How many twenty watt fixtures can run off a 100 watt transformer?
Is it less than five?
What overhead involved?
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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.
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.