grounding and bonding with subpanels

[alpinemike]

This is a 50 yr old resisence with the service (200 amp)on an outside pole.There is a distribution panel with a breaker that feeds a main panel in the house via 2" pvc, with no bonding wire in the pipe.There is a #6 ground wire to a ground rod. The ground wire is landed on the dist. panel neutral, not the meter neutral. The main house panel feeds a 100 amp sub panel via 4-3 romex. We installed that panel and put a separate ground bar in it. The neutal bar in the inside main panel has a #6 ground taken to the water pipe. When I take the wire off the water pipe , the resistance between neutral and ground goes to megohm. The wire into the house is old rubber coated stuff and would be impractical if not impossible to repull with a bond wire. I know the code says you have to keep the ground and neutral separate past the main panel. Anyway out of this mess, near as I can tell everything is ok like it is (there is no inspection in this area). Please review the code for bonding and grounding with sub panels involved. Thanks to all you gurus in advance

 

[infinity]

Re: grounding and bonding with subpanels

Your problem is that at the main panel in the house there is no ground/neutral connection. If you have no metallic paths between the outside pole and the house panel you may use the provisions of 250.32 which will allow you to bond the neutral and ground at the main panel and keep your 3 wire feeder. You said that your second panel was 100 amps fed with 4-3 NM cable. My concern is that you should have used a 70 amp or smaller OCPD for this feeder since 4-3 NM has an ampacity of 70 amps.

 

[alpinemike]

Re: grounding and bonding with subpanels

Thanks infinity, brain f**t on my part with the no.2, there is a 70amp breaker right there. I think 250.32 describes my situation, thanks again.

 

[suemarkp]

Re: grounding and bonding with subpanels

Am I reading this right: 200A service on pole, 200A main panel on house fed with 3 wires from the pole service, 100A subpanel in house fed from house main panel with 4-3 NM cable.

If so, then I see some issues:
1. The subpanel must be fed with 4 wires and not three (unless you only want a 120V panel). Only feeders to detached structures can use the 3 wire method.
2. The #6 to the water pipe is too small. This must be sized per 250.66 and would need to be #4 for a typical 200A service.
3. Where is the ground rod? You must have two at the house connected to the neutral bus in the house main panel. You may (not sure) also need one at the pole connected to either the distribution panel neutral or the meter neutral.
4. Your most critical problem is the lack of neutral-ground bond. Where were you measuring between the neutral and ground when the resistance went to infinity? If this was at the subpanel bus bars, then infinity has it right -- no neutral/ground bonding screw installed at the house main panel. You do need to install that since you only have a 3 wire feed to the house.

 

[infinity]

Re: grounding and bonding with subpanels

1. The subpanel must be fed with 4 wires and not three (unless you only want a 120V panel). Only feeders to detached structures can use the 3 wire method.

I would disagree. If there are no metallic paths between the strucutres (the distribution panel and the house panel) than 250.32(B)(2) would apply.

(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in each building or structure involved, and (3) ground-fault protection of equipment has not been installed on the supply side of the feeder(s), the grounded conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded.

 

[George Stolz]

Re: grounding and bonding with subpanels

Miscommunication is the devil, Bobby!

Alpinemike wrote:
The ground wire is landed on the dist. panel neutral, not the meter neutral.

That's legal. The meter isn't the service disconnecting means, the panel is. (250.24)

There is a distribution panel with a breaker that feeds a main panel in the house via 2" pvc, with no bonding wire in the pipe.

This is where it's getting fuzzy Mike. I see:

1. 200 Amp Service on a pole.

2. 200 Panel on a pole (maybe the same thing).

3. 2" PVC from this pole-panel to interior of the house's main panel. Violation of 225.31-ish, the disconnecting means for a remote structure must be outside or inside nearest the entrance of conductors. There is no EGC, which if there are no other metallic paths, is legal. What size conductors are in this pipe? What size breaker?

4. Main Panel in House. There is a #6 ground wire to the water pipe. Depending on the size of conductors, this could be legal or not. Mike never specified that this panel was fed with conductors big enough for 200A. I'm thinking there could be other circuits from the outside pole panel, and the house is less than 200A.

The neutal bar in the inside main panel has a #6 ground taken to the water pipe. When I take the wire off the water pipe , the resistance between neutral and ground goes to megohm.

You have an EGC somewhere in the house touching the water pipe. That's legal. The reason things are behaving that way is because the water pipe was landed on the neutral bar of the panel.

When everything was hooked up, juice in your meter was leaving the red probe, travelling through the EGC's until it reached water heater, or whatever was making contact with the water pipe. Then it was travelling back along the GEC to the neutral bar of the panel where it was landed. So you had continuity between neutral and ground until you removed the electrode conductor, breaking the circuit.

Install an appropriate bonding jumper between the neutral and grounding bar of this main panel. Reinstall the GEC, and supplement it with a ground rod.


5.

The main house panel feeds a 100 amp sub panel via 4-3 romex . We installed that panel and put a separate ground bar in it.

Is this new or old romex? Is there a EGC with it? The water pipe is not in this panel, is it?

From this point in your post, you went a little chaotic, so I am really confused.

Can you write out a "one-line diagram", filling in the gaps? When you hop back and forth, it makes it impossible for us to make sense enough to help you.

[ November 05, 2005, 08:19 AM: Message edited by: georgestolz ]

 

[suemarkp]

Re: grounding and bonding with subpanels

Originally posted by infinity:
I would disagree. If there are no metallic paths between the strucutres (the distribution panel and the house panel) than 250.32(B)(2) would apply.

What is not clear in the posters question is whether the 100A subpanel is inside the same structure as the main house panel, or a separate detached structure.