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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

George Anderson
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Jacob
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  • Imagine you mean a dual 30 amp breaker. A 30 amp breaker must use 10 gauge wire or larger. A 40 amp breaker must use 8 gauge wire or larger. A 30 amp breaker will reduce the power needed at best, probably tripping the breaker or if the breaker does not work right causing a fire. – crip659 Dec 03 '22 at 02:23
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    Odds are the old heater has one two-pole 30A breaker (normal electric tank heater) and the new "instant" one needs two, two-pole 40A breakers. Which your service may not be able to supply, depending on a load calculation that is utterly required before proceeding. – Ecnerwal Dec 03 '22 at 03:45
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    Be very careful here. On demand electric water heaters are incredible energy hogs. Be absolutely sure of the power requirements. like @Ecnerwal said, it may require 2 40amp 240v breakers. Next: No way can you run an on demand HW on a 30 amp circuit. – George Anderson Dec 03 '22 at 03:56
  • Depends on the gauge of the wire. – Hot Licks Dec 03 '22 at 13:37

2 Answers2

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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

Ecnerwal
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    I edited the title of the OP to make it closer to the actual question he was asking. Obviously I completely agree with you Ecerwal. And for the record I think on demand electric water heaters are a disaster. There is nothing wrong with a well insulated tank style WH, no less energy efficient than on demand, UV for your answer. – George Anderson Dec 03 '22 at 03:51
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    I have 400A service and a 25A tank-type water heater. I could run a 120A monstrosity, but it's nuts, IMHO. – Ecnerwal Dec 03 '22 at 03:56
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"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.

Harper - Reinstate Monica
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  • "until something blows up" - it should just trip the main breaker/fuse at that point, right? – 9072997 Dec 04 '22 at 03:11
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    @9072997 Breakers have some tolerance on overcurrent. The problems arise when the overload is marginal (maybe up to five or ten amps) when heat problems arise. At those high currents, the wire terminations are a weak point, and they must be torqued to the proper grip with a calibrated driver. – Paul_Pedant Dec 04 '22 at 11:04
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    @9072997 Hanging it all on a single point of failure is no longer done. It used to be done with split-bus panels, the breakers summed to more than the service size and it was all hung on the Load Calculation. Te minimum standard is Load Calc + main breaker. – Harper - Reinstate Monica Dec 04 '22 at 21:11