Electrical circuits run through walls, along skirting and under floorboards to all parts of your home, carrying current to feed the various fittings and appliances. All circuits run from the main fuse box or consumer unit, which in turn is fed from the mains supply from outside. Once you have learnt how these circuits are wired up, you can move and add lights, switches and sockets where you want. You may decide to run electricity to areas that previously were without — for example the loft, outbuildings or into the garden. Bear in mind that new wiring will involve a certain amount of redecorating afterwards.
Fitting a new consumer unit
A consumer unit is a comparatively modern piece of electrical equipment which combines the necessary double pole main switch with a single pole fuseboard in one casing. It should be situated as close as possible to the electricity company meter, to which it is connected by the meter tails which provide the supply. The switch is double pole so both the live and the neutral supplies are cut off simultaneously when the main switch is turned off. The fuses in the consumer unit, which protect individual circuits and equipment from excess current caused by a fault or overloading, interrupt the current in the live circuit.
Deciding number of circuits
The size of consumer units varies from two-way, for an installation with only two-circuits, to 12-way ; the most common unit installed in a home is eight-way. Generally the size is determined by the number of circuits involved when the house is built. So when an additional circuit is required, perhaps for a shower heater or power point in your garage, there is no spare fuseway. As the wiring regulations do not permit any fuse to supply more than one circuit, you have two alternatives: either fit a main switch and fuse unit next to the consumer unit or replace the unit with a larger one with more fuses. A separate switch and fuse unit needs a separate mains connection and since electricity companies will not allow more than one pair of leads connected to their meter, a special terminal box has to be fitted on the consumer’s side. Another disadvantage is that this will only provide one extra circuit. It is more satisfactory to replace the consumer unit and normally it is a waste of time and money to fit one smaller than an eight-way; this will, for example, supply two lighting circuits, two ring circuits, a cooker, an immersion heater and leave two spare fuseways for future expansion.
There are three principal types : one with fuses that can be rewired or fitted with cartridge fuses, another fitted with miniature circuit breakers (mcb) and a third which accepts either fuses or mcbs. The circuit breakers give the best protection, while cartridge fuses are next best ; most units have wire fuses because they are cheapest. New consumer units now often incorporate mcbs and an ELCB.
The current required to blow a wire fuse is twice the rating of the fuse, which means a 10amp current is required to blow a 5amp fuse. A cartridge blows at one and a half times its rated current and an mcb operates when only one and a quarter times its rated current flows in the circuit. An mcb also operates much quicker than a fuse and reduces the risk of fire or damage to the circuit or appliance. If price is a consideration, it is worth remembering an mcb costs about three times as much as a fused unit. You do not repair an mcb; you simply press a button or turn on a switch to reactivate it — as long as you have dealt with the problem that caused it to break the circuit. Fuse wires and cartridges must be replaced and it is vital to keep spares.
Circuit fuses and mcbs are made in five ratings, each with a different colour code, for domestic use: 5 (white), 15 (blue), 20 (yellow), 30 (red) and 45 (green). Consumer units do not usually have provision for a 45amp fuseway, which occupies the space of two standard fuseways in some units and is needed only for a circuit supplying, for example, a large cooker with a loading of 17kW or more. If you are changing your existing unit for a larger installation, you will probably find your present fuses or mcbs will be suitable for your new unit and will save you some expense.
Before you remove your existing consumer unit or fuseboard, the power must be disconnected at the mains by the withdrawal of the service fuse and you must give your electricity company at least 48 hours’ notice, in writing, that you require a temporary disconnection.
electricity companies are obliged by law to isolate a consumer’s installation from the mains on request during normal working hours.
You can usually make the change in one day and if electricity is disconnected at 9am you can have the supply restored at 5pm. If by midday you realize you cannot complete the job in time, you can always ask for reconnection to be postponed.
