Sharing the same Grounding Counterpoise

[Sparky2791]

Good Day to all!
I am working on an project which is additions to and renovations of an Elementary School. We are upgrading the service size with a brand new 2000A, 277/480V service with GFCI protection for the main. We are also installing a new Stand-by emergency generator with switched neutral transfer switches. My question is..... Can the new main service and generator share the same ground rod system? We use a series of 3 ground rods 6' apart in a triangle with a #6 GEC back to the service equipment neutral bar. We are connecting the standby generator as a separately derived system to avoid the whole GFI protection at the main issue. Obviously the reason we also have switched neutral transfer switches. I see no reason in Article 250 which prevent me from sharing this between the 2 instead of placing a separate ground ring for each system. Thank in advance for your reply.

 

[tom baker]

First to clarify
*The main has GFI protection, not GFCI
*Your " Standby emergency generator", is it a Emergency Generator, Legally required standby generator or a optionally required standby generator? There is a huge difference between the requirements
* What is the reason for the triad of ground rods 6 ft apart? If you use ground rods, the most effective spacing is twice the length.
* Are you using a ufer ground? it is for more effective than a ground rod, which are then not required.

 

[kwired]

All grounding electrodes need tied together to eliminate potential between them, this effectively makes one grounding electrode system anyway.

 

[Sparky2791]

First to clarify
*The main has GFI protection, not GFCI
*Your " Standby emergency generator", is it a Emergency Generator, Legally required standby generator or a optionally required standby generator? There is a huge difference between the requirements
* What is the reason for the triad of ground rods 6 ft apart? If you use ground rods, the most effective spacing is twice the length.
* Are you using a ufer ground? it is for more effective than a ground rod, which are then not required.

Correct GFCI was a typo on my part.
Legally required (it's a school)
Triad is a company standard I follow. Likely has to do with 250.56 insuring the 25 ohms or less plus it cannot hurt to have more than 1 also 250.56 indicates 6' is minimum.
No we are not using a UFER ground.

 

[Sparky2791]

All grounding electrodes need tied together to eliminate potential between them, this effectively makes one grounding electrode system anyway.

The switched neutral transfer switches actually make it 2 systems?

 

[tom baker]

Please review section 250.50, and especially 250.30 (4)

 

[jusme123]

The switched neutral transfer switches actually make it 2 systems?

He realizes that, he stated the GEC system is all bonded together, regardless of which system is connected to it

 

[roger]

He realizes that, he stated the GEC system is all bonded together, regardless of which system is connected to it

Agreed, the code requires it. As far as the triads, in my experience doing three point FOP testing, triads are not magical in terms of lowering resistance to earth, it is a waste of time and materials.

Roger

 

[nhee2]

Agreed, the code requires it. As far as the triads, in my experience doing three point FOP testing, triads are not magical in terms of lowering resistance to earth, it is a waste of time and materials.

Roger

INFIDEL!!!

 

[tom baker]

Correct GFCI was a typo on my part.
Legally required (it's a school)
Triad is a company standard I follow. Likely has to do with 250.56 insuring the 25 ohms or less plus it cannot hurt to have more than 1 also 250.56 indicates 6' is minimum.
No we are not using a UFER ground.

Research shows the best distance apart for ground rods is twice the length. I had a paper about this from AEMC but I sent it to Mike Holt.
You are better off to use a Ufer ground and with that no ground rods are needed. Mr Ufer submitted a paper to the IEEE on this in 1963 about the effectiveness of a CEE. If you have a qualifying CEE you are required to use is, see 250.52 (3). I have been on jobs where the CEE connection wasn't made and the contractor had to go back and chip out the concrete, and make the connection. If you have a vapor barrier under the footing, leave 20 ft out and make the connection, the resistance of concrete is very low, its hydroscopic and makes an excellent ground.

