It is NEVER safe to run a 120 V rated element on 240V, but it is always safe to plug a 240V rated element into 120 V power, because its output is reduced to a quarter of its standard rating. So, if both 120 V and 240V are available, then you can easily achieve two levels of control by operating ONE 240V element at two different voltages. Please remember that - use only 240V rated heater elements with dual voltage control, and only one at a time!
In North America, where both voltages are available, they have differently designed sockets so that devices cannot be accidentally plugged into the wrong voltage. In order to use both voltages for heater element control, you will have to build a short "jumper" cord that has a plug for one voltage on one end and the socket for the other voltage on the other. This cord must be made of heavy-duty materials and wire rated for 20 amperes. This kind of wire can be found at larger hardware and electrical supply stores. Connect a 240V plug to the element and make the jumper with a 240V receptacle for this, and a 120V plug at the other end. Use this jumper cord when using 120 V power, and a standard 240V extension cord when using 240V. This will prevent any 120V appliance accidentally being plugged into 240V.
Most 240V power supplies are rated at either 15 or 20 amp, though larger circuits exist for stoves and clothes dryers. Each circuit capacity has its own design of plug and socket. You will have to determine the size of your circuit and buy the appropriate plug. Two common and useful sizes of 240V water heater elements are 3800W (15.8 amp) and 4500W (18.75amp), both of which require at least a 20-amp circuit.
3500W (14.6 amp) and smaller elements, all of which draw less current, are also available.
Using a 3800W element as an example, you will have 3800W available for rapid heat-up, and when the element is plugged into the 120V supply, the power will drop to 950W for reflux operation.
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