ECN Forum Forums Code Related Discussion NEC & other Code issues Grounding questions

On a recent new service inspection, I ran into a number of conditions I was unclear about. I would appreciate some opinions (with code references) about the following:

1. On 100 and 200 amp services, are 2 ground rods required if a copper water main is unavailable?
2. On a 400 amp 3 phase CT service, can the neutral be bonded to the ground in the CT can or should it be bonded only at the main disconnect? Could it be bonded at the Ct can and not bonded in the disconnect.
3. Can couplings be used as short nipples with chase nipples on each end?
4. Do all nipples larger than 1 1/2" require a ground bushing or only if you use KOs?
5. If ground bushings are required, are meyer's hubs OK without ground bushings?
6. Is it OK to have 6 - 3/0 cu conductors with 2 - 3/0 neutrals in a 3" GRC service mast for a 400 amp panel with 75º rated panel?

Welcome to the forum!

1. Two ground rods, or other made electrodes are required per the NEC (250.56)if you forego testing of the first rod. Some power companies require two rods regardless of the resistance of the first rod.

2. The neutral must be bonded to the enclosure at each enclosure it passes through up to and including the service disconnecting means.250.92

3. If this is part of the service raceway, I don't think it is possible to easily bond this correctly.

4. All metal nipples that are part of the service raceway must be properly bonded, regardless of size.250.92(A)(1)

5. Only if the hub is listed for bonding purposes. If memory serves me correctly, a plain Myers hub is not listed for this purpose, you'll need one with a lug on it. Check out http://www.crouse-hinds.com/ and do a search for "fittingd" followed by a product category search for "Myers Hubs" to see the differance.

6. Maybe. I can see this one causing a few arguements. See 315.(B)(4) to determine if your neutral is a current carrying conductor, then check out table 310-15(B)(2)(a). If it is not a current carrying conductor and depending on the type of conductor insulation, I would say that this is OK (see 240.4(B)

If you're ready to pony up about $30, visit http://www.iaei.com & buy a copy of "Soares Book On Grounding." This book, in my opinion, is the definitive text on grounding & bonding.

Tom

[This message has been edited by Tom (edited 08-23-2002).]

[This message has been edited by Tom (edited 08-23-2002).]

[This message has been edited by Tom (edited 08-23-2002).]

Tom, OK, I'm tired and sort of slow anyway, but did you mean it when you said the "neutral" must be bonded at each enclosure?
Don

Tom,
Thank you for your welcome to the board and for your reply. On answer #2, I understand article 250.92 to require all service equipment to be bonded together. I do not se mention of the neutral in this area. I thought that the neutral could only be bonded to ground at one point to prevent ground loops?

On my question #6, regarding 6 - 3/0 cu conductors with 2 - 3/0 neutrals in a 3" GRC service mast for a 400-amp panel with 75º rated panel. The conductors are THHN. The service is 208Y/120 volts. It seems that if you count the neutrals then the wire size must be increased. What would determine if the neutrals are current carrying? I realize that if the service has a balanced load there will be no load on the neutral. Is there an article that addresses when a neutral is and isn't considered current carrying?
Thanks again,

Gregory

I don't have my code book handy so correct me if I'm wrong. I'm going from memory.

1. 1 electrode required. If available metal water pipe, metal building frame, encased electrode and grnd rings will all be used.
If metal water pipe is used a made electrode is also required. If the ground rod is the only available electrode it must have resistance of 25 ohms or you must add an additional rod within 6 ft. That's the nec requirement as I recall, but many times the local utility provider will dictate.

2. The requirement is 1 place only at the service only(with a couple of exceptions), if the disconnect is part of the CT can some will argue it's the same. I do know the logic behind not putting it in the meter portion is that the utility co will apply seals to that section rendering accessability difficult.

3. As long as it meets any grnd/bonding requirements of the situation.

4. No, Service entrance conduits require it.
In other locations it is basically required only on concentric/eccentric ko's for 250V and above.

5.Can't recall

6.In your follow up, you said THHN. A service mast is a wet location. THHN is dry/damp loc.
It should be THW or THWN.
For your question ? are you asking about derating for # of conductors?

Sparksalot,

Perhaps I stated it backwards & it might not mean the same thing.

Each enclosure & metal raceway must be bonded together. This is usually accomplished, for enclosures, by making a connection to the neutral. 250.92(A) & (B)(1).

Tom

gregoryf Wrote:


I suspect you are thinking about the location of the Grounding Electrode Conductor connection to the Grounded Conductor (Neutral) of the service entrance conductors. That connection may be made at any point between were the service entry neutral attaches to the utilities conductors and the buss bars in the service equipment enclosure.

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).
(1) General. The connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

As for how to bond the service raceway and meter sockets that is covered by 250.92. I cannot find a definition for the term threaded bosses but I bet it covers hubs. If hubs are not covered then I would think that bonding that occurs in the meter base would be enough. If not I guess you could use an over length screw on one corner of the hub and attach a lug on that screw inside the can.

250.92 Services.
(A) Bonding of Services. The non?current-carrying metal parts of equipment indicated in 250.92(A)(1), (2), and (3) shall be effectively bonded together.
(1) The service raceways, cable trays, cablebus framework, auxiliary gutters, or service cable armor or sheath except as permitted in 250.84.
(B) Method of Bonding at the Service. Electrical continuity at service equipment, service raceways, and service conductor enclosures shall be ensured by one of the following methods:
(1) Bonding equipment to the grounded service conductor in a manner provided in 250.8
(2) Connections utilizing threaded couplings or threaded bosses on enclosures where made up wrenchtight
(3) Threadless couplings and connectors where made up tight for metal raceways and metal-clad cables
(4) Other approved devices, such as bonding-type locknuts and bushings
--
Tom

Tom,

The hubs supplied by the meter socket manufacturer bond the service raceway to the meter can, they are listed for that purpose & as near as I can tell, it is because of the threaded fasteners used to attach the hub to the enclosure.

Threaded boss? I think that is similar to a "hub" & is part of the enclosure & not removable. Maybe someone else can enlighten us.

T

I'd really like to know the answer to the meyers hub question too!!!