Can I use existing 30 amp wiring that was serving a traditional water heater for an on-demand WH that requires 40 amps?

Question

My tankless water heater requires 2x40 amp breaker but the lines that run to my existing hot water heater are 2x30 AMP breakers If I hook it up will it work

Answer 1

No, of course not.

You need new wiring and new breakers. Or you need to return the new water heater for one that fits your available power supply.

See NEC Article 220 load-calculation for entire house about that load calculation. Then you'll know if you can add and swap breakers and wire and go, or if your new water heater requires a service upgrade from your power company, which may be enough expense to make you reconsider the very minor "savings" that are oversold on those.

Regarding the overhyped standby loss, data here: https://diy.stackexchange.com/a/244644/18078

Answer 2

"Dual breakers" don't mean what you think.

That 30A breaker you see powering your tanked water heater... is actually a single 240V breaker. It has 2 handles because of the way North American (and older Philippine) power works. So it supplies 30A @ 240V.

Doing tankless on-demand heating requires a stupendous amount of electricity. Remember that point, we'll come back to it twice.

As such, the heater requires much more power than your previous heater. So that thing you called a dual breaker there? Your new heater requires two of THOSE. And your heater is pretty small by tankless standards - possibly too small.

The #2 mistake in tankless is going too small

The tankless heater's ability to raise temperature is controlled by the flow through it. The slower the water flows, the more time the heater has to cram heat into the water. Draw too fast, and the heater doesn't have enough time to make the water as warm as you like, and it arrives at the faucet tepid.

What happens next isn't pretty. There's a family meeting, and the rest of the family tells the installer "OK, you tried that stupid thing, it doesn't work, now get rid of it and get a real water heater". Nobody wants to hear any science or coping skills or common sense.

How to solve this? a) enlarge the heater to be sufficient for the flow demanded; which requires enormous amounts of electricity. b) reduce flow by installing flow restrictors on spigots and/or low-flow shower heads. But you'll need to do this before the family experiences tepid-water problems; afterward will be too late.

Can your house handle all this?

Your old water heater was 4500W (derating to 5625W, or about 23.5 amps) and your new heater will be 80A. This is 57 amps more.

Does your electric service really have spare room for 57 amps of additional load? I super doubt it. Builders don't generally leave you that much spare capacity.

I realize many people just slap more stuff onto their panel until something blows up, then get an electrician and spend thousands for a service upgrade at that point... however that can start a fire, and it's cheaper/easier to do a NEC Article 220 Load Calculation right from the start. That, compared to your service size, will tell you what your house can handle. Water heater decisions can flow from that.