Most electric hot water heaters are designed for 240 VAC. The de facto standard size heating element is 4,500 watts, which corresponds to 18.8 amps and 12.8 ohms. Although most electric hot water heaters have both an upper and lower heating element, they are usually wired for non-simultaneous operation. That is, only one of two heating elements is "ON" at any given time (see link below). This gives the best performance for the money. According to the NEC, the minimum size wire required for this load is AWG #10 (American Wire Gage) with either a 25 or 30 amp circuit breaker.
I have yet to see an electric hot water heater cause a fire. The heating elements are surrounded by water. The heating element will open in a mater of seconds if it is not cooled by water. There is an adjustable thermostat for each heating element (usual range: 120 °F to 160 °F). The controlling upper thermostat contains an Energy Cut OFF (ECO) that activates at 180 °F, and it must be manually reset. In addition, all electric water heaters must contain a temperature/pressure relief valve that activates at 210 °F and/or 150 psi.
GSW - Johnwood Technical Brief: Sequence of Operation for Dual Element Electric Water Heaters October 4, 2005, TB: 105
U.L. Standard 174 applies to electric water heaters not to exceed 120 gallons or kilowatt input not to exceed 12 KW.
ANSI Z21.22 Standard applies to Temperature and Pressure Relief Valves.
For Maximum Resolution, Click on the Body of the Picture.