Generator EGC for generator panel

[bjenks]

Here is my design situation: a new generator is being installed that will feed a generator panel with 16 breakers feeding out to 4 pole ATSs. I will have a disconnect in the panel and there will be an electronic breaker in the generator that is about 75 ft from the building. Since the first disconnect is in the generator and it is a seperately derived system I believe that I have to bond the N-G in the generator and not at the panel (tell me if I am wrong). I was planning on driving an auxillary ground rod and bond it to the frame of the generator. Now it seems to me that I have to run an GEC from the generator back to the GES and a EGC from the generator to the panel. I am wondering if this is legal or not recommended: I would like to run just one EGC/GEC to the generator panel. Then run EGC to all the ATS and a GEC to the GES. Is this legal or poor engineering? I am wondering about running a GEC in the same conduit as the conductors when it is acting as a EGC too. If the breaker wasn't inside the generator I would have done the N-G bond in the panel and wouldn't be asking this question. Maybe I just have to spend the extra money and run an EGC and a GEC. The generator will have Ground Fault monitoring which might be another obstacle. thanks for you input.

 

[7anoter4]

You abbreviations [N-G, GES, EGC] even if they are according to NEC art.250 are difficult to decode.
See attached a sketch according to interpretation of the Code by University of Massachusetts Amherst College of Engineering for 4 pole ATS
See:

http://www.ecs.umass.edu/ece/hollot/ECE497DS06/ESD_2.pdf

In my opinion the built-in generator switch does not change the principle:
Generator Yoke Grounding -from utility [service] grounded grounding conductor.
The Load Neutral Grounded conductor is fed from ATS once from Generator and once from Service simultaneously with live phases.
At Generator end has to be a Grounding Electrode to connect only the Neutral Grounding Conductor. Two breaking place does not change the system.

 

[waross]

The basic design is to leave the neutral solidly connected through the transfer switch so that the neutral/ground connection at the main panel serves as the generator ground also. This allows ground fault detection equpment to function correctly.
A separate ground conductor is run from the main panel ground to the generator frame.
This basic connection has worked well for generations.
There are field conditions and demands that may make another grounding system mandatory. I have found it well worth the trouble to try to eliminate such conditions so that the simple, basic, dependable grounding system may be used.
When the basic connection can not be used, the system becomes more expensive and more complex. This also implies less dependable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree with you, waross, without any doubt, but for 3 pole ATS only. Four pole ATS- [I am not sure I know why 4 poles is better than 3!] is another situation. Of course, the grounding protection is more complicated –may be it requires 2 or 3 different protection location-one for Generator one for the Service or Load. I think-but I am not sure-the Code takes into consideration that the Utility may feed some circuits of Generator or Primary Motor –Diesel, Turbine or else so the ground current has to return to Service Ground and only Neutral Grounding will return to the Generator. Any way, I don't like to break the neutral and of course the neutral grounding.

 

[bjenks]

I would much rather use a 3 pole too. However, for Hospitals you have to have two levels of Ground Fault Protection. For >800A at 480V requires GFP. Look at these two situations that I have created showing how a GFP could get activated during a ground fault. In order to solve this problem you have to go with a 4 pole or some other form of neutral isolation. Once I do this it is now a seperately derived source. Then I have to bond the neutral to ground somewhere from the seperately derived source and the first OCPD which in my case is located at the generator. Just like a transformer you have to bond to the building which is actually an grounding electrode conductor (GEC) and still run a equipment grounding conductor (EGC). For transfomers the GEC bond might not be that far. For my situation the nearest location is over 150 ft. Further I still have to run a EGC now to the new panel. However I am really running two ground conductors that are tied together at the generator, but ran to two different location. I was wondering if there was a better solution like running only a GEC to the first panel and then split it out for the GEC and then a EGC. I hope this make more sense and remember there will be about 15 ATSs off the generator panel that have to all be 4 pole because the Main utility disconnect is 2500A

 

[bjenks]

7abiter4: I looked at Figure 9-8B and agree this is typical for a 4 pole. Only difference is for my case the ATS would be a panel then an ATS, but same application. If you notice a grounding electrode is shown on the generator. which would also be bonded to the neutral of the generator and at the generator. Now you have to connect it back to the utility grounding electrode system. I have given it careful though and I guess I have to run two seperate ground conductors one will be in the conduit with the conductors which is my EGC to the panel. I will then run outside the conduit my GEC that will go back to the utility GEC. This way when on utility the generator is properly grounded to the grounding electrode system.