We have an Whirlpool 50 gal 240v water heater (4500watt elements) that was installed by a plumbing company in 2016. We had zero problems with it until last week when it started tripping the breaker.

After flushing and inspecting the tank (it looks good) I replaced both heating elements and thermostats.

The breaker still continued to trip.

I tested the wire for continuity and visually inspected what I could reach including in the attic, it seems fine. But the breaker for the water heater is only 15amps, which I thought seemed low for a water heater this size. So, not wanting to change anything I replaced the breaker with an identical new 15amp breaker.

The breaker still continued to trip. Not random, not immediately, but after the heater had been operating to reheat water, like after a shower.

Some online research leads me to think the correct breaker should be 25amps (stated in manf install guide for 240v 4500w). So for a test I swapped the 15amp breaker with a 20amp I had on hand (this is in a 100amp sub panel that can support this increase) and ran a temporary 10ga wire (in wall wire is 12ga) to the heater.

The breaker has not tripped since. It does not get warm, nor does the wire. The water heater is working fine.

After investigating all possibilities it seems the most likely issue is an undersized breaker. So my question is:

Is there another possibility at this point?

Could it be we were somehow skating by all this time on an undersized breaker?

Is it possible the old elements somehow stayed under 15amp but then failed causing the breaker issue and the new elements pulled just enough more to continue tripping the undersized breaker?

  • Were any changes made in the service panel or to the wiring prior to it starting to fail? It should have consistently tripped a 15A breaker. In fact, a 15A 240V breaker is a "black swan", hardly ever seen, so I suspect it was actually on a 120V breaker. Was the breaker single or double width? May 28, 2021 at 4:40
  • 1
    It is a double and nothing changed prior to this issue.
    – user135671
    May 28, 2021 at 11:48
  • Does the WH have two heating elements each 4500 W, each on a separate breaker (originally 15 A, now 20 A), and each originally on #12 copper wire in the wall? May 28, 2021 at 17:44
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    There are two 4500w elements. There is one double breaker and one 12ga wire.
    – user135671
    May 28, 2021 at 20:40
  • If there is one 20 A breaker and one #12 cable, then it must be the case that both 4500 W elements are not heating at the same time. How does this work? May 29, 2021 at 3:23

3 Answers 3


TL;DR 25A is the right size

First of all, your hunch is correct. 4500W/240V = 18.75A. That should be on a 25A breaker because it is a continuous load - if you pulled water out at a rate that the water heater could not keep up with (or barely keep up with) then the elements would stay on "forever".

You also did the right thing changing the wire. 12 AWG is fine for 20A, but since you really need 25A here, 10 AWG is the right size. It is a good thing you did not have 14 AWG previously (to match a standard 15A circuit) because that would have been undersized considering what was actually happening - an 18.75A load.

So why didn't it trip? Breakers are not some magic device that trips at exactly the stated max. current. In fact, you wouldn't want that because an appliance that needs a little extra power at startup would frequently trip the breaker. So a 15A breaker might not trip on an 18.75A load for an hour. Another 15A breaker might trip on that same load in 1/2 hour due to manufacturing differences. But any 15A breaker will trip very quickly if it gets a 50A load. All of this is factored into the electrical code. For example, a 12 AWG wire is rated for 20A, but in reality it can carry much more for short periods of time, so a 20A circuit with 25A for a little while isn't going to suddenly burn up the 12 AWG wire.

My guess is that your breakers were just barely tolerant of the 18.75A current flow but gradually became more and sensitive and tripped sooner. Then you replaced it with another 15A breaker, which was probably closer to the 15A specification than the original and it (correctly) would trip after a little while but not immediately. Then you replace with a 20A breaker which can handled 18.75A "forever" but which is still undersized (but would be correct for 12 AWG wire) and 10 AWG wire (which is the right size).

So put in the 25A breaker and permanently switch to 10 AWG wire and you're all set.

I would check anything else done by that plumbing company for safe & correct installation. While a plumber is not an electrician, the crew putting in a water heater should know what to do and what size breakers & wire to use. If the 15A breaker and 12 AWG wire were from the previous water heater, the installers should have told you it was not correct and either swapped it at the time or stopped work until you got an electrician to switch to 25A/10 AWG. But they likely didn't because they wanted to get in, get out, get paid. So I'd worry about other "shortcuts".

