Generator Ground

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I

IllinoisElectrician

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Ok this might sound like dumb question but here goes..

Why do you have to install a grounding electrode by a generator if your transfer switch connects the generator neutral to the service neutral and you also have an EGC? I feel like that's not a seperately derived system... According to 250.20(D) (Especially FPN No. 1) it's saying that it is not considered a seperately derived system if the generator neutral and service neutral are solidly connected.

Just so you don't have to look, FPN No. 1 Says "An alternate power source such as an onsite generator is not a seperately derived system if the grounded conductor is solidly interconnected to a service supplied system grounded consudctor. An example of such situations is where alternate source transfer equipment does not include a switching action in the grounded conductor and allows it to remain solidly connected to the service supplied grounded conductor when the source is operation and supplying the load served."

So here's my question. Does the switcing action of the transfer switch not count as "solidly connected" and therefore it's a seperately derived system and need supplementary grounding? Or is it saying that as long as the switch bonds the neutrals then it's not a seperately derived system and therefore doesn't need grounding? Also, wouldn't the EGC from the panel with the transfer switch to the generator be sufficient ground (EMT into that panel, and EMT from panel to gen), just as good if not better than a supplementary grounding electrode? The frame is connected to an EGC, the neutral is common with the service neutral. Why the grounding electrode?

Please help me understand, I'm rather new and I've just been taught to ALWAYS install a grounding electrode, and I realize this is a safe practice but I just want to know exactly what 250.20 means.

Thanks :)
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
When the grounded conductor (neutral) is not switched by the transfer equipment the system is not considered separately derived and a grounding electrode at the generator is not required.

When the grounded conductor (neutral) is switched by the transfer equipment, then the system is considered separately derived and a grounding electrode is required at the generator in accordance with 250.20(B) & (D). See also 250.30(A)
 
I

IllinoisElectrician

Member
When I've installed them the gen neutral is switched through the transfer equipment and pounded in the electrode, but can you install them just landing the gen neutral on the main panel bus? That way it's not "switched" and now I can avoid the grounding electrode?

I just don't know if not switched means "permenantly connected" or "never connected" it's just unclear to me.
 
tom baker

tom baker

First Chief Moderator
Staff member
Location
Bremerton, Washington
If the neutral is not transfered "solidly connected" then no ground rods are required.
Electricans and inspectors don't understand this, so they require ground rods. Now the mfgs (Generac for one) get caught so they put on the instructions "ground rods at generator required" as they don't understand the code either, and roll over.
Now that is in the instructions, 110.3(B) applies and now the inspector makes you install ground rods
Cummins/Onan understands ground rods are not required and they have a great single point ground drawing in the manual.
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
Installing the generator as a separately derived is the safest way. If some removes the grounding at the utility source for maintaince or what ever reason while operating on the emergency source they will be at risk for serious harm. Hence the reason for 700.8(B), 701.9(B), & 702.8(B).
 
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