Generator Grounding Electrode & GEC installation

[Richard S]

We have a project were we are installing an outdoor diesel generator for an optional standby system. Generally we use 4-Pole automatic transfer switches so the generator is considered a separately derived system (SDS). Because it will be a SDS, we will install a grounding electrode at the generator. Because of the required concrete work, this will be a concrete encased electrode (Ufer) and GEC will be installed per 250.30.

The question I have is if we need to connect this new grounding electrode to the existing grounding electrodes within the building per 250.52 & 250.58?

 

[texie]

We have a project were we are installing an outdoor diesel generator for an optional standby system. Generally we use 4-Pole automatic transfer switches so the generator is considered a separately derived system (SDS). Because it will be a SDS, we will install a grounding electrode at the generator. Because of the required concrete work, this will be a concrete encased electrode (Ufer) and GEC will be installed per 250.30.

The question I have is if we need to connect this new grounding electrode to the existing grounding electrodes within the building per 250.52 & 250.58?

I would say yes because even though it is outside it is not a structure but rather equipment. This goes back to the old argument of what is a structure. The 2017 has made an attempt to clarify this with new language in the definition of a structure.

 

[Richard S]

I would say yes because even though it is outside it is not a structure but rather equipment. This goes back to the old argument of what is a structure. The 2017 has made an attempt to clarify this with new language in the definition of a structure.

This generator will have (2) output breakers and will feed (2) separate ATS's located in adjacent buildings. Based on the need to connect the new grounding electrode for the generator to the existing grounding electrodes within the buildings, then basically both buildings will share a common grounding electrode system?

 

[texie]

This generator will have (2) output breakers and will feed (2) separate ATS's located in adjacent buildings. Based on the need to connect the new grounding electrode for the generator to the existing grounding electrodes within the buildings, then basically both buildings will share a common grounding electrode system?

Gee, good question. Not you have me wondering. I don't see how it would be required to go to the second building. But then that begs a question of why would one need to do at the first building. After all, both are feeders. Maybe my first reaction was wrong.

 

[DrSparks]

We have a project were we are installing an outdoor diesel generator for an optional standby system. Generally we use 4-Pole automatic transfer switches so the generator is considered a separately derived system (SDS). Because it will be a SDS, we will install a grounding electrode at the generator. Because of the required concrete work, this will be a concrete encased electrode (Ufer) and GEC will be installed per 250.30.

The question I have is if we need to connect this new grounding electrode to the existing grounding electrodes within the building per 250.52 & 250.58?

All grounding electrode systems for a building need to be tied together prevent a potential difference between the two systems.

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[kwired]

I think the question is when is it far enough away that you should call it a separate building/structure? And when you do that it gets it's own GES at that separate structure.

 

[necGuru]

How far away is the generator from the building?

 

[Richard S]

How far away is the generator from the building?

The generator is only about 100' from the second building it will also serve. Basically the generator is in an equipment yard for one building that the client occupies. It will provide standby power for that building, but also provide standby power for the adjacent building that the client also occupies. About 100' between generator and adjacent building.

 

[Richard S]

All grounding electrode systems for a building need to be tied together prevent a potential difference between the two systems.

This makes sense and corresponds to 250.50 "All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system." The 2014 Handbook has a commentary addressing this in 250.30 and Exhibit 250.13.

Right now I'm thinking about the best and most practical way of achieving this bonding for the grounding electrodes. The generator will have (2) 400A output breakers that feed the (2) ATS. Per 250.122 EGC will be #3 CU. Raceways will be for the most part PVC. I think installing the bonding conductors in the feeder raceways would be most practical. If I increase the size of my EGC so that it is sized appropriately for a bonding conductor, could it serve as both an equipment ground and bonding conductor? I know that GEC's need to be continuous, but what about bonding conductors? Lastly what size would bonding conductor need to be? I could always size for 3/0 CU for CYA, but I'd like to figure out correct size. One building has a 1200A service and the other a 4000A service. The GEC for the services to their existing grounding electrode is 3/0 CU.

 

[shortcircuit2]

All you need is an EGC (#3 copper for each 400-amp feeder) between the generator and the transfer switch because the overcurrent protection is at the generator...(2 x 400-amp breakers.)

Given that the GES for the generator is "at" the generator...it would not need to be bonded to the GES "at" either building that it serves.

The above is based on the NEC

Now if we dive into NFPA 780 or the Motorola R56 Grounding standard...and the site grounding is designed for lightning protection, NFPA 780 and R56 require common bonding so everything is at equipotential. Then bonding in your installation is supported.