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Most garden lighting works at 12 volts, so you need to locate transformers in the garden and use flexible low voltage cable from the transformer to the garden lights themselves. Cable loses voltage according to its length and the number & power of garden lights you connect to it, so keep low voltage cable runs as short as possible to optimise the performance of your garden lights. The best ground burial low voltage cable to use is a round, double-insulated rubber type with a tough outer sheath impervious to moisture and salts in the soil to suit most garden lighting applications. The round shape seals well through cable glands or membranes into junction boxes to avoid moisture getting in. The cable is usually run under bark or gravel mulch to aid flexibility in locating garden lights, but we recommend the same cable for use under decking and over pergolas to resist condensation as well as rain. Protective conduit may be used where cable might be vulnerable, for example where it crosses under a grass path where lawn edging during maintenance might damage the cable. Black clips (T9950 Box 50) are used for neat clipping of cable over pergolas and along deck joists. 3 sizes of ground burial low voltage cable as standard; 2.5mm2 (T9912 - 50m pack; T9913 - 25m pack) is used for most spike mount spotlights, recessed uplights and other ground mounted halogen garden light fittings. Wire 50w and 75w garden lights direct from the transformer position to reduce cable voltage drop to a minimum. Thinner 1.5mm2 (T9918 - 50m pack, T9919 – 25m pack) ground burial low voltage cable is used for lower power applications such direct cabling to one or two 20w halogen garden lights over short distances, typically steplights or pergola downlights. The thicker 4mm2 cable is used for longer cable runs or cabling to higher power lights or groups of lights – typically for wiring 75w spike spotlights further than 5 metres from a transformer. "mm2" refers to the cross-sectional area (the thickness) of the copper conductor in the cable, not the overall diameter of the rubber sheath.
"Cable voltage drop" occurs in all electric cables, but in a 12 volt garden lighting system special care must be taken to limit this to avoid loss of light output resulting from voltage reduction at the lamp. Some manufacturers recommend a maximum of 1 volt drop, but this would result in a loss of 30% of the rated light output of a halogen lamp - a 50 watt lamp would only give 35 watts worth of light (see graph). The loss of voltage is higher the longer the cable run and the higher the lamp wattage. To limit this loss of light output, keep cable voltage drop to under 5% (0.6 volt in a 12 volt system) by sticking to shorter cable runs wherever possible.
The table gives the maximum length of cable run for typical garden lighting wiring requirements for halogen lamps - stick to these cable lengths to get full brightness from your outdoor lighting system.
How long is a piece of string?!!!!! A lot will depend on the combination of lights and cables you are using. For most installations 2.5mm2 cable is the general purpose choice (T9912 - 50m pack; T9913 - 25m pack) and allowing 8 metres of cable per light is a good rule of thumb for the amount you will need unless there are special circumstances. Always allow 1m loop of slack cable for spike spotlights curled up by the transformer to allow for flexibility in positioning. Allow the same when wiring moonlighting downlights or spotlights in trees to allow for growth.
Copyright 2010; Information based on products supplied by Lighting for Gardens Limited and is advisory only. The company accepts no responsibility for incorrect use or application of information given. Light fittings can become hot in use; exercise appropriate precautions. Exterior electrical installations should be undertaken by a qualified electrician.