Two Utility Sources and 3-pole transfer switch

[Colorado_EE]

Hello,

We have a 2500A, 3-pole, 4-w, 480v/277v transfer switch (MTS) with main bonding jumper on the normal. The ATS has a 2500AT/2500AF breaker with GFP on the main and swtich only on the emergency source. It feeds an MCC inside a building that has a 2500A main with GFP.

We would like to bring in a second utility to the emergency source (not generator but utility). I use 250.24 for the main service and was thinking to apply for the emergency, but am running into issues:
1. I need Disconnect/OCP/main bonding jumper ahead of the MTS with GFP
2. We have the neutrals tied at the ATS, therefore we will have two bonding jumpers, one at the Disconnect and one at the MTS.
3. Can I treat them as not separately derived, therefore not needing the bonding jumper at the OCP/disconnect? Note we are industrial so I can take exception to the GFP requirement, though I do have GFP on the MCC.

Any help is appreciated.

 

[Smart $]

Need clarification...

Your first sentence says you have an MTS. Then your second sentence says an ATS has a 2500A main breaker. Which is it?

Likely doesn't make any difference to your query if one or the other... just want to get the seeming conflict resolved from the get go.

 

[Smart $]

PS: Welcome to the forum. :thumbsup:

 

[Colorado_EE]

Need clarification...

Your first sentence says you have an MTS. Then your second sentence says an ATS has a 2500A main breaker. Which is it?

Likely doesn't make any difference to your query if one or the other... just want to get the seeming conflict resolved from the get go.

Sorry, we have an MTS not an ATS.

 

[Smart $]

Sorry, we have an MTS not an ATS.

First off, I'm no expert in GFP. If someone reads this thread that is, I ask you to step up with a reply.

Just speculating, I believe you are required to have a bonding jumper at both service disconnect locations when in different enclosures and would take precedence over existing GFP if it presents a problem as designed. If it does present a problem, you can take the exception if qualified and defeat GFP on the one service, or you can redesign your service(s) so the GFP will work (I assume the whole point is to not have to change out the MTS).

 

[Colorado_EE]

First off, I'm no expert in GFP. If someone reads this thread that is, I ask you to step up with a reply.

Just speculating, I believe you are required to have a bonding jumper at both service disconnect locations when in different enclosures and would take precedence over existing GFP if it presents a problem as designed. If it does present a problem, you can take the exception if qualified and defeat GFP on the one service, or you can redesign your service(s) so the GFP will work (I assume the whole point is to not have to change out the MTS).

Correct, we don't want to change the MTS. I wonder if we can classify as a non-separately derived system (NSDS) similar to a generator but I don't see this in the code. It seems we have to classify as separately derived system (SDS). If thats the case, we need an MBJ and GFP at each disconnect. If this is the case, then we have grounds tied to bond at multiple locations. Certainly if I was starting from scratch I would go 4-pole, but I want other solutions than possibly spending 8-weeks for a new transfer switch.

 

[Smart $]

Correct, we don't want to change the MTS. I wonder if we can classify as a non-separately derived system (NSDS) similar to a generator but I don't see this in the code. It seems we have to classify as separately derived system (SDS). If thats the case, we need an MBJ and GFP at each disconnect. If this is the case, then we have grounds tied to bond at multiple locations. Certainly if I was starting from scratch I would go 4-pole, but I want other solutions than possibly spending 8-weeks for a new transfer switch.

Wish I could give you another solution, but I don't see one... other than defeating the existing service GFP under the exception. You cannot claim each as an SDS because the grounding is based on being a service supplied system.

 

[chris kennedy]

Not a SDS, your not switching the neutral.
Is POCO OK with this design?

 

[Strathead]

OK a few things. First off though, your description has the potential for confusion so if I misunderstand then I apologize up front.

You use the term "utility", "second utility" and " separately derived system", while at the same time referring to being industrial and not using the term service. So let me clarify a couple things. A Service is the feed from a utility company. If a Power company provides power to a central location and the medium voltage is customer owned, then the 480 and 208 secondaries are separately derived systems. Even though from the connection point on they are treated exactly the same, their terminology is different.

So all of that said, it is unclear whether you have a power company feed, and want to add another power company feed or other. For this purpose it likely matters. What is probably more important is whether the two power sources come from the same 480 volt source, which is unlikely. If they don't then I don't think you want the two neutrals to be touching period. I am not sure about code, but I suspect it is prohibited. As such, your only option would be a 4 pole transfer switch.

 

[fmtjfw]

Not a SDS, your not switching the neutral.
Is POCO OK with this design?

The likelyhood of two separate POCOs have the same ground potential on the grounded conductor is pretty slim. The likelyhood that they can deliver mucho amps if you interconnect them is pretty high. We used to have a building served by two separate POCOs -- with receptacles from both POCOs in the same room. Plugging one piece of computer gear in to one and another piece into the second would do marvelous things to the communications cables between the two.

I found the same effect on truck mounted telephony equipment fed by separate generators way back when I was in the Signal Corps. You don't quickly forget being lit up when disconnecting a multi-pair steel-wire shielded telephone cable with one had clinging to the side of the shelter and the other on the now "energized" cable connector.

 

[Colorado_EE]

OK a few things. First off though, your description has the potential for confusion so if I misunderstand then I apologize up front.

You use the term "utility", "second utility" and " separately derived system", while at the same time referring to being industrial and not using the term service. So let me clarify a couple things. A Service is the feed from a utility company. If a Power company provides power to a central location and the medium voltage is customer owned, then the 480 and 208 secondaries are separately derived systems. Even though from the connection point on they are treated exactly the same, their terminology is different.

So all of that said, it is unclear whether you have a power company feed, and want to add another power company feed or other. For this purpose it likely matters. What is probably more important is whether the two power sources come from the same 480 volt source, which is unlikely. If they don't then I don't think you want the two neutrals to be touching period. I am not sure about code, but I suspect it is prohibited. As such, your only option would be a 4 pole transfer switch.

Utility (S1) = POCOand Second Utility (S2) = Remote generating station (approx. 10MVA) which steps power up to MV, power approx 1mile, and then steps down at site to 480V. We have two transformers at 480v/277v tied to each end of the transfer switch fed from S1 and S2.

 

[Colorado_EE]

Wish I could give you another solution, but I don't see one... other than defeating the existing service GFP under the exception. You cannot claim each as an SDS because the grounding is based on being a service supplied system.

Sorry, you are right. We would have a Service Supplied System (SSS) and an SDS from our generator station, which arguably could be classified as either an SSS or SDS.

 

[Colorado_EE]

Not a SDS, your not switching the neutral.
Is POCO OK with this design?

This is right, but don't we have to classify as SDS and switch the neutral, or leave as is?

 

[GoldDigger]

This is right, but don't we have to classify as SDS and switch the neutral, or leave as is?

How do you switch the neutral when it is not connected?
IMHO you need to run a neutral from genny to MTS and switch the neutral (if bonded at the genny), or run a neutral and an EGC and remove any bond at the genny.

 

[Smart $]

Sorry, you are right. We would have a Service Supplied System (SSS) and an SDS from our generator station, which arguably could be classified as either an SSS or SDS.

If the local transformer of S2 is not POCO/PUC controlled, whether there is a solid connection to S1 determines whether S2 is an SDS. With the grounded neutrals not being switched, S2 is an alternate power source*, but it is definitely not an SDS.

We already covered being a service, if the transformer is POCO/PUC controlled.

*Code does not assign a defined term to this source. Some refer to it as a non-SDS source.