Preparing for installation
Some preparatory work can be done a day or so before you have the electricity cut off. Check you have all the materials and equipment you require. Remove the cover from the new unit and the fuses or mcbs, if they are bought already fitted. This leaves only the main switch, terminal banks and the copper busbar to which the fuse units are screwed. Consumer units are made entirely in metal, or plastic, or a plastic casing with a timber frame. The metal and all-plastic units have knock-out holes at the top and bottom and at the back. If it is a plastic case on a timber frame, you should either drill holes or cut recesses for the cables in the timber frame (at the top or bottom or both, depending on whether your circuit cables run up from ground level or down from ceiling height); if the cables are channelled into the plaster, they can bc brought into the unit through the back of the timber frame. If the unit is to be mounted on a combustible material a sheet of non-combustible material should be fixed between the unit and the wall. Strip cable Strip about 50mm (2in) of sheathing from the 16sq mm cable and take about 13mm (kin) of insulation off each conductor. Connect the red (live) conductor to the L terminal and the black (neutral) conductor to the N terminal. Tighten the screws and replace the insulated sleeves, where fitted. Make sure you have enough cable for the electricity company official to connect up. Your existing earthing lead should be connected to the earthing terminal on your new unit after the supply has been disconnected. If you are fitting a new earthing lead, you can fit it to the earthing terminal but don’t connect it to earth; the electricity company official will do that.
Check all the circuits are labelled on the existing consumer unit. If not, by switching on all lights and appliances and then withdrawing fuses one by one, turning off the mains switch before pulling out each fuse, you can identify the circuits. Label them by attaching self-adhesive stickers on the cover of the consumer unit and then prepare a second set of labels which will go on the cables.
When the electricity has been cut off, take off the cover from the existing consumer unit and remove the fuses. Loosen the terminal screws on each fuse holder in turn and disconnect the live circuit wires, which should all be red. As you disconnect, wrap the appropriate identification label round each circuit cable. Where two cables were connected to one fuseway in a ring circuit, use a piece of adhesive tape to bind them together until you reconnect to the new unit. Release the wires from the neutral and earthing terminals. Remove the frame or casing of the unit, replaster any holes and make good any decoration as required.
Take the frame, or casing, of the new unit, hold it in position on the wall and mark the fixing holes. Drill and plug the holes and screw the unit into position. Don’t forget to fix a noncombustible sheet between the unit and the wall if you are mounting on a combustible material. Feed the circuit cables into the casing and connect them to the fuseways (an mcb unit may require the mcbs to be positioned first). Starting at the main switch end of the unit, connect your cooker circuit (if you have one) or the first of your ring main circuits. Connect the red conductor to the first live terminal, the black to the first terminal on the neutral bank and, after slipping green PVC insulating sleeving on if it does not have it, connect the bare earth conductor to the first earthing terminal. The end of the cable sheathing must be within the case or the frame. Continue to connect up in descending order of rating: 30, 20, 15 and 5. Any spare fuseways must be fitted with blanking plates until required. The circuits should, ideally, be rearranged where necessary when the current rating of any new circuit is known.
Fitting fuse bases
Screw the fuse bases — they are sometimes called shields — into their correct position according to their rating on the live busbar. This is important because it would be dangerous, for instance, to fit a 30amp base and fuse carrier to a 5amp circuit. The fuse bases and carriers are sold together, so when you buy your new equipment you must know exactly what your circuit ratings will be. The bases are manufactured to accept only a carrier of their own rating (or sometimes smaller) — you can never fit a 15amp fuse carrier into a 5amp base and so on. The colour coding of bases and carriers helps to eliminate mistakes and aids quick identification when you are changing a fuse wire or a cartridge if a circuit blows. When the bases are installed, replace the terminal cover if there is one and insert the fuse carriers. Finally replace the fuse cover (having taken care to identify the circuits with adhesive labels inside the cover) and wait for the electricity man to call to reconnect you. It is unlikely he will test your work unless new circuits have been added at the same time as you installed the new consumer unit: but the decision is his and you must be prepared for him to check out the work.
Earth sleeving Until recently sleeving on earth wires was always green in colour; this has now been standardized to green/yellow, although you are still likely to come across the old sleeving, particularly in older houses.