Installing your products
First and foremost all new external electrical works must be installed by a PART
P registered electrical contractor. The contractor must supply you with full certification
once the work is completed. For a mains 240v supply they should use SWA ( steel wired armour
) cable. If a 12v low voltage system is being installed then flexible
round double insulated cable can be used (
NOT PVC CABLE) Each new garden supply should also be on its own RCD.
Where alarms and CCTV installations are concerned it is more efficient to have a dedicated RCBO protecting each individual circuit, especially where mains voltage exterior lighting is concerned. Cable should be buried to a safe depth to avoid any unnecessary risk due to garden forks or spades.
Equally as important and probably the weakest link in any exterior cable run is the joining of cables. Each joint will need to be 100% water tight and this can be achieved by the use of Magic Gel poured into exterior weather-tight junction boxes once the termination is complete or you could use heat shrink kits. See FIG 1 and FIG 2 below. Burial resin mixes are also available to use in weatherproof junction boxes and this product sets to a very hard resin and not ideal if you need to get to a joint later on, but ideal as a permanent water tight fix.Where possible try and keep all low voltage transformers and joint boxes above ground, this will reduce the risk of water ingress.
FIG 1 Magic Gel
FIG 2 Heat shrink
Mains 240v or 12v ?
Another thing in determining the type of light fitting you can use is the type of installation you are going to have in the garden ie mains 240v or 12v. There are pro's and cons to both system which we will explain below.
Mains voltage systems are in some ways easy to use as most electricians are already familiar with using 240v and probably less so with a 12v lighting system. The obvious advantage to a mains supply is there is no need for a transformer to step the load from 240v down to 12v. There is also next to no restriction on the lamp wattage that you can use in a mains 240v supply. You also have a much wider choice of lamp source including, fluorescent and metal halide lamps.
However one of the main disadvantages is that nearly all the 240 volt cables in the garden must be SWA ( steel wired armoured ) or have some other suitable protection against accidental damage like buried conduit. Only the last 2 metres to the light can be a flexible cable. This generally means that more junction boxes are required.
Another big disadvantage is that 240 volt
lamps blowing will often trip out a circuit breaker, whereas this rarely happens
with 12 volt lamps as the current surge is absorbed by the transformer.
Installing mains 240v light fittings
The low voltage system is much safer to use in a garden environment and has a more flexible cable system. Should you accidental hit a 12v cable with a spade or fork the worst that will happen is the lighting system will trip out. A little bit of dampness might not affect a 12volt circuit but it could render a 240 volt circuit unusable. The low voltage lamps tend to be brighter than an equivalent mains halogen lamp and there is also a wider selection of beam angles.
One disadvantage is that you have to decide on what total wattage lamps you need before choosing the correct transformer as you can't under load or over load a transformer as this will stop it operating at it's maximum potential. See FIG 1 below on how to load a 12v transformer.
Example of transformer loadings
Installing Low Voltage Light fittings
A transformer will be required when installing low voltage light fittings. There are also some LED light fittings that require a transformer and the following rule applies to both. To work out the size of the transformer required to run the light fittings you take the lamp wattage and times it by the amount being used. For example you have 4 x 5w LED ground spikes and 5 x 3.5w LED buried lights, 4 x 5 = 20 and 5 x 35 = 17.5 now add the 17.5 to the 20 and you have a total of 37.5 watts so the transformer required would be a 40va. You should always allow about 10% extra on the transformer for start up current. Using low voltage cable is both cheaper and much safer in the garden than a mains 240v supply. Try not to put any more than 12 lights on any one run of cable to avoid voltage drop and possible loading issues. Every cable joint should be made 100% watertight by using a heat shrink kit, IP rated junction boxes and also a resin sealing compound. Please also ensure that all ground mounted light fittings have adequate drainage channels beneath them especially in clay based soil beds.
Can you mix 12v and mains 240v ?
It is possible to use both 240 volts and 12 volts together in the garden – the only
difference is that instead of just a junction box there is a transformer there as
well. 240 volt GU10 LED lamps are generally not as bright as the 12volt LED MR16 lamps but they have
a warmer quality, which might suit some situations better than the crisp look of
low voltage lighting. Conversely you might need the low voltage crispness to bring
out the best in foliage.
With the advent of technology LED lamps are now used extensively in the garden environment due to their low running cost, cool to touch, and high output. LED lamps can be used in both the mains 240v supply and a 12v supply system. Voltage drop is less of an issue with LED lighting and on a 12v system a transformer can be placed nearly 30 mtrs away from the light fittings which gives the installer greater flexibility.
The other type of LED system is the dedicated LED light fittings which has to run off a transformer and be wired in a very specific manner.
1. In series
2. In parallel
Wiring either the wrong way round could blow all the lights so care must be taken and all instruction should be followed to the letter, please see example FIG 2 below.