My Malibu transformer is sold as 500W unit which is referring to the 12V
output. The label on my transformer says INPUT: 3.5A max @ 120VAC which
would calculate to be 420 Watts.
The OUTPUT is labeled: 500W total @ 12V, BUT it is really two 250W outputs
or about 20.8A each output.
If the OP has 3 equal outputs of 200 Watts each then that would be 16.6
amps
each which would be reasonable for the 12ga low voltage wire they sell.
Kevin

Thats a good one, good for more watts out than the input wattage. The
trasnformer could not be more than about 90% efficiant so 380 watts should
be about the maximum output of the transfromer.

The 120 volt circuit should be good to power atleast 2 of the transformers
if that is the only load on them. If there is some other load but not much
he should still be ok with it.