I just came across this thread and happen to work at STC.
All STC valve require 18 gauge wire. The current draw depends on the valve, and as a result, the coil you are using. We have 12VDC, 24VDC, 24VAC, 120VAC, and 220VAC coils for all of our valves.
If you are still interested in more information, let me know what valve and coil you are using and I'd be happy to help.