 Calculating transformer ratings
A
frequently asked question is “how many lights can I run from
one transformer”? In fact, 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 lights”? This is simply a matter of multiplying the
wattage and numbers of the lamps used in the luminaries (light
fittings) 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
a luminaire in the future.
-
Multiply the lamp wattages for each type of fitting or lamp
rating as shown in the examples
-
Allow at least 20% spare capacity in finalising the transformer
choice.
-
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
-
Check that the total load is at least two-thirds of the transformer
rating you propose to use; using a big transformer with only
a small load (few lights with low lamp wattages) as this can
result in “overvoltage” which reduces lamp life.
Examples of how to calculate transformer rating
|
Type of luminaire
|
Number of luminaires
|
Lamp wattage
|
Total wattage
|
|
Spike mount spotlight
|
3
|
50
|
150
|
|
Wall mount spotlight
|
2
|
20
|
40
|
|
Spreadlight
|
2
|
20
|
40
|
|
Total
|
7
|
|
230
|
|
Spare capacity
|
|
|
70
|
|
Transformer rating
|
|
|
300
|
|
Type of luminaire
|
Number of luminaires
|
Lamp wattage
|
Total wattage
|
|
Recessed uplight
|
2
|
75
|
150
|
|
Underwater light
|
1
|
35
|
35
|
|
Steplight
|
4
|
20
|
80
|
|
Tree mounted downlight
|
2
|
20
|
40
|
|
Total
|
7
|
|
305
|
|
Spare capacity
|
|
|
95
|
|
Transformer rating
|
|
|
400
|
Tips on transformer location and cabling
-
Keep cables runs from transformers to light fittings as short
as possible to ensure optimum performance
-
Locate a transformer centrally amongst a group of lights
it is to power
-
If you want 2 sets of lights in an area to be operated by
separate switches, you must provide a separate transformer
for each group of lights and a separate mains supply to each
transformer.
-
Hide transformers behind plants or landscape features, under
decking, or in outbuildings and other enclosures; paint them
and junction boxes to blend with their surroundings.
-
Where you can’t hide a transformer, in lawn or gravel
areas for example, consider using in-ground burial transformer
kits
|
