Multiwire circuits tripping because neutrals shared,(?) Main breaker installed wrong.

Mikros

Member
Location
Colorado
I have a Milbank meter/panel combo pedestal feeding a sub-panel. (240/120v 1-ph 3w)

In the Milbank pedestal panel, there is a 60A 2pole breaker that feeds a separate sub-panel next to it.
The 60A 2P breaker is installed in a position where the two poles are fed from the same leg in the Milbank panel. (so, 120v to ground on each pole, but basically zero volts between them)


The multi-wire branch circuits in the subpanel have been nuisance tripping. They are 20A 2pole breakers, and share a neutral, to feed 120v circuits.
The sub-panel was obviously meant to be fed 240/120 and share neutrals, but in this case the two main lugs are fed from the same leg of the service.

I can see how this could possibly overload a neutral, but is there any reason they would trip the 20A breaker? I don't see that right off...

I can easily correct this by trading positions of a few of the breakers in the Milbank panel, but was just curious if it might cause a breaker to trip.
Thanks, sorry if this is a dumb question :) Jeff
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
The multi-wire branch circuits in the subpanel have been nuisance tripping. They are 20A 2pole breakers, and share a neutral, to feed 120v circuits.
I don't have an answer to your question, other than to point out that the question has an error. You don't have multi-wire branch circuits in the subpanel. By definition, a MWBC must have voltage between any two of its ungrounded conductors. What this installation represents is a code violation. 210.2 says that all branch circuits must be installed in accordance with that article. 210.4 allows us to create MWBCs. But nothing else in 210 allows circuits to share neutral conductors.

I agree with your proposed solution. I would be curious as to whether it succeeds. I would not count on it, but would instead do a thorough test after your work is done. It may just be that something else is happening in the subpanel, other than being fed by the same phase on both of its incoming lines.

I'll add my "welcome to the forum."

 

Mikros

Member
Location
Colorado
I don't have an answer to your question, other than to point out that the question has an error. You don't have multi-wire branch circuits in the subpanel. By definition, a MWBC must have voltage between any two of its ungrounded conductors. What this installation represents is a code violation. 210.2 says that all branch circuits must be installed in accordance with that article. 210.4 allows us to create MWBCs. But nothing else in 210 allows circuits to share neutral conductors.

I agree with your proposed solution. I would be curious as to whether it succeeds. I would not count on it, but would instead do a thorough test after your work is done. It may just be that something else is happening in the subpanel, other than being fed by the same phase on both of its incoming lines.

I'll add my "welcome to the forum."

Right, they were meant to be multi-wire, but do not meet the definition because of the error. I also think along the same lines as you with the nuisance tripping, that it is probably a different issue. It just made me wonder if that set up would trip breakers for some reason. I'm actually there to add a 240v pump from the sub panel and discovered the existing error while I was planning the job. The nuisance tripping was mentioned to me, but not in my scope.

Thanks for the welcome, I have looked at this forum for clues on things for years, but this is my first contribution.
 
I don't think it is the way the wiring is set up
After all it was working at some stage
Tell me about the loads
And the expected amp draws on them
Anything changed recently ?
Have you checked for anything obvious ?
Loose/dirty/corroded connections ?
 

JFletcher

Senior Member
Location
Williamsburg, VA
Are you sure that double 60 feeds the subpanel, and that one wire wasnt moved to another breaker on the same leg? In any event, Id do what you plan to get 240V to the subpanel.

The nuisance trips, I dont see them coming from that wiring error.

Welcome.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
When a 120/240 panel is fed with 120 only, all the 'legs' are the same. I saw several panels wired up that way in a power plant we were building, and another in a house locally that only had 120 coming in to it.
I understand that, but if the meter combo has 240V how could a 2-pole breaker pick up the same leg for each pole? OP said the 60A 2-pole was in the combo and feeding a subpanel.
 

iwire

Moderator
Staff member
Location
Massachusetts
The panel is only being fed by one hot and the two hot buses of the panel would be connected together, making them both the same. The neutral stays on the neutral bus like usual.
You are missing the point.

How is it physically possible to install a two pole breaker in a way it only connects to one phase?

I think the OP is reading a backfeed and one side of the 240 is actually dead.
 
You are missing the point.

How is it physically possible to install a two pole breaker in a way it only connects to one phase?

I think the OP is reading a backfeed and one side of the 240 is actually dead.
The breaker is installed as usual. Both hots of the breaker are 120 if the two sides of the panel are jumpered together. With no jumper, one side of the two pole breaker would be dead as you say.

Don't take this as advocating such an installation, I am just trying to explain it based on what I have seen.

In this case the 'jumpering' is done by feeding both legs of the sub from the same leg of the meter panel.
 

iwire

Moderator
Staff member
Location
Massachusetts
The breaker is installed as usual. Both hots of the breaker are 120 if the two sides of the panel are jumpered together.
That does not seem to be the case does it?


I have a Milbank meter/panel combo pedestal feeding a sub-panel. (240/120v 1-ph 3w)

In the Milbank pedestal panel, there is a 60A 2pole breaker that feeds a separate sub-panel next to it.
The 60A 2P breaker is installed in a position where the two poles are fed from the same leg in the Milbank panel. (so, 120v to ground on each pole, but basically zero volts between them)

My bet is the problem is with the two pole 60, either it's open internally or the bus connection is bad.
 
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