Two feeders from transformer secondary to seperate LV Switchgear lineups

[mull982]

I'm looking at an application where the customer wishes to have a single transformer feed two separate LV 480V switchgear lineups located in separate buildings. The transformer is a 2000kVA transformers with a 13.8kV primary breaker and 480V secondary. The first LV Switchgear is located right next to the transformer and would likely be close coupled with bus to the transformer. Each LV switchgear lineup has (4) feeder breakers)

In trying to feed the 2nd LV switchgear lineup I see two options that would not be NEC code compliant:

Option 1: Tap the incoming bus of the 1st LVA lineup to cable feed the 2nd lineup. From what I can tell this would be a violation of 240.21 ( Tapping a tap) as well at 450.3 Note 2 requiring breakers to be grouped in a single location. Is it still considered a tap if you use cable with same ampacity?

Option 2: Run 2 separate feeders from the transformer secondary to the two LVA lineups. This however appears to violate 450.3 Note 2 as well.

In looking at this I see two options that perhaps will be NEC code compliant but wanted to get feedback from others.

Option 3: In the first lineup have a main breaker for that lineup but before that main breaker have a feeder breaker to feed over to the 2nd lineup. The 2ne lineup can then be MLO. You would have to essentially double the ampacity of the first switchgear lineup. (800A to 1600A). The other option would be to just have (5) feeder breakers in the first lineup with no main breaker (one feeding 2nd lineup) so that both lineups would not have mains. This would not be idea for coordination.

Option 4: If I put a CT on the secondary bushings of the transformer and I have those CT's connected to a relay on the 13.8kV primary breaker can I count those CT's as a secondary main protection? If so then then I can run the two separate feeders from the transformer secondary to the lineups because the feeders will be on the load side of the CT's which are serving as the secondary transformer protection and secondary conductor protection. Is there any reason why I can count these secondary CT's as the secondary protection. To me this seems like the best approach.

 

[electrofelon]

I'm looking at an application where the customer wishes to have a single transformer feed two separate LV 480V switchgear lineups located in separate buildings. The transformer is a 2000kVA transformers with a 13.8kV primary breaker and 480V secondary. The first LV Switchgear is located right next to the transformer and would likely be close coupled with bus to the transformer. Each LV switchgear lineup has (4) feeder breakers)

In trying to feed the 2nd LV switchgear lineup I see two options that would not be NEC code compliant:

Option 1: Tap the incoming bus of the 1st LVA lineup to cable feed the 2nd lineup. From what I can tell this would be a violation of 240.21 ( Tapping a tap) as well at 450.3 Note 2 requiring breakers to be grouped in a single location. Is it still considered a tap if you use cable with same ampacity?

I agree not code compliant.

Option 2: Run 2 separate feeders from the transformer secondary to the two LVA lineups. This however appears to violate 450.3 Note 2 as well.

But it would not be a note 2 violation if secondary protection is not required. Are you a supervised location (note 3)? Although you would still have to comply with your 240.21(C) requirements, which may be possible if the one set is "outside".

In looking at this I see two options that perhaps will be NEC code compliant but wanted to get feedback from others.

Option 3: In the first lineup have a main breaker for that lineup but before that main breaker have a feeder breaker to feed over to the 2nd lineup. The 2ne lineup can then be MLO. You would have to essentially double the ampacity of the first switchgear lineup. (800A to 1600A). The other option would be to just have (5) feeder breakers in the first lineup with no main breaker (one feeding 2nd lineup) so that both lineups would not have mains. This would not be idea for coordination.

I think you can do that per 240.21(C) (3). But note you need a some sort of disconnect at the second building (could be 2-6 rule).

Option 4: If I put a CT on the secondary bushings of the transformer and I have those CT's connected to a relay on the 13.8kV primary breaker can I count those CT's as a secondary main protection? If so then then I can run the two separate feeders from the transformer secondary to the lineups because the feeders will be on the load side of the CT's which are serving as the secondary transformer protection and secondary conductor protection. Is there any reason why I can count these secondary CT's as the secondary protection. To me this seems like the best approach.

that might work for the transformer protection per 450.3(A) note 4 (this is out of my knowledge area, I dont know if what you propose is compliant with that note), but I dont see any such provision for the 240.21(C) conductor protection.

 

[jim dungar]

Option 4: If I put a CT on the secondary bushings of the transformer and I have those CT's connected to a relay on the 13.8kV primary breaker can I count those CT's as a secondary main protection? If so then then I can run the two separate feeders from the transformer secondary to the lineups because the feeders will be on the load side of the CT's which are serving as the secondary transformer protection and secondary conductor protection. Is there any reason why I can count these secondary CT's as the secondary protection. To me this seems like the best approach.

Yes you can use CT's on the secondary to trip the primary side breaker. 240.15(A) recognizes that a relay and current transformer(s) are equivalent to a circuit breaker trip unit.

