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An increasing number of electrical appliances are designed to be powered at 12 volts, rather than the usual 230 volts. Normally such appliances will be provided with their own transformer to generate the correct voltage from the mains.

In general, the transformer needs to be mounted as close as possible to the appliance, because the lower voltage implies a very high current. Therefore, installations of this type are very often no different in installation than mains-powered systems; most of the cabling will be at mains voltage anyway. As an example I will describe the increasingly-common 12-volt halogen track lighting systems.

Track lighting systems are appropriate where a large number of small, intense lamps are required, for example to emphasise a number of paintings hung on walls or to illuminate a large work surface. Most systems allow lamps to be clipped into place or removed, and pointed in the appropriate direction.

Normally the lamps are 12-volt halogen types, and they produce a very bright, white light. To get the 12 volts requires a transformer; the system should be provided with information about how many lamps can be supplied by one transformer. Tracks can often be connected together to extend the system.

The wiring from the transformer to the tracks, and between tracks, will carry a very large current at 12 volts. Thus these cables will be very large and heavy, and need to be kept as short as possible. In addition, the transformer is ugly, and needs to be mounted out of sight, but not in a confined space (because it will overheat) or concealed under floorboards (unless you can get at it for maintenance).

The transformer is mounted on the wall just below the ceiling, and within a few inches of the first track. This keeps the amount of thick, 12-volt cable to a minimum. The transformer could also be mounted in the ceiling void if you have access to it from upstairs. This would put it completely out of sight, but may make it tricky to get to.

The transformer is connected to an existing ceiling junction box; I am assuming we are replacing an existing pendant lampholder here. The lighting cable can be concealed in the wall as normal, and brought to the transformer by a flex outlet. The transformer is probably supplied with a built-in cable, so there has to be some mechanism for connecting it to the installation cable.

Note that while it is possible to conceal the 12-volt cables in plaster as well, we don't have reliable figures for how this affects their current ratings, so this may not be a good idea. In any event, these cables should probably be short enough that you don't need to conceal them.

Modern (indoor) electrical wiring fittings (outlets, lightswitches, etc.) come in two general type: flush mounting and surface mounting. In a flush-mounted fitting, the wiring and connections are below the surface of the wall, and only a small thickness of the fitting protrudes.

This system is ideal when used with fully-concealed wiring. The mounting boxes are usually made of steel, and are very cheap (since no-one will ever see them). These boxes are called patresses.

Surface-mounted fittings are mounted entirely on a wall surface, and may be used with either concealed or surface wiring. Large surface-mount boxes look very ugly, and are best avoided where possible. However, because they don't require any chiselling away of brickwork, they are much easier to fit.

For outdoor systems, fittings are usually designed to join up to standard 20 mm circular truncking. Normally there will be an insulating bush at the junction to keep the system watertight.

You can be an electrician without knowing much about electricity. It seems odd, but it's true. But if you do know the principles, you can do safe and practical work without memorizing a whole heap of regulations, because they're mostly derived from standard principles anyway. The key features of electricity are voltage, current, resistance, power, and frequency.