Ground connection to cold water pipe -- main panel on detached shop

[bob_mp]

Hi,
I am getting some information from the building department on grounding requirements that I don't understand and want to know if I am missing something.

I have a 400 amp main panel on my detached shop. It has 2 200A main breakers that feed two subpanels, one inside the shop and one inside the house. The house will be connected to the main panel through 3/0 conductors through PVC conduit. I have the main panel connected to the UFER ground in the shop and to a ground rod (next to the service entrance).

Before I upgraded the service (from 200A), I had a continuous #4 conductor that went from the ground connection at the main panel, through the house, to the cold water pipe. I had a separate ground connection (#4) that went from the main panel to the house subpanel.

I was told by the building inspector is that I don't need to connect the grounds between the main panel and the house subpanel and that I needed to connect the ground at the house subpanel to the cold water pipe and a ground rod near the house.

The lack of a ground connection between buildings could result in a difference in potential between the shop ground (and neutral conductor) as compared to the cold water pipe, which doesn't seem right (or desirable).

I am planning on having a phone in the shop which creates a metallic connection between the buildings per NEC 250.32, which (I think) means I need to run a ground wire between the main panel and the house subpanel in any case.

A few questions:

1. Where does the ground connection to the cold water pipe get made? Is it from the subpanel in the house or back to the main panel?

2. Is there a section of the code or other reference that clarifies this?

3. What size conductor is needed? Is it a #4 conductor or do I size it based on Table 250.66, which seems to suggest 1/0 Cu conductor?

Much obliged for the help.

Bob

 

[sberry]

I would want a wire panel to panel.

 

[Ryf]

I was told by the building inspector is that I don't need to connect the grounds between the main panel and the house subpanel and that I needed to connect the ground at the house subpanel to the cold water pipe and a ground rod near the house.

you don't need to but you can, but you do need to do the ground at the subpanel and cold water pipe as the inspector requested.

I ran the ground because I ran cat5e as well, I also put a ground rod in for the hell of it at the shop side, between the risk of lightning and just plain "easier to do now than later". inspectors are not perfect and its all about their interpretation of the law you are governed by, and despite the clear ignorance of some of them you need their ok to move on. do what he says and do more if you feel its to your benefit.

the general answer about grounds etc. is the shortest possible route. the panels have to be tied in the house and tying the main to the water and rod is to get your second ground so they don't have to question resistance.

IMO, it would be easier to run the ground in the house and put a second rod more than 6 ft away from the first than running a second wire across my house to tie into the water pipe. I would decide this by "whichever costs less"

 

[CNGsaves]

OP . . . it would be most helpful to let GJer's know what COUNTRY and STATE that you're in as electrical codes vary by location.

Now would be a good time to Update GJ Profile with your Country and State, at minimum . . . or State and City if within USA.

 

[ddawg16]

1. Where does the ground connection to the cold water pipe get made? Is it from the subpanel in the house or back to the main panel? If possible, you want the water pipe to be connected to the main panel ground....but in your case, if I'm understanding it right...your just going to connect it to the ground buss in your sub-panel. Remember, one of the reasons for tying into your plumbing is to make sure all your pipes are at ground potential. Some houses will actually have plastic pipe feeding the house...it only goes to metal where it comes out of the ground. You will want the #4 from your main panel going to the ground buss in your sub's....but DO NOT bond the ground buss and neutral buss in the subs together if the sub is not in the same building as your main. So...your sub in the garage can have the neut and gnd bonded together...but the house can not.

2. Is there a section of the code or other reference that clarifies this? Hopefully one of the sparky's will chime in.

3. What size conductor is needed? Is it a #4 conductor or do I size it based on Table 250.66, which seems to suggest 1/0 Cu conductor? For the ground? #4...unless your area is different....one of the reasons we need to know where you are.

 

[1Garageman]

OP . . . it would be most helpful to let GJer's know what COUNTRY and STATE that you're in as electrical codes vary by location.

Now would be a good time to Update GJ Profile with your Country and State, at minimum . . . or State and City if within USA.

I wish they would make that a requirement!

 

[MrMark]

He's in CA, we only have one electrical code: 2008 NEC. There are VERY limited local amendments and those have to be justified based on local conditions.

You need a ground from the house to the main panel. Otherwise you have no fault path back to the main panel.

If there is a metallic cold water pipe available at either location you bond it as part of the grounding electrode system. You have to bond it within a certain number of inches of where it enters the building. I forget the number. There is a section of this.

Remember the electrode system is not for clearing faults, it is for lightning and static and reference.

 

[ddawg16]

5 Feet....

I grabbed the page from the building code for LA County that covers the electrical....chances are it's the same up there.

 

[theoldwizard1]

I grabbed the page from the building code for LA County that covers the electrical....chances are it's the same up there.

Don't forget, as long as no "major renovations" are being performed, there is no requirement to meet current code.

If it passed inspection at the time it was installed, then it is "good". The whole ground rod/double ground rod issue is a non-issue for most homes in the US.

 

[wyliesdiesels]

you don't need to but you can, but you do need to do the ground at the subpanel and cold water pipe as the inspector requested.

I ran the ground because I ran cat5e as well, I also put a ground rod in for the hell of it at the shop side, between the risk of lightning and just plain "easier to do now than later". inspectors are not perfect and its all about their interpretation of the law you are governed by, and despite the clear ignorance of some of them you need their ok to move on. do what he says and do more if you feel its to your benefit.

the general answer about grounds etc. is the shortest possible route. the panels have to be tied in the house and tying the main to the water and rod is to get your second ground so they don't have to question resistance.

