4 pole transfer switch and GEC

Catman800

Member
I have question regarding article all of 250.30, 250.50 and 250.58.

Background:
I also have pictures of the installation that I could send if needed.
I have the contractor and his engineer telling me this is OK. I am not convinced of this.

I have a 500KW, 3 phase, 480v generator that is the stand-by power for a new building. The generator is clearly a separately derived system due to the 4 pole auto transfer switch. The grounding electrode system for the bldg. consists of the water pipe, foundation, and bldg. steel which are properly bonded and a GEC is properly terminated to the 800 amp utility disconnect main bonding jumper.
The contractor installed 2 ground rods local to the generator and did not bond to the grounding electrode system for the bldg. My opinion is that the separately derived system (the 500 kw generator) shall be connected to the same grounding electrode system for the bldg. served.

Questions.
1. Are the rod electrodes at the generator adequate to perform their intended function?
2. Does the generator need to be connected to the building's grounding electrode system?
3. In your opinion, should I pursue this further with the contractor?
 

ceb58

Senior Member
I have question regarding article all of 250.30, 250.50 and 250.58.

Background:
I also have pictures of the installation that I could send if needed.
I have the contractor and his engineer telling me this is OK. I am not convinced of this.

I have a 500KW, 3 phase, 480v generator that is the stand-by power for a new building. The generator is clearly a separately derived system due to the 4 pole auto transfer switch. The grounding electrode system for the bldg. consists of the water pipe, foundation, and bldg. steel which are properly bonded and a GEC is properly terminated to the 800 amp utility disconnect main bonding jumper.
The contractor installed 2 ground rods local to the generator and did not bond to the grounding electrode system for the bldg. My opinion is that the separately derived system (the 500 kw generator) shall be connected to the same grounding electrode system for the bldg. served.

Questions.
1. Are the rod electrodes at the generator adequate to perform their intended function?
2. Does the generator need to be connected to the building's grounding electrode system?
3. In your opinion, should I pursue this further with the contractor?
This is from 250.30
(8) Bonding. Structural steel and metal piping shall be
connected to the grounded conductor of a separately derived
system in accordance with 250.104(D).
Now the question is did they run 5 wires to the ATS? Did they bonded the Neutral and GEC in the generator? If so they should have ran a EGC from the generator to the transfer switch where it was bonded to the cabinet with the EGC from the service disconnect. This would tie all grounding systems together.
 

Smart $

Esteemed Member
For grounding under Article 250, we can start at 250.35 and (A) therein, which says 250.30 applies in this scenario. However, because the source is separate from the building supplied, we must also consider 250.32(B)(2), for which compliance is dependent on whether the supply is with or without overcurrent protection where the feeders originate. What it boils down to is 1) if there is overcurrent protection at the generator, the grounding conductor run with supply conductors is sized per 250.122, or 2) if there is no overcurrent protection at the generator, the grounding conductor run with supply conductors is sized per 250.66 (i.e. GEC size).

IMO, a grounding electrode system is required at the generator either way, but one is specifically required under 250.32(C), 2011 edition.
 

Smart $

Esteemed Member
... 2) if there is no overcurrent protection at the generator, the grounding conductor run with supply conductors is sized per 250.66 (i.e. GEC size).

...
I guess I left out some details on this condition...

A disconnect with ocp would be required at the building, either outside or inside nearest entry point... and the building GES needs to be connected to this disconnect's grounding bus (or neutral if this is where the system bonding jumper is located). The system bonding jumper (N-G) can be connected at this disconnect or at the generator, but not both.
 

ceb58

Senior Member
I guess I left out some details on this condition...

A disconnect with ocp would be required at the building, either outside or inside nearest entry point... and the building GES needs to be connected to this disconnect's grounding bus (or neutral if this is where the system bonding jumper is located). The system bonding jumper (N-G) can be connected at this disconnect or at the generator, but not both.
I also left that out also just to keep from mudding the waters. It would be nice to see photos of what the OP has.
 

ceb58

Senior Member
In the drawing it states there is a breaker on the generator that is not shown, correct? If so then it looks like a text book install. If you follow your green line from the generator through out the drawing it winds up at the service disconnect where the utility neutral and the GEC's are bonded. So for all intents and proposes, through the grounding bars, the generator is tied to the building grounding system through the EGC. I know that there are times that you must switch the neutral but even then, when it is installed correctly, you will never break the relationship between the utility neutral and the generator neutral.
I have the contractor and his engineer telling me this is OK
What dose the inspector say?
In your opinion, should I pursue this further with the contractor?
From what you have shown in the drawing, No. I don't know where you stand in the food chain where you work but.......... the contractor and especially the engineer may get a little bent out of shape with the powers to be with some one, unless you are authorized by your company, to over see the construction and installation of equipment, questioning their installs. I'm just say'en.
 

Smart $

Esteemed Member
I contend that the GEC must be connected to all the electrodes present at the bldg being served, must be used. (250.50) Not just a ground rod at the generator.
There are many out there that confuse this issue the same as you have. You have a grounding electrode system (GES) at/for the building. If the generator is outside and remote to the building, it is a separate structure under the NEC, and requires its own GES per 250.50. There is no requirement in the NEC which says separate GES must be interconnected as one large GES.

250.58 says where feeders supply a bldg and are required to be connected to a grounding electrode, the same electrode must be used.
That is for grounding at the building, i.e. at the load end of the feeder.

Instead of installing a GEC to bldg steel, like this, the contractor drove 2 ground rods local to the generator. Is this adequate?
Yes, and the 2 ground rods are in compliance with GES requirements for a separate structure.

Here's a revised diagram... View attachment EOF revSMART.pdf
 
Top