Hi everyone,

We are building the service for an apartment building and are getting several different stories about where to bond the neutrals and grounds together. Here is the scenerio: CT cabinet with trough mounted directly to the side of it. 3 4" conduit runs to feed 3 400A disconnects. The neutrals all will land in the CT cabinet. The engineer is saying all neutrals and grounds (cold water, grounding electrodes, and grounds going to each 400A disco) should be bonded in the trough. Another theory is that the neutrals and grounds should be bonded in each disconnect. Does anyone know what the real deal is? Your input is appreciated.

It gets bonded in each service disconnect but the handbook also has an example with a bonded neutral bus in the trough and the bonds in each disconnect that is troubling to me. 2002 HB ex 250.12

edit to add pic




I think you will have neutral current flowing in the enclosres and nipples in this example.

[This message has been edited by gfretwell (edited 08-10-2006).]

are you saying that the neutrals should be bonded together in the trough but not the grounds? To me that would make no sense because they are already bonded together in the CT cabinet. If they get bonded in each disconnect does that mean we need 3 seperate cold water grounds and 3 seperate sets of grounding electrode conductors? We were liking the engineers suggestion because we could bond everything in the trough so we would only need to pull 1 cold water and 1 to the rods.

thanks for the pic, i see what you are saying about the neutral current, we happen to be using pvc and pulling a ground so i guess that alleviates that concern. Looks to me that this indicates the engineers method would be acceptable.

Where you bond the neutral and where you land the ground electrode conductor are two separate things. You can attach the ground electrode conductor anywhere from the service point to the service disconnect but the neutral should onl;y get bonded in the service disconnect. Also you can tap in supplimentry electrode conductors and the GEC to each disconnect. In reality only the biggest one needs to be unspliced.

[img] http://members.aol.com/gfretwell/250ex26.jpg [/img]

[This message has been edited by gfretwell (edited 08-10-2006).]

[This message has been edited by gfretwell (edited 08-10-2006).]

well in the picture the neutrals and grounds are bonded in the trough, not in the disconnects. What you are saying though, is the neutrals pass right through the trough into each disconnect, while a ground bar can be installed in the trough to land the cold water ground, ground rod, and the individual grounds to each disconnect. One other thing, would the grounds to each disconnect be sized by 250.66 or 250.122?

thanks for the additional pic, it seems they indicate completely different points of bonding the neutrals and grounds, the first in the trough, the second in the individual disconnects

well i guess in the first pic they are bonded both in the trough and in the disconnects

Bonding in both places is troubling. I wonder if they have fixed that example.
I do see people who assume everything in that cludge is part of the service disconnect and they bond everything to everything every chance they get. If you have a metal raceway back to the transformer where everything gets bonded again it is probably a distinction without a difference.
This is one area where you can have academic discussions about where the current is flowing but in reality it is probably everywhere in all of those cans, raceways and grounded/grounding conductors to some extent.

Multiple points of bonding on the line side of the service disconnects are pemitted by the code.
Don

crl22191 wrote: it seems they indicate completely different points of bonding the neutrals and grounds, the first in the trough, the second in the individual disconnects.

Why is this difficult? The neutral wire is the bonding wire at all places along the service until you hit the main disconnect(s). Your neutral will be bonded the chassis in the meter enclosure, the trough, and in both disconnects. The trough is not a disconnect, so there is no need to isolate the grounded and grounding wires yet. The GEC is attached to the neutral in the trough because it is the only neutral large enough in this example. If a larger neutral (2/0) was run to one of the disconnects, the GEC could have been connected there which may make people feel more comfortable.

Yes, there will be current flowing in the metal service raceways, troughs, and enclosures, but this is how most service equipment is.

If you are at an IAEI metting you will always find several guys who say "parallel neutral" when you talk about bonding the neutral and have metal paths. I agree it isn't really very important in cases like this.