If you use extension cords between transformers and receptacle, use 14-gauge if the receptacle is on a 15-amp breaker at the main box, or 12-gauge if a 20-amp breaker. A GFI-protected receptacle is a good idea.
Pulling 10 amps from your transformer, set at 18 volts, will be pulling under 2 amps from your 120-volt receptacle.