Grounding Electrode on Generator

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gumbyElvis

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I am installing a transfer switch on a generator for a home owner. I do not have the transfer switch yet but the one I am looking at does not switch the grounded conductor so it is not a separately derived system.

I plan on feeding the transfer switch via the 240 volt receptacle on the generator, so I will have my grounding conductor and grounded conductor from the plug connected at the main bonding jumper in the homeowners service.

So far so good, I hope. My question is about a requirement to hook the generator to a grounding electrode. I know that this is the function of the main bonding jumper in the service, so why would I want to introduce yet another grounding connection? I have looked at article 250, I also have Mike's grounding and bonding dvd set and do not see a grounding electrode connection to the frame of the generator as a requirement.

Am I missing something?
Rob
 
gumbyElvis said:
... I do not have the transfer switch yet but the one I am looking at does not switch the grounded conductor so it is not a separately derived system ... Am I missing something?
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Your direct question re: the grounding electrode has already been answered.

Re: "missing" ...

I'd just add that you'll need to ensure that the generator's neutral is isolated from the ground at the generator ... otherwise you'll end-up with dual bonds -- creating parallel neutral paths -- and a 250 violation.
 
Rob, This is a sticky situation. I went through this in late 04 early 05 after hurricane charley. The manufacturers of the generator state that a ground at the generator is required for warranty and some of my local inspectors agree. Then I attended mike's ceu course in July 05. This exact scenerio came up. Here are the questions that came up. Why would you want to create a second path to ground? Why would you want to route the voltage to ground through your $xxxx.xx generator? The way I understand it, by the NEC, no secondary ground is required.


Todd
 
toddpatrick said:
Rob, This is a sticky situation.
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It sure can be.

Some gensets might indeed need to be grounded to function properly -- or perhaps their neutral is inherently grounded and simply can't be isolated; in those cases you'd need to install a neutral-switching ATS and follow the sep-derived rules.

Other manufacturers make it easier on you. (e.g. Kohler actually states in their installation instructions that their gensets are designed to properly function either way; and that it's up to the installer to determine the proper configuration so as to allow the installation as a whole to be code-compliant)
 
dereckbc said:
Since it is not an SDS application, there is no GEC requirement.
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I'll jump in to play devils advocate.

If you supplied a separate detached residential garage with 4 wires according to 250.32(B)(1), you would have a separate neutral and equipment ground and yet 250.32(A) says that garage still needs its own electrode.

Since 250.32(A) says "structure" and not just building, why would a generator be different than a garage building. . SDS or not doesn't make any difference to 250.32(A).

David
 
Also, what is the big deal about grounding it? If you had a 4-wire feeder to a building, that equipment ground would need to be connected to an electrode at the detached building.

So put a rod in the ground next to the generator and run a GEC from the frame (or it may even have a grounding lug on the frame) to that rod. The bonding jumper in the main service will be a secondary path to ground and also connect your neutral to the equipment and earth grounds.

If you remove the plug from the generator that goes to the house, then your grounding system will become isolated from the neutral and the house. So if you do that, think through it (and it isn't so bad because both the AC conductors are now floating and have no reference to earth no neither one should shock you).
 
suemarkp said:
Also, what is the big deal about grounding it? If you had a 4-wire feeder to a building, that equipment ground would need to be connected to an electrode at the detached building.
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True, but the neutral at that same out-building would NOT be bonded to the EGC at the out-building (nor would neutrals be bonded to ground at any other subpanel).
 
Here is an illustration of the earthing of an unswitched neutral transfer switch system from Ed MacLarren, take note that the Generator frame is not bonded to the winding(s).

Gen4.gif


Roger
 
David,
Since 250.32(A) says "structure" and not just building, why would a generator be different than a garage building. . SDS or not doesn't make any difference to 250.32(A).
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You don't run supply conductors to the generator. 250.32(A) only applies to structures that are supplied by feeders or branch circuits.
250.32 Buildings or Structures Supplied by Feeder(s) or Branch Circuit(s)
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Don
 
I've taken the liberty to "annotate" a Mike Holt drawing to hopefully illustrate why bonding the neutral at the non-SDS genset is a bad idea.

Note: All drawing markups assume that the premises are powered by the utility and the generator is secured.

Image 1 is the original drawing of an non-SDS system:
image1.PNG



Image 2 shows the impact of a 2nd neutral bonding jumper added at the genset (circled in green)

You can easily see how the parallel neutral paths are formed.
image2.PNG



Image 3 goes one step further by actually opening the "proper" neutral conductor and illustrates how all neutral current would then attempt to flow via the EGC / bonding system.
image3.PNG
 
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NoVA Comms Power said:
True, but the neutral at that same out-building would NOT be bonded to the EGC at the out-building (nor would neutrals be bonded to ground at any other subpanel).
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Nor should it be at the generator in this case. I don't see this generator as any different than a 4-wire feed to panel. The only thing that grounding requires is connecting a GEC and electrode to a wire. That wire is the 4th equipment gruonding conductor in both cases.

Because you have to ground (earth) the generator locally doesn't mean you have to put a neutral-ground bond locally.
 
suemarkp said:
... Because you have to ground (earth) the generator locally doesn't mean you have to put a neutral-ground bond locally ...
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Exactly.

It appears that we were in complete "violent agreement" all-along. :smile:

And to be completely honest ... I'd mis-read some of the other posts ... and my reply quoting you was as a result of my own mis-reading.
 
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