Is it possible to run a water-heater on a undersized breaker for years?

Question

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?

Answer 1

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".

Answer 2

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.

Answer 3

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.