Subpanel ground/neutral

I was tripping AFCIs. After an investigation, I found that the ground and neutral in my subpanel were reversed (in other words, the ground from the main went to the neutral bar and the neutral from the main went to the ground bar). After reversing them, AFCIs do not trip.

Questions: Was that a real issue (given that the two are tied together at the main)? Should the neutral and ground in the subpanel be tied together like in the main?
 

rnatalie

Member
Location
Herndon, VA
Yes, that would cause AFCI and GFCI trips. The problem here is that the neutral from the load side of the AFCI and/or GFCI should never touch another circuit or ground. There are special circuits that detect neutral-to-ground leakage and trip.

The only place in most situations that the ground and neutral should EVER be connected together is at the service disconnect (commonly, the "main" panel).
 
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Yes, that would cause AFCI and GFCI trips. The problem here is that the neutral from the load side of the AFCI and/or GFCI should never touch another circuit or ground. There are special circuits that detect neutral-to-ground leakage and trip.

The only place in most situations that the ground and neutral should EVER be connected together is at the service disconnect (commonly, the "main" panel).

Thank you - That is what I thought. Mistakes happen. This is why these forums are so important.
 

lordofthisworld

Senior Member
Location
Massachusetts
I was tripping AFCIs. After an investigation, I found that the ground and neutral in my subpanel were reversed (in other words, the ground from the main went to the neutral bar and the neutral from the main went to the ground bar). After reversing them, AFCIs do not trip.

Questions: Was that a real issue (given that the two are tied together at the main)? Should the neutral and ground in the subpanel be tied together like in the main?
No, Neutrals and ground can not be tied together at sub-panels
 
My question is, how did the AFCI know that it’s white pigtail was connected to the ‘green’ wire vs the ‘white’ wire? Both went back to the same location. It doesn’t matter that they were swapped accidentally.

A detail is missing.
I was wondering too. Perhaps it was indirectly making it trip by putting current on the EGC system which then made it sense a N-G bond in the branch circuit that it didnt quite pick up before? Thats all I got.
 

rlundsrud

Senior Member
Location
chicago, il, USA
I'm assuming that this was in conduit and the equipment grounding conductor buss was mounted directly to the panel. My guess is you were getting parallel paths between the EGC conductor and the conduit causing your trip.

I should add that this was happening at the receptacle itself between the ground strap and the neutral..
 
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synchro

Senior Member
Location
Chicago, IL
Occupation
EE
It's somewhat puzzling to me that the AFCI was tripping with the neutral and ground bar connections reversed. Nothing apparently changed on the load side of the breaker (either hot or neutral), just where the neutral pigtail on the line side was tied. Either way any voltage on the neutral pigtail must be very small and I don't know why the breaker would trip even if there were a few volts AC on the neutral line going into the breaker.
However, because all of the neutral current on the subpanel was going through equipment grounds perhaps there are some marginal connections (like through painted knockouts, bonding screw, etc.) that cause fluctuations if not actual arcing in the conduction path, causing the AFCI to trip. Kind of reaching for straws here but that's all I've got.
 

kwired

Electron manager
Location
NE Nebraska
It's somewhat puzzling to me that the AFCI was tripping with the neutral and ground bar connections reversed. Nothing apparently changed on the load side of the breaker (either hot or neutral), just where the neutral pigtail on the line side was tied. Either way any voltage on the neutral pigtail must be very small and I don't know why the breaker would trip even if there were a few volts AC on the neutral line going into the breaker.
However, because all of the neutral current on the subpanel was going through equipment grounds perhaps there are some marginal connections (like through painted knockouts, bonding screw, etc.) that cause fluctuations if not actual arcing in the conduction path, causing the AFCI to trip. Kind of reaching for straws here but that's all I've got.
I think that is a good place to start.

Neutral pigtail is going to carry all the current the branch circuit neutral is carrying - with a single pole device that is the same current as the ungrounded conductor, if the load neutral lands on the breaker anyway, GE units may not apply.
 
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