Main verses sub panel question

Originally Posted By: phughes
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This has probably been answered before, but I need to ask again to help me understand the reason behind the rule.


In a sub panel, the ground and neutral must be isolated. This makes sense.

In the main service panel, is it required that the neutral and grounds be on separate buses, and if so why?

In the main panel the neutral should connect to earth ground, and also connect to the panel cabinet, and the panel connects to the ground bus. So doesn't that make it all the same electric potential.


Originally Posted By: jpope
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Quote:
In the main service panel, is it required that the neutral and grounds be on separate buses, and if so why?

No.

Quote:
In the main panel the neutral should connect to earth ground, and also connect to the panel cabinet, and the panel connects to the ground bus.

Although they are bonded at the service equipment, the grounded conductors (neutrals) and grounding conductors (grounds) perform a completely different functions within the circuit/system.

Quote:
So doesn't that make it all the same electric potential.

That is the reason for bonding, yes.


--
Jeff Pope
JPI Home Inspection Service
"At JPI, we'll help you look better"
(661) 212-0738

Originally Posted By: escanlan
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Hi Peter, Jeff,


The IRC does not address the question of using the service entrance panel bus bars for either or terminations of neutrals and equipment grounds. I could not find anything specifically in the NEC for this also. This would be a good question for Joe Tedesco!!

Where are you Joe???

I can say that my work in the military and civilian commercial world always had a specific connection arrangement. The Neutrals conductors from branch circuits and equipment grounding conductors were always placed on separate bus bars and never mixed. Also the grounding electrode conductors were always tied to the neutral bus bar. This was done for several reasons:

1. A failure of the bonding from the neutral bus bar to equipment bus bar left you without equipment grounding only. If they were mixed and this occurred you would have a very dangerous situation.

2. In the event a negative equipment grounding situation existed causing noise, etc. with the equipment grounds then they could be easily lifted and abatement methods used. If they were mixed on the bus bars it would require extensive rewiring to do this. If the cabling in the service panel were properly terminated there would not be sufficient wire length in many cases to just move terminations around.

3. In the event the need arose to change the service entrance panel and make it a subpanel (very rare but possible) mixed bus bars would make it very difficult (see number 2 above).

4. Then there is the aesthetic and workmanship issues. It just looks better and is a mark of a more professional job.

I have not had the opportunity to see a mixed bus bar connection arrangement. Don't know how I would write it up or if I would. It would be nice to have a professional electrician comment on this from a codes stand point.

Manny (Emmanuel) Scanlan


Originally Posted By: bbadger
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At a service panel (the enclosure that contains the service disconnect switch) the NEC allows the grounded and the grounding conductors to connect to the same terminal bar, or you may install a separate grounding terminal bar.


For the new here the grounded conductor is required to be a white or marked white conductor. We often call it the 'neutral' which it may or may not be.

The grounding conductor is required to be bare or green.

I agree that it looks better with separate terminal bars at every panel it is only required at sub panels.

Other issues to consider are no more than one grounded conductor to a terminal

Quote:
408.21 Grounded Conductor Terminations.
Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.



And for the grounding conductors no more than one conductor to a terminal unless the terminal is specifically identified for more than one wire.

Quote:
110.14(A) Terminals. Connection of conductors to terminal parts shall ensure a thoroughly good connection without damaging the conductors and shall be made by means of pressure connectors (including set-screw type), solder lugs, or splices to flexible leads. Connection by means of wire-binding screws or studs and nuts that have upturned lugs or the equivalent shall be permitted for 10 AWG or smaller conductors.
Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.



--
Bob Badger
Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: jtedesco
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Quote:
408.40 Grounding of Panelboards
Panelboard cabinets and panelboard frames, if of metal, shall be in physical contact with each other and shall be grounded. Where the panelboard is used with nonmetallic raceway or cable or where separate grounding conductors are provided, a terminal bar for the grounding conductors shall be secured inside the cabinet. The terminal bar shall be bonded to the cabinet and panelboard frame, if of metal; otherwise it shall be connected to the grounding conductor that is run with the conductors feeding the panelboard.


Quote:
A separate equipment grounding conductor terminal bar must be installed and bonded to the panelboard for the termination of feeder and branch-circuit equipment grounding conductors. Where installed within service equipment, this terminal is bonded to the neutral terminal bar.

Any other connection between the equipment grounding terminal bar and the neutral bar, other than allowed in 250.32, is not permitted. If this downstream connection occurs, current in the neutral or grounded conductor would take parallel paths through the equipment grounding conductors (the raceway, the building structure, or earth, for example) back to the service equipment. Normal load currents on the equipment grounding conductors could create a shock hazard.

Exposed metal parts of equipment could have a potential difference of several volts created by the load current on the grounding conductors.

Another safety hazard created by this effect, where subpanels are used, is arcing or loose connections at connectors and raceway fittings, creating a potential fire hazard.



--
Joe Tedesco, NEC Consultant

www.nachi.org/tedescobook.htm

Originally Posted By: escanlan
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Thank you very much Joe Tedesco!!


I really need to buy the searchable CD version of the NEC!!!

Manny (Emmanuel) Scanlan


Originally Posted By: pdickerson
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Hello all,


It is my understanding that if a main panel feeds a sub panel in a defferent building that is not electrically connect to the location of the main panel (railing, sidewalk, phone wires, etc.), then the sub panel should have its own grounding rod, and the grounds and neutral should be bonded together in the sub panel. Is this right?


Originally Posted By: jpope
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My understanding is;


If the remote building is fed with a four wire system, the grounds and neutrals remain isolated and a GES is still required.

If the remote building is fed with a three wire system (no EGC) and there is no other metallic path between bldgs (fence, phone lines, etc., etc.), the grounded (neut) and grounding conductors should be bonded at the panel and a GES installed.

Let's see what our experts say.


--
Jeff Pope
JPI Home Inspection Service
"At JPI, we'll help you look better"
(661) 212-0738