Generator bonding of the grounded conductor

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InspectorPatches

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Electrical Inspector/plans reviewer 1
Hi everyone, I've recently been hired as a Electrical Inspector after years in the trade and currently have plans that I'm reviewing that my gut is telling me should be addressed. They are renovating a building and adding 2 back up generators. On the plans it notes to not bond the neutral cond. to the ground. My concern, and I could be totally wrong but I'd rather be safe than sorry, is that in a loss of normal power and a fault on phase A that the path the current would have to take would be too great to trip the breaker if the only bonding jumper is at the main. I've read 250.30 for the past few days and am just having a really hard time grasping the actual requirement. I'm thinking they need to bond neutral and ground at generators and switch the neutral at the transfer switch. I've attached a partial pic of the schematic in question with what I'm thinking worst case scenario with current drawings. Go easy on me I've been working towards being an inspector and the last thing i want is to approve something that could be potentially dangerous in the future and currently have no one to reach out to with questions like this. I'm not looking for someone to do my job for me as much as I'm looking for a better understanding. Im having a hard time getting the pic to attach I'll try again later.
 
The system can be set up as an SDS or a non-SDS. Sounds like you're describing a non-SDS system which would be code complaint because the neutral is not switched when on generator power. This is a design issue not a code issue.

Welcome to the Forum. :)
 
ok so from what I'm gathering is that the generator itself being a generator isn't considered a SDS unless the neutral is switched. So the SDS requirements are dependent on how its wired. How though is that safe, unless, does the neutral current return through the utility or does it still need to make its way back to its source the generator? In the case of a fault would the fault current go back through the utility be sufficient enough to trip the breaker at the generator?? Or would it have to return through the main bonding jumper to get back to the generator neutral? I'm sorry if these might be redundant or unnecessary questions, I tend to overthink a lot of things when it comes to this stuff, but only because I have the most respect for electricity and all the work involved with it. I apparently haven't met the threshold to upload pictures yet but still working on it.
 
(y)
That is correct for a system like yours, a non-switched neutral, non SDS installation.
 
After doing some more research I'm pretty sure I have a way better understanding. Thank you. But just on a side note does anybody else think that generators should be considered SDS and be required to meet NEC as a SDS? I just think that the path for fault wouldn't be adequate if having to go all the way to the main jumper. but I also haven't ran the actual numbers to figure that out it just "feels" like it wouldn't be enough and I get that feelings don't mean anything when it comes to facts lol.
 
InspectorPatches said:
After doing some more research I'm pretty sure I have a way better understanding. Thank you. But just on a side note does anybody else think that generators should be considered SDS and be required to meet NEC as a SDS? I just think that the path for fault wouldn't be adequate if having to go all the way to the main jumper. but I also haven't ran the actual numbers to figure that out it just "feels" like it wouldn't be enough and I get that feelings don't mean anything when it comes to facts lol.
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You are suggesting to require that all generators should have the neutral switched thereby making it an separately derived system. I doubt that would ever get approval from the code panel. You would need some substantiation to show that a non separately derived system is not safe.
 
Dennis Alwon said:
You are suggesting to require that all generators should have the neutral switched thereby making it an separately derived system. I doubt that would ever get approval from the code panel. You would need some substantiation to show that a non separately derived system is not safe.
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I almost feel like that would be the sure fire way to be safe about it but I 100% agree with you. There would have to be some concrete evidence to prove that that path for fault wouldn't be good enough. Its more of a theoretical question I guess. I was simply asking if anyone else thought that the path the fault current would have to take would be too great of a distance to trip the breaker. And/or if there was a formula or way to determine if the path would be adequate. I've never done the math or even know of a way to determine that. Just a theory thought I'm having
 
I see no reason why a generator should be required to be setup as an SDS. The extra length of travel for the fault current is not an issue. We build highrise buildings and often feeders that go to the roof can be 750' long (or longer) and the MBJ is still in the service in the basement.
 
I also wonder how much impedance of the fault path matters with a generator. They generally dont have a huge available fault current. On a feeder fault, the breaker may trip on thermal overload or the generator stalls. A small branch circuit fault on a large generator may trip the branch breaker via magnetic trip. A very long path in the generator feeder EGC may prevent the mag trip from working, but lack of high available fault current should limit damage.
 
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