Why is this Breaker Tripping

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Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
I went to a call that was for the breaker to a water heater that was tripping. Little background.....

This is a mobile home and of course the service panel is outside.
In the house panel the bus was burnt where the WH breaker was so a small disconnect was mounted on the outside main panel. Now this was done by friend of HO and not at all correct. They just ran #10 from the load side of main breaker to the disconnect. The disconnect has a 30A breaker for the WH. A 10-2 UF wire was ran to the WH.

When the 30A breaker in the disconnect is turned on it trips immediately. After checking I thought the water heater had a short but found it doesn't. After further checking, I found that the feed from the main breaker to the disconnect is shorted. Then I discovered the main breaker is bad. One leg is dead. The conductors to the disconnect, when disconnected, are not shorted.

My question is why or how would only the breaker for the WH trip and not anything else in the home? How could just losing one leg of the main cause a short on a 240V load?
I'm going back to replace the outside panel/disconnect and will know for sure if that is all that is wrong. I just can't seem to grasp why it affects only the WH breaker.
 

Jraef

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Location
San Francisco Bay Area, CA, USA
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After further checking, I found that the feed from the main breaker to the disconnect is shorted.
How do you mean "shorted"? One leg is going to ground? Or leg-to-leg shorted?


The conductors to the disconnect, when disconnected, are not shorted.
this contradicts what you just said.

My question is why or how would only the breaker for the WH trip and not anything else in the home? How could just losing one leg of the main cause a short on a 240V load?
I'm going back to replace the outside panel/disconnect and will know for sure if that is all that is wrong. I just can't seem to grasp why it affects only the WH breaker.
My guess is that with the main breaker is not just OPEN on one leg, it was grounded on one of the LOAD side terminals of that breaker, but the main contact was open, so no short circuit as long as there were no 240V loads. With it only good on one leg on the feed to the WH, it was only hot on one side until you close the WH breaker, at which time it got back TO that grounded breaker terminal through the water heater elements. In other words the WH breaker was completing the circuit to ground.

For that to work, ALL of the other 1 pole breakers in that panel would have had to be on the one good leg, or else whatever was on that dead leg wasn't working either. If none of those were 240V, nothing would have tripped.
 

Little Bill

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Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Well all the cards fell in just the right places! After I got the outside disconnect/panel changed and a new breaker for the water heater it tripped as soon as I turned on the breaker.:rant:

Seems whoever ran the new 10-2 to the WH the other day clamped the NM connector down too hard and penetrated the insulation causing a short from one leg to ground. I had checked earlier but the wires were disconnected from the water heater at the time.
I had looked the UF over and couldn't see any damage and was about to run a new wire. When I took it out of the NM connector I saw where it was pinched.
Seems everytime I checked for shorts/faults everything was not in place to show it.
With the exception of when it showed to the disconnect from the main.

Case closed!:thumbsup:
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Well all the cards fell in just the right places! After I got the outside disconnect/panel changed and a new breaker for the water heater it tripped as soon as I turned on the breaker.:rant:

Seems whoever ran the new 10-2 to the WH the other day clamped the NM connector down too hard and penetrated the insulation causing a short from one leg to ground. I had checked earlier but the wires were disconnected from the water heater at the time.
I had looked the UF over and couldn't see any damage and was about to run a new wire. When I took it out of the NM connector I saw where it was pinched.
Seems everytime I checked for shorts/faults everything was not in place to show it.
With the exception of when it showed to the disconnect from the main.

Case closed!:thumbsup:

Overtight Romex connectors have been more than one PIA problem
 
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