Hello.
I have had a tankless water heater installed in my house now for 2 years. The unit keeps tripping the circuit breaker whenever the hot water is turned on for an extended time (15 minutes).

The unit is rated for 220 volts or 240 volts, however, the sticker on the unit only contains the wattage and amperage for 220 volts, which are 54a and 11.8kw.

My electrician would only install a breaker capable of handling 54 amps (70 amp breaker and AWG6 wire). He says the reason it continues to trip is due to the fact Canada uses 240 volts, which will make the water heater draw more current than the sticker on it indicates. He wouldn't go higher because he needed to cover his butt.

Can I increase my wire size to AWG4 and breaker to 80 amps without burning my house down?

Thanks/Dan

There is no way that should be tripping a breaker at 240v.
Everything residential, with few exceptions, is 240v now in North America. Canada and the US have the same systems.

Your tankless is rated at 54A @ 220v. This is 4.07 ohms.
The same 4.07 ohms at 240v is ~59 amps and 14,138 watts.

True, it should be on an 80a circuit, but 59a should not trip a 70a breaker, ever.

Unless the #6 is in conduit the whole way it is not even rated for 70a, at least as far as the NEC is concerned, but this should not affect the breaker tripping either. Unless of course the circuit run is extremely far like 100'+.

Did anyone ever come out and check the running amperage?

Yes the running current was checked. If I remember correctly, it was 61 or 63 amps. As for the run, it's about 25' of #6 wire that is in a flexible conduit similar to BX cable.