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I am trying to do a simple low voltage lighting design. I intend to use 3 18w incandescent bulbs just to light a small area. I want to know if there is a problem using a 120w transformer for this or should I use something much smaller like 75w? The distance from the transformer to the first light will be about 20ft, and each light is separated by 10ft. As far as I can tell I should use 12gauge wire for this.

Again the question is should I use a 75w transformer or 120w transformer and what is the problem using the 120w if I should not use it?

Thanks,

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You can use either.

54 watts is the load from the bulbs. The transformer must be larger than the load.

In theory you could use any transformer 54 watts or greater. In practice though inefficiencies such as splices and voltage drop mean that exact 1:1 ratio doesn’t work anymore, so upsize 20% to take into account inefficiency. Meaning in this case use any transformer greater than 67 watts.

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All things being equal, the larger transformer is better.

It will run cooler and have more design headroom.

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