New electric water heater (with tank) is tripping the circuit breaker--sometimes

[MarkW]

Greetings. I installed a new AO Smith 50 gal water heater at my friend's house. All went ell for the first day, then he called to tell me that his circuit breaker for the water heater (at the main panel) had tripped. It is a two-element 240V water heater, 4500W elements and the circuit breakers are twin 30A.
Trying to troubleshoot things, here's what I found:
-- With hot water (approx 120F) in the tank:
--- When I set upper and lower thermostats to full "LOW" (so neither should be calling for heat): Top element shows 120V across the contacts,. Bottom element shows 0 V.
--- When I then turn the top thermostat to about 135F (bottom thermostat still "low") , I hear the top thermostat click on: Top element shows 240V across the terminals, the bottom element shows 0V
--- When I then turn the top thermostat to "LOW" and the bottom thermostat to
about 135F (which should cause the top element to turn off and all the power to go to the lower element): The top element shows 120V, the bottom element shows 240V.

The circuit breaker didn't trip while I was doing this.

So, my questions:
1) As I understand it, I should never be seeing power to both of these elements at the same time. Is that right? So, what would cause the top element to sometimes have 120V sometimes and sometimes have 240V, but never be entirely off?
2) How can I troubleshoot this?
3) Why didn't the circuit breaker trip when I went through the various thermostat cycles (but it does eventually trip at some point)?

My present theory: Maybe the top element is grounded (somehow) and receiving power from one leg all the time (why?). When the top thermostat calls for power, the top element gets the proper 240V across the opposing phases. When the top thermostat closes, the element is still receiving power from one leg and it is completing the circuit through the ground (so, 120V)


Any tips or advice would be appreciated. Thanks
Mark

 

[WorthFlorida]

For 240 volt connections, disconnecting one leg is normal. Are you measuring one leg to ground or across the element?
Number one error DIYer' s do is turning on the power to the WH before the upper element is fully submerged in water. Just takes seconds to damage the element. Possible there is a "floating short" in the upper element.
If you have a loop volt meter check the current, each element draws about 18 amps.

Or,
Replace the circuit breaker. Internally they can go bad and after years of never being switched off and in a damp basement, some corrosion internally is possible.

 

[Sponsor]

 

[Fitter30]

Harbor Freight and others have a clamp on amp meter for under $20. Good diagnostic tool.

6 Function Mini Clamp Meter

Amazing deals on this 6 Function Mini Digital Multimeter at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

 

[John Gayewski]

The elements can run at the same time it's not a normal operation for it, but a deep draw will have both elements running to try to keep up. It's been a long time since I've looked at the wiring for these, but there are different wiring diagrams for them for different operations. It really depends how your is meant to run.

My guess is an element is bad. But also check the ladder diagram.

 

[WorthFlorida]

Residential water heaters only one element operates and requires 30 amp, 10 gauge wire. Two elements at the same time will draw 36 amps.
Commercial water heaters are built for multi element operation when they are three phase.

Light-Duty Commercial Electric Water Heaters | Light-Duty Commercial Electric Water Heaters | Lochinvar

 

[MarkW]

For 240 volt connections, disconnecting one leg is normal. Are you measuring one leg to ground or across the element?

I was measuring across the terminals, wires still connected to the element.
Thanks. I looked at it again today and yes, only one leg is switched. Which works fine, unless an element has shorted to ground. In which case, some part of the element will be turned on (though just using 120V) all the time regardless of what the thermostat is commanding. I guess that's one reason there's a high-temp cutoff and, if all else fails, a TPR valve.

Number one error DIYer' s do is turning on the power to the WH before the upper element is fully submerged in water. Just takes seconds to damage the element.

"Winner, winner, chicken dinner!" Yep, when I got there today and put a meter on the disconnected top element it showed no continuity across the terminals and one terminal was shorted to ground. I removed the element and it was clearly burned up. When I installed the tank I thought I was being careful to avoid being "that guy" who turns on the power before the elements are both in water, but obviously I screwed up. Today, after replacing the element, I waited longer and double checked to see that water would come from the (high) TPR valve before I turned the power on. Everything works fine.

Thanks very much for the reply and suggestions.

Harbor Freight and others have a clamp on amp meter for under $20. Good diagnostic tool.

6 Function Mini Clamp Meter

Amazing deals on this 6 Function Mini Digital Multimeter at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Thanks for the suggestion. I've got that very model, and it works fine. If I earned my living doing this I'd surely invest in Fluke meters and "the good stuff", but sometimes the HF models are okay.

Mark

 

[Fitter30]

Ohm meters only put out 4 vdc and under to measure resistance varying on scale used. When checking for a ground a ohm meter might not find one because voltage is so low unless it's very good ground. A megger that puts out 500-1000 volts to measure resistance will find it.