Neutral current at service

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frofro19

Member
Location
VA.
Occupation
Master Electrician
If there can be neutral current flowing on all metal parts of a sub-panel if grounds ans neutrals ties together on the old 3 wire systems (prior to 2008), what prevents this on a service disconnect. Wouldn't the disconnect and the meter have the same thing going on?
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Yes it would. That's the one place that objectable current is allowed, there is not much you can do if using metal conduit. That's why PVC is preferred between the meter and 1st means of disconnect w/OCPD.
 

frofro19

Member
Location
VA.
Occupation
Master Electrician
I always use pvc in those applications but would the pvc eliminate the objectionable current? Can there still be neutral current on the disconnect even with pvc between the meter and the disconnect?
 

JoeStillman

Senior Member
Location
West Chester, PA
Objectionable neutral currents are ordinary load currents that are flowing in something other than the neutral conductor. In a properly grounded system, only fault currents flow in the equipment grounding system. The neutral and ground are only connected by one single conductor; the system bonding jumper, and it doesn't carry ordinary load current.

If you put a bonding jumper in a sub-panel, then the ground and neutral from the main to the sub are connected in parallel and the equipment grounding system can carry ordinary load current - objectionably.

Just remember that there must only be one connection from the neutral to the equipment ground and it's the system bonding jumper.

Wonder why sub-panels were allowed to have bonding jumpers before?
 

JoeStillman

Senior Member
Location
West Chester, PA
If there can be neutral current flowing on all metal parts of a sub-panel if grounds ans neutrals ties together on the old 3 wire systems (prior to 2008), what prevents this on a service disconnect. Wouldn't the disconnect and the meter have the same thing going on?
The disconnect and meter pan don't have the same thing going on because there is typically no ground wire in parallel with the neutral at that point.

The neutral is grounded at both ends in that part of the circuit; back at the transformer and the one and only one system bonding jumper. If there is no supply side bonding jumper, there is no path for neutral current other than the neutral itself. I guess you could say the earth itself is a parallel path, but the impedance is so high that the amount of current is negligible.
 
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