National Electrical Code Articles and Information

1. It applies to boilers for operation at 600V (nominal) or less, in which heat is generated by the passage of current between electrodes through the liquid being heated [424.80].

2. This is a trick question. In all cases, the boiler must be suitable for the installation [424.81].

3. The branch circuit conductors and OCPDs must be sized at least 125% of the load served [424.82].

4. They can be sized at 100% if you meet the three conditions of 424.82(1), (2), and (3).
   
5. Overtemperature control, if in normal operation there is no change in the state of the heat transfer medium [424.83]; Overpressure control, if in normal operation there is a change in the state of the heat transfer medium [424.84]. Follow the logic, here, and it makes sense that you'd have these two controls. If you're not making steam, then overtemperature is a concern. If you are making steam, then overpressure is a concern.