 

[mull982]

But it would not be a note 2 violation if secondary protection is not required. Are you a supervised location (note 3)? Although you would still have to comply with your 240.21(C) requirements, which may be possible if the one set is "outside".

The "supervised location" has always been a bit of a gray area for me. This being an industrial plant I'm assuming that they have qualified electrical staff and thus its qualifies as a supervised location. Is it a general assumption that most industrial plants with electrical staff can be considered supervised locations or are more details needed such as the type of maintenance and frequency of maintenance performed.

If I do qualify this as a supervised location then yes you are correct secondary protection of transformer is not required. I can tap (2) feeders from transformer secondary as long as feeder conductors adhere to 240.21(C) as you mention.

One of these feeders will be close coupled to transformer (may even be a short bus run) that is inside of a building (a few feet) while the 2nd feeder will leave the building and be routed outside to the 2nd building. Based on the requirements of 240.21(C) if I have a main breaker in each LV Switchgear then I can run these two separate feedrs?


Do you think this is the best approach or is there a better means that I am not seeing?

 

[mull982]

Yes you can use CT's on the secondary to trip the primary side breaker. 240.15(A) recognizes that a relay and current transformer(s) are equivalent to a circuit breaker trip unit.

Thanks for this reference. I was not aware of this reference.

Although its NEC compliant it would not provide a way to isolate each LV Switchgear lineup individually unless you had Main breakers at each Switchgear. Since electrofelon pointed out above that the (2) secondary feeders would be NEC complaint if its a supervised location then I don't know that there is any added benefit to this CT on the secondary. My original thinking was that this CT could satisfy the transformer secondary protection requirements if there were no other options in this case.

 

[david luchini]

One of these feeders will be close coupled to transformer (may even be a short bus run) that is inside of a building (a few feet) while the 2nd feeder will leave the building and be routed outside to the 2nd building. Based on the requirements of 240.21(C) if I have a main breaker in each LV Switchgear then I can run these two separate feedrs?


Do you think this is the best approach or is there a better means that I am not seeing?

How is the feeder to the second building going to comply with 240.21(C)? The second building would have to be awfully close to meet the 25' length requirement.

The Supervised Industrial Installation section would allow up to 100' secondary conductor length (or differential relaying, or calculation under engineering supervision.)

 

[electrofelon]

How is the feeder to the second building going to comply with 240.21(C)? The second building would have to be awfully close to meet the 25' length requirement.

The Supervised Industrial Installation section would allow up to 100' secondary conductor length (or differential relaying, or calculation under engineering supervision.)

I was thinking of outside unlimited taps, but you have to get it outside somehow . The only realistic way to do that is to combine two tap rules, which is questionable.

 

[david luchini]

I was thinking of outside unlimited taps, but you have to get it outside somehow .

If the secondary conductors don't originate outside, you can't use the 240.21(C)(4).

 

[mull982]

If the secondary conductors don't originate outside, you can't use the 240.21(C)(4).

240.21(C)(4) says " Where the conductors are located outdoors or a building or structure except at the point of load termination". Does this imply that the conductors must originate outdoors and not indoors? I have always thought that they have to be outdoors as long as any indoor portion (such as feeder origin) is under a certain percentage of the total feeder length? ( I don't recall reference off hand)

 

[david luchini]

240.21(C)(4) says " Where the conductors are located outdoors or a building or structure except at the point of load termination". Does this imply that the conductors must originate outdoors and not indoors? I have always thought that they have to be outdoors as long as any indoor portion (such as feeder origin) is under a certain percentage of the total feeder length? ( I don't recall reference off hand)

I don't know of any provision of the indoor portion being a certain percentage of the total length of the feeder.

"Except at the point of load termination" implies to me that the conductors must originate outdoors.

 

[mull982]

I don't know of any provision of the indoor portion being a certain percentage of the total length of the feeder.

"Except at the point of load termination" implies to me that the conductors must originate outdoors.

I'm not sure where I thought I saw that reference but perhaps I am misting.

So it sounds like the best practical way to accomplish this is to install a CT at secondary of transformer to satisfy secondary feeder protection and then have Main breakers in each switchgear lineup to be able to isolate mains. Added benefit of secondary CT will be differential on transformer and Arc Flash mitigation of downstream Switchgear.

 

[mull982]

I'm thinking the best option may be to go with option #3 I listed above by having one secondary feeder terminate in the first switchgear lineup and in that Switchgear lineup have a feeder breaker just before the Switchgear 1 Main that feeders over to the Switchgear 2 which will also have a main breaker.

This will meet 240.21(C)(3) since feeder length will be less than 25ft and all OCPD will be grouped together.

Is the breaker feeding over to Switchgear 2 located on the line side of the Main breaker in Switchgear 1 considered a "Tap" which would violate 240.21(C)? Must the feeder breaker over to Switchgear 2 be located on load side of Switchgear 1 main breaker?


I would likely make Switchgear 1 a 2000A lineup with Switchgear 2 feeder and lineup rated at 1200A. Common transformer is rated 2000kVA.