  • Thanks for the confirmation. It does bother me the plumber never called this issue out.
    – user135671
    May 28, 2021 at 11:51
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    A question for manaseehkatz.... why a 25 amp breaker? Common (and approved) practice where I live (Western USA), is 10ga wire and 30 amp breaker. The breaker is to protect the installed wiring, not the device. Not challenging your answer, just curious. May 28, 2021 at 14:21
  • I agree that in general a 30A breaker would be fine with 10 AWG wire. But breakers are also to protect the appliances. If the manual indeed says "stated in manf install guide for 240v 4500w" then 25A it is. Just like there are situations allowed by code in the other direction - a larger breaker for certain types of motors, etc. than would normally be allowed for a particular wire size, because the manufacturer (and UL, etc.) have determined that the way the appliance works requires the larger breaker but that the timing of how long the higher current is used doesn't require larger wire. May 28, 2021 at 14:24
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    The Whirlpool manuals I can find online don't give breaker specs. Whirlpool discontinued the product line so no support online. My hunch is they never made them - there is a lot of cross-labeling in the basic appliance business. I found Rheem specs in the installation manual here and page 8 shows for a 4500W heater @ 240V the recommended breaker 25A. Going up on wire size is totally safe. Going up on BREAKER size contrary to manufacturer instructions is against code. May 28, 2021 at 14:35
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    @GeorgeAnderson I don't understand why they allow 30A for 3500w, but see WAC 296-46B-422 010 Water Heater Circuit "....Overcurrent protection must comply with NEC 422.11(E)". So for 4500w in WA round up to next largest size over 150%, 4500/240=18.75, 18.75*1.5= 28.125, round up to 30A May 28, 2021 at 15:49

I bet it was a lineside power "issue".

I.E. the power company changed your voltage on you, due to system upgrades. US electrical power has a surprisingly wide delivery tolerance.

Water heater elements are resistors. So your resistor is (assuming it's rated for 250V)...

 W = V I   (Watt's Law) 
 4500W  = 250V x I 
 4500/250 = I
 18 amperes = I  (current) 

 V = I R (Ohm's Law) 
 250V = 18A * R 
 250/18 = R 
 13.888 ohms = R  (resistance)

That suggests its resistance is 13.888 ohms. That won't change.

However, that doesn't mean your voltage was really 125V. Let's say you're out at the end of a distribution and getting 110/220V. How does that change current?

 V = I R 
 220V = I * 13.888 ohms
 15.84 amps = I 

Well, heck, that's within the margin-of-error of a 15A breaker. UL only requires 135%.

But then, the power company upgrades their infrastructure and you are getting 123/246V:

 V = I R 
 246 volts = I * 13.888 ohms
 17.7 amps = I

Now your heater is drawing 17.7 amps instead of 15.8. That pushed the breaker over the edge.

Your wire needs to be good for 125% anyway.

So #12 wire should never have been installed. 18A is expected, and you're required to derate that by 125%, so that means the wire needs to be good for 22.5 amps. 12 gauge wire is not, so it needs to be #10.

Generally for dedicated circuits, manufacturers are better off aiming for 12A, 16A or 24A - which when derated 125%, match nicely with standard wire sizes of 15, 20 and 30 amps.

  • Interesting. I don't know if the power company has made any upgrades or changes recently. Could certainly be as possible as manassehkatz-Moving 2 Codidact's answer. But seeing as I've observed zero other issues with anything electrical I think I'm leaning toward his answer being the most likely. Thank you for your insight.
    – user135671
    May 28, 2021 at 20:51
  • "zero other issues" - Actually, in this scenario (bump from 110V to 123V), you might not see any other issues. Clothes would dry a little faster (assuming you have a typical electric dryer). But not much else would change if everything else was installed properly because everything is designed to handle the full range 110V to 125V. And much (but not all) of modern consumer electronics is designed to handle 100V to 240V in order to work worldwide with just a change of the actual plug. Incandescent lights would be a little brighter, if you still have any, but not obvious except side-by-side. May 28, 2021 at 21:18
  • Good point. I just went and checked and the outlets are at 123v. It's been a minute but I seem to recall things being around 117 in the past.
    – user135671
    May 28, 2021 at 21:34

You have a lot of tolerance issues you could have been dealing with. Voltage could be a little low and element resistance of the element could be a little high. If you change 5% each way (which isn't ideal but within tolerances) into an Ohms Law Calculator you end up with 17A. 113% is pretty much right in the middle of the "unknown" trip status of an Inverse Time circuit breaker on the time scale, which means it could hold forever without tripping. Scroll down this page for chart.

Push the tolerances 5% the other directions you're over 20A, around 140% of nominal breaker rating, which should trip most inverse time circuit breakers in the 10 minute or less range, which is part of what justifies the requirement to over size wire and circuit breakers.

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