I'm going to kick this over to the electrical wizard But since the plumber both pipes and connects the power to a heater let me describe the process.
The electrician installs a 220 breaker in the box and runs a three wire lead over to where the plans show a water heater will set. We bring in the heater, pipe it and connect the three wires in the recessed panel on top of the heater to the lead from the breaker box using wire nuts. After we finish piping the unit we pressure up the heater to check our work and move on.
If I've missed something We'll let our resident electrical expert take it from there. Regards, Tom