#22 gauge would be fine. One thing, be careful. You used the term, "DC transformer". That's a misnomer. A tranformer puts out AC, a power supply or power pack will put out DC. Atlas switch machines run on AC, usually 12 to 18 volts, and be sure that you do not use a sustained voltage on them, they burn up really quickly when you do. Use either a momentary switch closure or a capacitance discharge unit to provide a quick pulse.