I'm replacing a defective 277 volt, single phase, 3kw water heater with a 240 volt water heater. Is it safe to hook-up?

Several concerns:

1. First is that using an appliance intended for a 240 volt circuit on a 277-volt line is a code violation, assuming the water heater manufacturer's instruction manual specifies power must be 240 volt.

2. If you decide to proceed anyways, the heater element will draw about (277/240)^2 = 1.33 times the power that it's designed for, so internal fuses and/or the heating element may overheat and burn out in short order.

3. 240 appliances expect to be connected with two hot leads (each 120 volt relative to ground and 180 degrees out of phase with each other) plus a ground, plus (sometimes) a neutral - this neutral may be required so that the appliance has 120 volt power for ancillary circuits such as a timer. When connecting to 277 volts the leads are hot, neutral and ground, so there is no provision for a 120 volt circuit. If your heater requires such a connection the 277 volt wiring won't work.

4. If there is an internal transformer in the water heater that is intended to convert 240 volts to 120 volts in order to power electronic controls, applying 277 volts could over-power the electronics.

Bottom line is: I wouldn't recommend it.

Totally agree with ebaines!

You can always check specifics with the product manufacturer... otherwise, just like ebaines, I can't recommend this!

Mark

I think you need an electrician with license to replace that heater. 277 v is usually for commercial or industrial and it is a single hot but 220v is two hot .

"but 220v is two hot". True but, AC power today is 120/240.