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CALCULATING GARDEN LIGHTING 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" as they are called in the professional lighting trade) 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. A transformer size is normally given in VA - volt amps - but this is for all intents and purposes the same thing as watts when rating a transformer.

Examples of how to calculate transformer rating

Type of light # of lights Lamp wattage Total wattage   Type of light # of lights Lamp wattage Total wattage
Recessed uplight 1 50 50   Spike spotlight; 2 50 100
Underwater light 1 35 35   Wall up/downlight 2 40 80
Steplight 2 20 40   Wall spotlight 1 20 20
Wall spotlight 2 20 40   Spreadlight 2 20 40
Total 7   165   Total 7   240
Spare capacity     35   Spare capacity     60
Transformer rating     200   Transformer rating     300

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 surface-mount, in lawn or gravel areas for example, use a ground burial transformer or ground burial transformer kit to power 12 volt recessed uplights or spreadlights

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). 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 if you are using a small number of low power lights such as Navigator Minor, models which are made especially to work from a wide range of both a.c. and d.c. voltages, you should use a regulated 12 volt 10w DC power supply such as the 10w B9403 to ensure stable voltage. If you are using an a.c. transformer for a larger group of led garden lights or if you are using higher power led lights such as the Eye or Maxor models which are designed for ac operation, then ensure the transformer is loaded to at least 80% of it's nominal rating, eg. 8w with a 10w transformer, 16w with a 20w transformer etc

See also:
Garden lighting transformers
Power supplies for led garden lights

Copyright 2013; 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.

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