How many twenty watt fixtures can run off a 100 watt transformer?

Is it less than five?

What overhead involved?


Provided you don't have ridiculously long runs of wire between the transformer and fixtures, a 100W transformer should be able to run five 20W fixtures OK. Make sure you use the correct Voltage light bulbs in the fixtures.


Are you hooking lights directly to a transformer? If so, why? That's not something the average homeowner should ever need to do! :) @Niall C's is probably as good as can be given without more information about the circumstances. If you are making long runs with the line, the voltage drop associated with long lengths could be an issue. For lights, they'll just be a little dimmer, but motors (or heaters, and some other devices) will draw as much power as they need, which could result in overheating or burning out the equipment.

  • Hey thanks for the info! @Niall C's is good enough, I will up vote you when I can.. – jtzero Nov 13 '11 at 1:33

Sorry but I have not got the rep points to add a comment. Just take the wattage of the 5 lamps and add them together. That is the total watts used. You can try to go to a lower wattage lamp if the wattage adds up too much, but if you do this then later on somebody might decide there is not enough light and put higher wattage lamps in, therefore overloading the transformer and eventually burning it up.

Also, you might want to put a fuse kit on the 12V(?) side. The total of amps on 100W 12V is 8.3A (Wattage divided by the voltage, or 100 / 12 = 8.3A) If 24V then it would be 4.16A. The fuse kit will help you from burning up the transformer if you overload it.

  • The accepted answer already implies the same answer as yours. So there is no need to add more answers unless you really think the chosen one is misleading or needs more elaboration. Welcoem to DIY! I +1 to boost your reps. – Piotr Kula Nov 24 '11 at 14:12
  • I would have just commented with my second paragraph to Niall's comment if I had the ability to add comments, which hopefully will be a couple of days. Thanks for the rep vote and I will try to follow the faq's closer next time. - – lqlarry Nov 25 '11 at 0:01

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