What size fuse in amps should a hot water heater have? What size fuse in amps should a hot water heater have? SAVE CANCEL. already exists. always turn off the power at the breaker box/fuse panel BEFORE you attempt to do any work AND always use a Hot Water Heater Blowing Fuse - Plumbing - DIY Home Jan 20, 2012I am sure the Fuse box is there because it is not in line of sight of the main panel. I am not sure that you need a fuse or a switch can be used. The new heater may be pulling higher amps on start up and weaken the fuses and then they blow. I have an electrical water heater in the barn up north. Sitting right next to the well pump.