split-level cookers

In the UK, regulations regard the separate hob and oven sections of a split-level cooker as one—and therefore on one circuit—providing that both sections are within 2m of the control unit.

Following these rules, you can install the two sections up to 4m apart as long as the control unit is midway between them. And if one of the sections is installed more than 2m away from the control unit, it can share the circuit but needs a second control switch. In all cases, the cable fitted on the cooker side of the control unit must be the same size as the circuit cable.

Electric shower unit circuit

An electric shower unit is an instantaneous electric water heater in which the water is heated as it flows over the element unit. To provide adequate hot water the element has a loading of 6000 watts or in some cases 7000 watts, with respective current demands of 26 and 29 amps.

To install a shower unit you will therefore need a 30 amp fuse way, a length of 30 amp twin and earth PVC sheathed cable, a double pole isolating switch, and a length of sheathed flexible cord or cable to connect the shower unit to the isolating switch. In some cases a flex, or cord, outlet unit is required for the final connection to the shower unit.

Running the cable

On an electric shower installation, use a twin and earth PVC sheathed cable with a current rating of not less than 30 amps to match the circuit fuse or MCB in the consumer unit. The circuit cable running from the 30 amp fuse-way to the 30 amp double-pole isolating switch should run in the same way as that for a cooker.

It is dangerous to have an electric switch or socket within reach of wet hands, so you must install either a cord-operated ceiling switch in the bathroom or a wall-mounted switch outside.

A special 30 amp double pole switch with neon indicator is available for cord operation and is mounted on a standard, square-moulded plastic box. To install this unit, first remove the knockout blank in the base for the two cables; one is the circuit cable, the other is the shower unit connecting cable. Pierce a hole in the ceiling for the two cables, connect them to the switch, then fix the unit to a timber joist using wood screws.

Some shower units are supplied with a 3-core circular sheathed flex. Do not remove this; instead, run both it and the cable from the isolating switch into a cord outlet unit mounted on a square-moulded plastic box and fixed near the shower unit.

In rooms other than the bathroom mount a 30 amp double-pole switch outside the showering cubicle, in a position where it cannot be reached by anyone using the shower.

All the necessary controls are on the shower unit itself and are all nonelectrical, the heater circuit being controlled by a water-operated pressure switch.

Connecting up the switch

The 30 amp cord-operated ceiling switch is a double-pole switch and therefore has two pairs of terminals. Each respective pair is marked L for live and N for neutral. One pair takes the cable from the consumer unit, and these are marked SUPPLY or MAINS. The other terminals are for the cable to the shower unit, and are marked LOAD.

Connection at the consumer unit

With all the other wiring completed, your final task is to connect the cooker or shower circuit to the mains, either at a spare fuseway in the con- sumer unit—or if that is not possible— to a separate switchfuse unit. The installation of, and connection to, a switchfuse unit is described in the panel opposite.

To connect to a fuseway, turn off the mainswitch and remove the consumer unit cover. On some models you must first remove the fuse carriers or MCB’s.

Cut the new circuit cable, allowing about 300mm for the inside of the unit. Trim about 250mm of outer sheathing from the cable, strip about 9mm of insulation from the live and neutral wires, then sheath all but 9mm of the bare earth wire with green and yellow PVC sleeving.

Next, knock out a blank from the consumer unit case and thread in the three wires until there is about 50mm of sheathing within the unit. Connect the red wire to the terminal of the spare fuseway, and the black wire to the neutral terminal block—which will already have some neutral wires connected to it in any case. Connect the sleeved earth wire to the common earth terminal block. Carefully replace the fuse carriers or MCBs, if you have had to remove these. Then put back the cover.

Finally, fit the new 30 amp fuse unit and if the fuse is rewirable, check that it contains a 30 amp fuse wire. Replace the cover of the consumer unit, turn on the mainswitch and you are ready to test the new installation. If any appliances refuse to work recheck the wiring.

Connecting a fuse unit

Where there is no spare fuseway on an existing 30 amp consumer unit, or the circuit requires a 45 amp installation, you must install a switch-fuse unit.

In the UK, you cannot make the final connection to the meter yourself, but must contact the Electricity Board to do it for you. At the same time, they can check over the rest of your installation to ensure that it is safe.

A switchfuse unit is a one-way consumer unit consisting of a double-pole mainswitch and a single-pole fuseway fitted with either a fuse or an MCB. Both two-way and three-way consumer units are available, and you should consider installing these if future electrical extensions are anticipated.

Fit the unit to the wall adjacent to the existing consumer unit. Then connect three single-core, double insulated PVC sheathed cables—one with red, one with black, and one with green and yellow insulation—to the appropriate L , N and earth mains terminals. The cables should be cut to about 1m in length, and then left for the Electricity Board to connect up. The Board stipulates the cable size —-usually 16mm2—which is determined by the current rating of the Board’s service fuse.

To prepare the end of the circuit cable first cut it to the correct length, allowing a short amount for the inside of the switchfuse unit. Trim about 300mm of outer sheathing from the cable, strip about 9mm of insulation from the live and neutral wires, then sheath all but 9mm of the bare earth wire with green and yellow PVC sleeving.

Connect the wires to the appropriate fuseway terminals, insert the fuse unit if necessary then notify the Electricity Board that the circuit needs connecting to the mains.

Electricity Boards often insist that a 60 amp double-pole terminal block is installed between the consumer unit and the electricity meter. Though you can fit this yourself—but not wire it—it is _ generally easier to pay the Board to do it for you especially as they have to wire it up for you in the first place.

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