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I want to install a tankless water heater. Have a 125 amp panel, with two 30 amp breakers for the current water heater. Install manual recommends having two 40 amp breakers. Max draw is 75 amps. Can I just replace the two 30 amp breakers with two 40 amp breakers?

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    Does the breaker for your water heater take up two "slots"? A 240V breaker for a stove or water heater looks like two single breakers with their handles tied together. The tankless will need two 240V breakers taking up 4 slots. – JPhi1618 Nov 9 '15 at 14:49
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Not likely will it be possible to replace the two 30A breakers with 40A breakers.

The wire size (AWG) for a 30A circuit should be 10 gauge. Proper wire size for a 40A circuit is 8 gauge if the run from the service entrance to the tankless heater is a reasonably short distance.

So do not simply replace the breakers like you asked. The breaker's primary function is to protect the wiring from over current so as to prevent a fire in overload or short circuit conditions.

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