One Genset, Two Services

[soloelelctrician]

I have a question about wiring a genset for a farm that is fed from 2 services.

Customer bought a 250A genset to feed his farm. Main farm service is 400A. Opposite end of the farm has a separate, 200A service, from a separate utility transformer that feeds 2 hog barns. Hog barn was wired with a 200A 2pole ATS 10 years ago to feed barns in case of outage. Recently customer ran underground wire from main farm service to hog barn service and wants to power both services from the new genset located in the hog barn.

I understand that this genset should be installed as a SDS, which means the ATS and main farm service will need a neutral break. Cost is driving factor and it'd be about 5K to put in new ATS guts with a neutral break and a 3P manual switch for the farm. How important is it to have a neutral break option in this situation? Will a shared neutral path from the two services really be an issue?

 

[qcroanoke]

I have a question about wiring a genset for a farm that is fed from 2 services.

Customer bought a 250A genset to feed his farm. Main farm service is 400A. Opposite end of the farm has a separate, 200A service, from a separate utility transformer that feeds 2 hog barns. Hog barn was wired with a 200A 2pole ATS 10 years ago to feed barns in case of outage. Recently customer ran underground wire from main farm service to hog barn service and wants to power both services from the new genset located in the hog barn.

I understand that this genset should be installed as a SDS, which means the ATS and main farm service will need a neutral break. Cost is driving factor and it'd be about 5K to put in new ATS guts with a neutral break and a 3P manual switch for the farm. How important is it to have a neutral break option in this situation? Will a shared neutral path from the two services really be an issue?

How are you going to hook the generator output to the 400 amp service in the main barn?
To me you would need a Manual transfer switch in front of the 400 amp service to disconnect the service.
And I'm not so sure you could legally do that with two Seperate services and one genset.
Let's see what others have to say.

 

[soloelelctrician]

There is an existing manual DPDT switch (no neutral break) on the load side of the 400A disconnect. Wires feed to multiple buildings after that. Load will never be that much, it was sized that large for a bin dryer about 5 years ago. I have read some other posts on gensets and I can't seem to determine if it matters that there are 2 utility transformers for the 2 services vs 1 xfmr and 2 services. Will neutral current flow to utility transformer if this generator is not separately derived? I do not want to put line man at risk...

 

[hillbilly1]

There is an existing manual DPDT switch (no neutral break) on the load side of the 400A disconnect. Wires feed to multiple buildings after that. Load will never be that much, it was sized that large for a bin dryer about 5 years ago. I have read some other posts on gensets and I can't seem to determine if it matters that there are 2 utility transformers for the 2 services vs 1 xfmr and 2 services. Will neutral current flow to utility transformer if this generator is not separately derived? I do not want to put line man at risk...

Very unlikely, but always a possibility the utility neutral current may flow through the generator if the utility hv ground/ Neutral line gets broken, and their hot is still connected.

 

[Fred B]

2 services will require 2 ATS. like described here:

Of course with 2 separate services from 2 separate transformers it would really require SDS configuration that will switch the neutral. A power outage could in theory could result in only one structure loss of power and sharing a neutral could energize to the down side of the system endangering the lineman.

Also a load calculation would need to be done to ensure the genset is not overloaded while powering the what looks like a potential of 600A between the 2 services. Potential for load shedding might also be considered.

Recently customer ran underground wire from main farm service to hog barn service and wants to power both services from the new genset located in the hog barn.

This factor really not your concern as it sounds like a start of a DIY by the farmer that he got lost in. If the customer unwilling to fund correct installation its a better run away than try to cob something together to save them money. see no CHEAP options.

 

[wwhitney]

Of course with 2 separate services from 2 separate transformers it would really require SDS configuration that will switch the neutral. A power outage could in theory could result in only one structure loss of power and sharing a neutral could energize to the down side of the system endangering the lineman.

I don't see how having one solidly connected conductor could energize a system in a way that would endanger a lineman.

But I do believe that with two separate services it would be a violation of 250.24(A)(5). Avoiding that violation would only require one of the two ATSs to switch the grounded conductor; the other could use a solidly connected grounded conductor.

Cheers, Wayne

 

[soloelelctrician]

2 services will require 2 ATS. like described here:

Of course with 2 separate services from 2 separate transformers it would really require SDS configuration that will switch the neutral. A power outage could in theory could result in only one structure loss of power and sharing a neutral could energize to the down side of the system endangering the lineman.

Also a load calculation would need to be done to ensure the genset is not overloaded while powering the what looks like a potential of 600A between the 2 services. Potential for load shedding might also be considered.

This factor really not your concern as it sounds like a start of a DIY by the farmer that he got lost in. If the customer unwilling to fund correct installation its a better run away than try to cob something together to save them money. see no CHEAP options.

So am I correct in getting equipment (ATS neutral break & a TPDT manual switch) to make the generator a SDS? According to Wayne I could in theory leave the 2pole ATS equipment because the utility is bonded there, and make the main farm service a neutral break.

Also I agree there are rarely any cheap options that make things compliant.

 

[Fred B]

So am I correct in getting equipment (ATS neutral break & a TPDT manual switch) to make the generator a SDS? According to Wayne I could in theory leave the 2pole ATS equipment because the utility is bonded there, and make the main farm service a neutral break.

Also I agree there are rarely any cheap options that make things compliant.

Without a second ATS at the second service, the gen set would only be operative if the power was out at the first service as well as the second service. (With the 2 services being fed from 2 separate transformers there is a higher likelihood of only one service being interrupted.) A manual transfer switch will not activate and start the gen set, relying on the first service ATS to start the generator.

Personally wouldn't want to use only one SDS transfer as seen too many issues with neutrals that by combining the 2 service neutrals would only add to the potential issues.

Now not sure from the poco as to frequency timing between the 2 transformers, unless specifically set up for synchronization, but in other applications when synching into the poco system a means to time (or synch) the frequency of the 2 sources is required and may result in damage if not done. This also effecting the neutrals. So to have the 2 source neutrals connected in common can lead to significant damages to POCO system. That can be also from the Gen to POCO on ones sided transfer.

one source with links:
Synchronization between two transformer neutrals means ensuring that the voltage levels, phase angles, and frequency of both neutrals are precisely matched before connecting them together, preventing large circulating currents that could damage the transformers or other equipment when paralleling them; essentially, they need to be "in sync" to share the load evenly. [1, 2, 3]
Key points about synchronizing transformer neutrals: [1, 2, 4]