 

[tom baker]

I typically see triads used at cell towers, personally I think the mass of concrete under a cell tower, used as a CEE makes a very low resistance ground. But the dirt worshipers seem to like ground rods.

 

[roger]

INFIDEL!!!

Roger

 

[kwired]

Correct GFCI was a typo on my part.
Legally required (it's a school)
Triad is a company standard I follow. Likely has to do with 250.56 insuring the 25 ohms or less plus it cannot hurt to have more than 1 also 250.56 indicates 6' is minimum.
No we are not using a UFER ground.

If you added any new footings you must use any qualifying reinforcement that is in them. If you did not add any new footings or do not have qualifying reinforcement in them then there is no CEE to connect to. Is still an option to place your own 20 feet of #4 copper in a footing that otherwise doesn't have qualifying reinforcement in it, which most will claim likely results in a better electrode than any driven rods will be.

And yes all separate systems still make a connection to a common grounding electrode system. We do this to prevent voltage differences between the associated non current carrying components of each system. If you ran steel raceways and had steel structure and other conductive components that possibly contact one another those systems inherently get bonded together anyway via such items. What you are isolating with the switched neutral in the transfer switch is the neutral current itself, and having GPF is one of biggest reasons to do so. If you did not have GFP there isn't as much reason to use separately derived standby system.

 

[tom baker]

KWired, very nice explanation.

 

[kwired]

KWired, very nice explanation.

Thank you

 

[Sparky2791]

If you added any new footings you must use any qualifying reinforcement that is in them. If you did not add any new footings or do not have qualifying reinforcement in them then there is no CEE to connect to. Is still an option to place your own 20 feet of #4 copper in a footing that otherwise doesn't have qualifying reinforcement in it, which most will claim likely results in a better electrode than any driven rods will be.

And yes all separate systems still make a connection to a common grounding electrode system. We do this to prevent voltage differences between the associated non current carrying components of each system. If you ran steel raceways and had steel structure and other conductive components that possibly contact one another those systems inherently get bonded together anyway via such items. What you are isolating with the switched neutral in the transfer switch is the neutral current itself, and having GPF is one of biggest reasons to do so. If you did not have GFP there isn't as much reason to use separately derived standby system.

Agree - In fact the GFP is the only reason I would treat the Gen Set as a separately derived system.

 

[Sparky2791]

If you added any new footings you must use any qualifying reinforcement that is in them. If you did not add any new footings or do not have qualifying reinforcement in them then there is no CEE to connect to. Is still an option to place your own 20 feet of #4 copper in a footing that otherwise doesn't have qualifying reinforcement in it, which most will claim likely results in a better electrode than any driven rods will be.

According to the NEC you only need to connect to CEE one time as part of the GES, not each time they are added to a building. This is done at the new service.

 

[kwired]

According to the NEC you only need to connect to CEE one time as part of the GES, not each time they are added to a building. This is done at the new service.

Correct, so do you already have a CEE? If not are you adding footings that may contain one?

You still likely to get better electrode out of a CEE than out of ground rods and if you have a CEE NEC does not require you to also use ground rods, though they are not prohibited.

 

[tom baker]

Likely has to do with 250.56 insuring the 25 ohms...

I realize you understand this requirement, but since there is confusion on this, I will explain for others. Its not section 250.56, but its 250.53 (2), Exception. We often hear the NEC requires a minimum of 25 ohms for ground resistance, what the NEC says is if you have a single electrode, it has to be supplemented except if the single electrode measures 25 ohms or less, a supplemental electrode is not required.
When adding electrodes in parallel a second electrode decreases the resistance by 60%, a third by 40, it the law of diminishing returns. In the trade we refer to this as "drive two and go home".

 

[tom baker]

Triad is a company standard I follow. Likely has to do with 250.56 insuring the 25 ohms or less plus it cannot hurt to have more than 1 also 250.56 indicates 6' is minimum.
No we are not using a UFER ground.

Please see IN following 250.53 A (not .56 in the 2020 NEC)