Can I use a 50 amp circuit breaker with a tankless water heater that requires 40 amps?

Yeah but its not recommended it says 40 for a reason if you do a 50 thats 10 amps over and would require 10 exyra amps to trip it and you would damage the device first

The instructions that come with it are part of the UL testing. If it says you need a 40 amp breaker then the UL and code requirement is a 40 amp breaker, Increasing the breaker 25% would result in a 50% overload being seen as a 16% overload that may take 5 minutes to trip and still be within NEMA, NEC and UL specs. A failure producing a 25% excessive current draw would never trip.

Just turn off the main and replace the $9 breaker! http://www.amazon.com/gp/product/B00068ZILM/ref=as...

Yes that could be done and it will work. The 9kw unit is rated for 37.5 amps at 240 volts but at 220 volts it will draw over 40 and changing the breaker to a 40 may cause nuisance trips. Use 45 amps if available.

The maximum draw should be less than 40a so that would get the job done, but as the other guys are saying, there may be too much overhead capacity to protect the circuit correctly.

Where did you find a tankless electric heater that only pulls 40a? It must be very small and intended to serve only one sink.

Sure you could use it provided you are running 6 gage wire on that circuit, but I suggest changing it out as soon as possible.

The purpose of the breaker is to protect the wire and connections, but also to protect the appliance.