Paul Furman wrote:

I had some trouble with setting up a transformer for my low volt garden
lighting. Here's what I learned:

The small (size of a pack of cigarretes) $25 12v transformer is an
ELECTRIC transformer that must be
mounted indoors and is easily damaged by moisture, the large (size of a
shoebox) $100 12v transformer is a MAGNETIC transformer suitable for
mounting outdoors and
more weather resistant.

The reason the small one blew my breaker is because the small wires go
to the 120v
wall plug & the fat wires go to the 12v lighting. The smaller voltage is
more sensitive to voltage drop thus the larger wires. The 120v leads
were really skimpy, less than most electric appliances.

They said the power usage when the transformer is plugged in is trivial
so it's not really necessary to switch the 12v side. The small
electronic type does not have a delayed startup like the large magnetic
one so you could switch the 120v side if you want to save a tiny bit
more energy and don't mind operating everything on one switch.

Having a too-small transformer isn't a big problem, no loss of
efficiency but if you only have say 10 watts of lights on a 150w
transformer it might provide irregular supply however they said a 40w
draw on a 150w transformer was no problem.

Just thought y'all might find this helpful.
Thanks Paul, good to get the imput, never know when it will be needed...