IMO, it would be easier to run the ground in the house and put a second rod more than 6 ft away from the first than running a second wire across my house to tie into the water pipe. I would decide this by "whichever costs less"

Incorrect. An EGC(ground wire) is ALWAYS required in a feeder thats coming off a disconnect, and feeding a subpanel. If the OP didnt have a main disconnect for the house feeder, THEN an EGC isnt needed.

1. Where does the ground connection to the cold water pipe get made? Is it from the subpanel in the house or back to the main panel? If possible, you want the water pipe to be connected to the main panel ground....but in your case, if I'm understanding it right...your just going to connect it to the ground buss in your sub-panel. Remember, one of the reasons for tying into your plumbing is to make sure all your pipes are at ground potential. Some houses will actually have plastic pipe feeding the house...it only goes to metal where it comes out of the ground. You will want the #4 from your main panel going to the ground buss in your sub's....but DO NOT bond the ground buss and neutral buss in the subs together if the sub is not in the same building as your main. So...your sub in the garage can have the neut and gnd bonded together...but the house can not.

2. Is there a section of the code or other reference that clarifies this? Hopefully one of the sparky's will chime in.

3. What size conductor is needed? Is it a #4 conductor or do I size it based on Table 250.66, which seems to suggest 1/0 Cu conductor? For the ground? #4...unless your area is different....one of the reasons we need to know where you are.

No, incorrect. Neutral and ground busses should NEVER be bonded in subpanels regardless of what building theyre in compared to where the main panel is. Subs always need a 4-wire feed and ALWAYS must have the neutral buss insulated from the panel enclosure and ground bar.

Now, on the other hand, if there wasnt main disconnects after the meter/before the subs, then the subpanels would actually be considered main panels and would only need 3-wire feeders and the neutral and ground busses should be bonded!

 

[ddawg16]

Incorrect. An EGC(ground wire) is ALWAYS required in a feeder thats coming off a disconnect, and feeding a subpanel. If the OP didnt have a main disconnect for the house feeder, THEN an EGC isnt needed.



No, incorrect. Neutral and ground busses should NEVER be bonded in subpanels regardless of what building theyre in compared to where the main panel is. Subs always need a 4-wire feed and ALWAYS must have the neutral buss insulated from the panel enclosure and ground bar.

Now, on the other hand, if there wasnt main disconnects after the meter/before the subs, then the subpanels would actually be considered main panels and would only need 3-wire feeders and the neutral and ground busses should be bonded!

We might want to double check that. It was my understanding that a sub in the same building as a main got the neut and ground bonded together.

 

[Aceman]

We might want to double check that. It was my understanding that a sub in the same building as a main got the neut and ground bonded together.

After the first disconnect on a 4 wire feed, the neutral and ground wires should never be bonded together again. That creates a parallel path and would put current on the ground wire.

 

[MTW]

Post # 10 wyliesdiesels is correct, on this often misunderstood subject.
Post # 12 Aceman is also correct

After the 1st service disconnect, neutral and ground should always be separate or not bonded regardless of where the other panels are located.

For the size of water pipe grounding electrode conductor, yes use table 250.66 #4 copper for 3/0 feeder conductors that feed the building. Even though it's a sub feed, it's treated like a main for the purposes of the grounding electrode system.

See section 250.32 for buildings supplied by feeders

 

[bob_mp]

Thanks for the help guys.

My original setup had a dedicated ground wire without splice to the cold water pipe in house from main plus a ground wire from the main panel to the subpanel in the house. It seems like this is still the way to go.

So do I understand correctly then that I size the Grounding Electrode Conductor to the cold water pipe based on the size of the feeders to the building, independent of the size of the main? I was required to run a #4 conductor for the old 200A service. Will this suffice for the new 400A service?

What's confusing me is that 250.66 specifies the Grounding Electrode Conductor size in terms of Service Entrance Conductor, not the feeder. I looked at 250.32 -- I couldn't find anything that indicated that 250.66 applied to the feeders, not the service entrance conductors?

Much obliged for the help.
Bob

 

[wyliesdiesels]

I see no one has gotten back to. Since i havent come across this before, I will read up and get back to u tonight if I have time!

 

[Aceman]

Thanks for the help guys.

My original setup had a dedicated ground wire without splice to the cold water pipe in house from main plus a ground wire from the main panel to the subpanel in the house. It seems like this is still the way to go.

So do I understand correctly then that I size the Grounding Electrode Conductor to the cold water pipe based on the size of the feeders to the building, independent of the size of the main? I was required to run a #4 conductor for the old 200A service. Will this suffice for the new 400A service?

What's confusing me is that 250.66 specifies the Grounding Electrode Conductor size in terms of Service Entrance Conductor, not the feeder. I looked at 250.32 -- I couldn't find anything that indicated that 250.66 applied to the feeders, not the service entrance conductors?

Much obliged for the help.
Bob

In the 2011 NEC, 250.66 states "The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder.......shall not be less than given in Table 250.66.......

 

[Kevin C]

In the 2011 NEC, 250.66 states "The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder.......shall not be less than given in Table 250.66.......

That prevents the ground and the neutral wires that connect to the main to the sub-panel from both carrying current (since they would be in parallel).

The electricity is not smart enough to only go down the neutral if a secondary path is available.



View media item 32743
If you have this type of Sub Panel... Never mind.

 

[Graham08]

Now, on the other hand, if there wasnt main disconnects after the meter/before the subs, then the subpanels would actually be considered main panels and would only need 3-wire feeders and the neutral and ground busses should be bonded!

This is how my 400 amp service is set up. Two 200 amp panels, without any disconnects. I have a ground rod at the meter, and one at each panel. The ground and neutral is bonded at each panel.