Most modern garden lighting uses a low voltage system comprised of 3 main components: a cable that distributes power to the areas to be lit; a mains to 12 volt transformer; the actual light fittings.
The first consideration though is to decide whether to opt for conventional or LED lighting or both, and to then calculate the size of transformer that will be needed to power all the lights, which is a factor of both the number and power of the light fittings to be installed.
We shall examine the two systems (LED and conventional) more closely in a bit, but for immediate purposes it is only necessary to know that both use the same underlying ideas.
Calculating the size of transformer required is simply a matter of counting up the wattages for all the actual lights (so, 3 x 10 watt deck lights plus 2 x 20 watt standard lanterns comes to 70 watts for example) and tells you what rating of transformer you need to purchase.
The transformer input is connected to a mains outlet and must therefore be placed inside. The transformer output is then connected to the cable which then leads outdoors.
Note that the cable itself adds some load additional to the load caused by the light fixtures. You should make an adjustment for this when sizing your transformer – the cable packaging usually states its load, and obviously the longer the cable the greater the load, which limits how long your cable can be and remain effective.
If your lights seem dimmer than expected and particularly so towards the far end of the cable then the cable is almost certainly over long. This is easily addressed by looping the cable back (if necessary attaching an extra section) and joining it close to the transformer. However, be certain that the two wires inside the cable are not cross connected – if you look carefully you should see a marking on one strand only which is used to help determine the correct polarity.
After you have connected the cable and laid it out then you can attach each low voltage light fixture by simply cutting and re-joining the cable incorporating the cable attached to the fixture.
As noted above, both LED and conventional incandescent garden lighting are now widely available, and it can be confusing to know which to get and how to combine the two different types.
LED lights consume an awful lot less energy and produce negligible heat. LED light is also brighter and more defined in comparison to regular incandescent light, which may be considered either a benefit or a bit hard on the eyes depending on what types of effect you want to obtain.
In general you cannot attach LED light fittings to a normal low voltage garden system, and the reverse also applies. In the first case, the LED light will likely fail within weeks and in the second, the LED transformer (commonly called an LED driver) will quickly self-destruct.
There are some exceptions to this and some LED lights can indeed be connected to a traditional low voltage lighting system, but these will state this clearly on the box. But in general, to incorporate both incandescent and LED low voltage garden lights you should install two cables with one wired up to a standard 12v transformer and the other to a constant voltage 12 volt LED driver, then wire in each type of light fitting to the correct cable.
There are significant advantages to running both kinds of garden lighting. LED light has a distinct quality that makes possible a whole new range of lighting effects and, when combined, both kinds of lighting system can be deployed so as to enhance or contrast each other, creating lighting effects beyond the capabilities of either one alone.