Separate Building Second Gen - Switched Neutral?

[W@ttson]

Hello,

I have a situation where I have 2 buildings.

Building 1:
Has the Service Entrance (480/277V, ~400A)
Has a Standby Generator
Has a 3 pole ATS with a solid neutral.Takes in the POCO and the Standby Gen feed.
The 3 Pole ATS feeds a Double Throw Safety Switch (DTSS), the DTSS also takes in a feed from a Roll up generator tap box.
The DTSS feeds a Distribution panel which in turn feeds Building 2.

Building 2:
Gets a feed from building 1 and the feed is received into a DTSS.
the DTSS second source is from a roll up generator.

Under normal every days conditions, the utility is feeding building 1 and building 2. In the case of loss of power, the ATS switches to the Standby generator. In the case that the standby generator fails, the client can roll up a generator at building 1 to feed building 1 and building 2. However, if the client cannot acquire a generator large enough to feed both building 1 and building 2, the client has the option to feed building 2 from its own roll up generator.

See attached quick sketch for an overview of the above. View attachment Multiple Gens Switched Neutral.pdf

Question:
Does the Double Throw Safety Switch at building 2 need to have a switched neutral? My concern is during the situation of building 1 utilizing roll up gen and building 2 utilizing roll up gen.Even if all of the generators are properly not having their neutrals bonded to ground, in the case of a fault at building 2, the fault path back to the generator at building 2, would need to go through the neutral - ground connection located all the way back at the service entrance. The neutral in that area would already be loaded with the loads of building 1 and I am worried that it could be overloaded.

 

[ActionDave]

Since the neutral only carries the imbalance between the phases I don't see it burning up because of a fault. If it comes to that you'd prolly burn up your generator too so having a switched neutral is no guarantee of totally avoiding a catastrophic failure somewhere.

 

[texie]

While your application is a little more elaborate than often seen, the same situation arises when you have 2 buildings each with their own service and have 1 genset suppling both. Each building having a single ATS. I have seen this arrangement a number of times.
The code does not seem to directly address this. I've always felt that this indirectly violates 250.24(A)(5). As such, I believe switched neutrals should be used and set the genset up as a SDS. But that is just my opinion and not sure if others would agree. I have never seen this situation debated.

 

[W@ttson]

While your application is a little more elaborate than often seen, the same situation arises when you have 2 buildings each with their own service and have 1 genset suppling both. Each building having a single ATS. I have seen this arrangement a number of times.
The code does not seem to directly address this. I've always felt that this indirectly violates 250.24(A)(5). As such, I believe switched neutrals should be used and set the genset up as a SDS. But that is just my opinion and not sure if others would agree. I have never seen this situation debated.

I agree that if it were two utility services you would definitely need the switched neutral because each service would require its own grounded neutral connection. This is a little different since I can very well force there to only be one neutral to ground connection.

 

[W@ttson]

Since the neutral only carries the imbalance between the phases I don't see it burning up because of a fault. If it comes to that you'd prolly burn up your generator too so having a switched neutral is no guarantee of totally avoiding a catastrophic failure somewhere.

that is an interesting point.

 

[synchro]

Question:
Does the Double Throw Safety Switch at building 2 need to have a switched neutral? My concern is during the situation of building 1 utilizing roll up gen and building 2 utilizing roll up gen.Even if all of the generators are properly not having their neutrals bonded to ground, in the case of a fault at building 2, the fault path back to the generator at building 2, would need to go through the neutral - ground connection located all the way back at the service entrance. The neutral in that area would already be loaded with the loads of building 1 and I am worried that it could be overloaded.

When powered through the POCO, the neutral conductors must handle the building 1 and 2 loads and also conduct the currents from any L-N faults "all the way back" to the service so that the circuit's OCPD can clear such faults. With a roll-up generator supplying building 2 through a DTSS with solid neutral, as you mentioned any L-ECG fault currents would flow through the EGC back to the bond at the service and return through the neutral to the generator supplying building 2. In each of these two cases the neutral conductors must handle both the load currents and the fault currents, and so I don't think a solid neutral generator connection through a DTSS at building 2 will increase loading on the neutral beyond what it's already designed to handle.