Garden Lighting Help sheet  No. 3: - Low voltage cabling layout

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. Our low voltage cable is a double insulated rubber type with a tough outer sheath impervious to moisture and salts in the soil to suit most garden lighting applications. 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 (T9951 Box 100) are used for neat clipping of cable over pergolas and along deck joists. We use 2 sizes of ground burial low voltage cable as standard; 2.5mm² (T9912 - 50m; T9913 - 25m) 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. 1.5mm² (T9918 - 50m) ground burial low voltage cable is used for low power applications such as led (L.E.D. - light emitting diode) deck lights or recessed lights and direct cabling to one or two 20w halogen garden lights over short distances, typically steplights or pergola downlights. 

What length of cable can I use from the transformer to a garden light?

“Cable voltage drop” occurs in all electric cables, but in a 12 volt garden lighting system 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 right). 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 below give 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.

Can I run longer cables to L.E.D. lights (light emitting diodes)?

Yes. L.E.D.’s consume a lot less power than incandescent lamps and are generally less sensitive to small voltage reductions, so cable runs can also be a lot longer. The table below gives suggested maximum cable runs. Note that L.E.D.’s are vulnerable to overvoltage, so unless you are using an L.E.D. garden light like Omnyx, which is made especially to work from a wide range of both AC and DC voltages, you should generally use a regulated 12 volt DC power supply to ensure stable voltage to your L.E.D. deck lights, recessed uplights or steplights

Tips on transformer location and low voltage cabling

1. Keep cable runs from transformers to garden light fittings as short as possible to ensure optimum performance

2. Locate a transformer centrally amongst a group of garden lights it is to power; e.g. a radius of 8 metres for 50w lamps 
3. To run 2 sets of garden lights operated by separate switches, you must provide 2 separate transformers & circuits. 
4. If the distance exceeds the maximum cable length recommended for a single garden light, revise the transformer layout, increasing the number of garden lighting transformers to reduce the low voltage cable runs, or use thicker cable.
5. If the run for multiple lights connected to one cable exceeds the maximum, divide the garden lights onto individual cables
6. In gardens with garden lighting or irrigation, garden with a fork rather than a spade until you are sure there are no garden lighting cables or irrigation pipes where you want to plant your new shrub!