Garden Lighting Help sheet  No. 2: - Calculating transformer size

A frequently asked question is “how many garden lights can I run from one transformer”? In fact, garden lighting transformers are available in a wide range of wattage ratings, so we need to look at the question in reverse: “what size of transformer do I need to power a group of garden lights”? This is simply a matter of multiplying the wattage and numbers of the lamps (bulbs) used in the garden light fittings (or “luminaire”) to be connected to the transformer, as shown in the examples below. In most cases we need to consider some spare capacity within the transformer rating so that we have scope to increase some lamp wattages as plants grow, or to allow for some flexibility in adding an extra spotlight or other garden light in future. This helps the garden lighting to “grow” with the garden.

1. Multiply the lamp wattages for each type of garden light or lamp rating as shown in the examples below
2. Allow at least 20% spare capacity in finalising the transformer choice.
3. Transformer ratings are available in 50 or 100 watt steps, so it is merely a question of choosing the next step up from the total wattage you have calculated
4. Check that the total load is at least two-thirds of the transformer rating you propose to use; avoid using a big transformer with only a small load (one or a few garden lights with low lamp wattages) as this can result in overvoltage”, which reduces lamp life and increases maintenance of your garden lighting system.

Examples of how to calculate transformer rating

Tips on transformer location and cabling

1. Keep cable runs from transformers to garden lights as short as possible to ensure optimum performance
2. Locate a transformer centrally amongst a group of garden lights it is to power
3. If you want 2 sets of garden lights in an area to be operated by separate switches, you must provide a separate transformer for each group of garden lights and a separate mains supply to each transformer. 
4. Hide garden lighting transformers behind plants or landscape features, under decking, or in outbuildings
5. If you can’t hide a transformer, in lawn or gravel areas for example, use ground burial transformer kits to convert 12v recessed uplights or spreadlights, or groups of low voltage garden lights to 240v operation 

What is the difference between a  transformer and a power supply ?

A transformer provides alternating current (a.c.) output of 12 or 24 volts; 12 volts a.c. is normally used to power halogen lamps in low voltage garden lights. A dc power supply provides direct current (d.c.) output (same as a battery) and is normally required for light emitting diodes (l.e.d.’s), in l.e.d. deck lights for example. L.e.d.’s consume a lot less power than incandescent lamps and are generally less sensitive to small voltage reductions but note that l.e.d.’s are vulnerable to overvoltage, so unless you are using an l.e.d. light like Omnyx, which is made especially to work from a wide range of both a.c. and d.c. voltages, you should generally use a regulated 12 volt DC power supply to ensure stable voltage.

NB: See also     Help sheet No. 1: Exterior & Ground burial transformers

Help sheet No. 3: Low voltage cabling layout