Bonding Older 3-wire sub panels

[knightm1]

Adding bonding conductor

Adding bonding conductor

Could you not just connect a correctly sized bonding conductor to the existing panel from the new main panel, remove any bonding at the old panel and still meet current code?

 

[Ken 6789]

I am not sure if this would be acceptable in this case, but can you bond an EGC to the sub-panel and run it to a nearby copper water pipe?

 

[wireman71]

Here's a better idea.. Never do remodel work unless all the walls and ceilings are exposed! And all insulation is gone!

 

[badabing]

NO, NO, NO! Neutrals and grounds MUST be kept separate in sub-panels. Otherwise neutral current is imposed on all of the grounding conductors and the metal panel enclosures. In the event of a bad neutral connection these items would be energized to system voltage with no way to clear the fault. This was probably in violation when it was originally installed, but I don't know that for sure. New feeders will need to be pulled to these panels to make this installation compliant. There is no other way.

I must ask.. I never really understood this, why is it that the ground and neutrals are bonded together in the main panel, but not after that? Although there are sepearte wires (neutral to the neutral bar, ground to ground bar) And are not touching in the sub panel... if you put your meter to them and test, it's still touching from the main panel... So what's the difference if it's bonded or not?! There must be something, possibly simple thats never been explained to me, or that i've just overlooked... Just wondering.

 

[Ken 6789]

I believe it's that way because the ground and neutral that are bonded at the service panel is closest to the transformer and there is a ground rod bonded to that neutral/ground bar in the service panel. Also, the water line EGC is bonded there as well. If we're permitted to bond the ground and neutral in sub-panels, and if the neutral comes loose, then the metal sub-panel box, metallic surfaces of appliances connected to that sub-panel will become energized. This will obviously become a shock hazard. If the neutral becomes loose in the main service panel, then the current will take the path of the EGC to the water line or ground rod, whichever having the least resistance, thus eliminating the shock hazard. This is my interpretation. Anyone that can explain this better, please jump in.

 

[al hildenbrand]

Badabing,

The separation of neutral and EGC after the main bonding jumper keeps all the unbalance current on the neutral.

The EGCs of all the branch circuits are interconnected, where they are in the same enclosure, thus forming a web, or, if you will, a net. The EGCs are also in contact with conductive systems like interior gas and water pipes, duct work, etc. The normal unbalance current, if it gets into the EGC will take ALL available paths back to the source.

That is, consider the definition:

2005 NEC Article 100

Grounding Conductor, Equipment. The conductor used to connect the non?current-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.

 

[haskindm]

If grounds and neutrals are put together downstream of the service equipment, there is now a parallel path for neutral current back to main service. Grounding conductors are not supposed to carry current, but in this situation they will. Then, in the event a neutral is lost, the grounding conductors will carry all of the neutral current. Not a situation that you want to encourage!

 

[George Stolz]

If the neutral becomes loose in the main service panel, then the current will take the path of the EGC to the water line or ground rod, whichever having the least resistance, thus eliminating the shock hazard. This is my interpretation. Anyone that can explain this better, please jump in.

Ken, the grounding electrodes are not effective at (and not intended to be) serving as an emergency neutral in an open neutral condition. They are solely for the purposes defined in 250.4(A)(1).

 

[stjohnbarleycorn]

ON the condition that in this old house, there are ONLY two wire circuits, and ONLY three wire feeders to the sub-panels, then bonding the neutral to the cabinet should have already been done, shouldn't it? I have not checked all the sub-panels I have seen in old houses but the ones I have checked have been that way. I know the problem is that additional circuits are or have been added that do have EG and the electrician who added them did not know what to do with the EG.
Believe it or not I came across one where the guy put it under a FUSED screw!!. How no one was killed I don't know, I was for the refer, and I was called in because people were getting shocked by it. They had installed a new refer with a EG and now the whole box was energized the old one was the two wire type. That was pretty scary.

 

[al hildenbrand]

in this old house. . . .bonding the neutral to the cabinet should have already been done

The classic commodity residential panels that have fuse pull-blocks and screw in plug fuses (think 60 Amp 240 / 120 Volt single phase) are manufactured with the neutral strap from the service / range lugs to the branch circuit neutral bar bonded to the panel pan.

The bond is usually a rivet and is behind the phenolic body that the fuses connect into, and / or the mounting rivets at the lugs at each end. The strap is un-insulated and, as manufactured, was never intended to be "floated". As long as the panel is used as a service disconnect things are fine, but on a new upgrade with a new service disconnect, the bond must be moved to the new service disconnect.

Still older panel configurations, porcelain fuse holders and knife switches used with Knob & Tube, are yet another story.