2000kVA Transformer Service Entrance Cable & Ground Requirements

[johnj0745]

I am designing an electrical line-up with a 4160V Delta -480V Wye , 2000kVA Pad-Mounted Transformer that is fed from a utility owned 46kV Delta -4160V Wye, 7000kVA transformer. I am wondering what are the specific Service entrance requirements that I need to follow for this facility that's located in Huntsville, Alabama? Also, on the secondary of the 2000kVA Transformer I am feeding a 3000A, 480V, 3P, 4W switch board with 9 Sets of 4-1/C 750 kcmil cables w/#2/0 Gnd (A-Phase, B-Phase, C-Phase, Ground, Neutral) that are being routed in a 9-4" PVC 3000PSI Concrete duct bank ~20' away.

1. Do I have to have neutral cable off the secondary of the transformer to the switchboard because it's considered a "service entrance"? The meter is actually on the utility transformer itself not on the actual pad mounted transformers. I read somewhere that I can have the ground serve as the "Neutral". Think this term was "grounded neutral"? Or possibly I could run the neutral to the Main breaker that is 4-Wire breaker and remove the neutral bus since I don't have any 277V loads? Trying to save some money anyway I can.

2. Can I use NEC Table B.310.15(B)(2)(7) to derate the cables in the duct bank from the transformer to the switchboard? It calls for 6 electrical ducts but I have 9. Also, is this the right table to use? I am planning on using single conductor cables and running (A-Phase, B-Phase, C-Phase, Ground, Neutral) in each conduit.

3. Is a #2/0 ground in each conduit sufficient on the secondary size of the transformer? I plan to tie the ground for the transformer and the switchboard to the ground grid utilizing a 4/0 however I just need to confirm the ground sizing in each conduit in the duct bank is sufficient. I heard there might be specific requirements for service entrance grounds. Also, I am not sure if the "full size ground rule" applies here since there's technically no upstream protection other than fuses on the primary.

 

[Dennis Alwon]

I am confused by the first sentence. You say the util is providing a trany f4om 46kv Delta- 4160V Wye. Then somehow you have a 4160V Delta - 480 V Wye. I assume that should be a 4160v Wye and not a Delta

I am designing an electrical line-up with a 4160V Delta -480V Wye , 2000kVA Pad-Mounted Transformer that is fed from a utility owned 46kV Delta -4160V Wye, 7000kVA transformer.

 

[johnj0745]

I am confused by the first sentence. You say the util is providing a trany f4om 46kv Delta- 4160V Wye. Then somehow you have a 4160V Delta - 480 V Wye. I assume that should be a 4160v Wye and not a Delta

The utility company is providing a delta-wye transformer. I can't just pull off the three phases of the wye transformer to feed a Delta-Wye Pad-Mounted transformer? Or would I have to do like a Wye-Wye transformer or Wye-Delta transformer? We only need 3P, 480V at the switchboard. I think there's higher ground fault risks associated with a delta system on the 480V secondary?

 

[infinity]

Where is the service point? The 3000 amp disconnect is the service disconnect?

 

[johnj0745]

Where is the service point? The 3000 amp disconnect is the service disconnect?

There will be a fused cut out at the substation that is attached to bus bars that jumpers over to the utility transformer secondary.

 

[augie47]

#1 Sounds like your 4160-480 is a customer transformer (not the utility service transformer) therefor you are dealing with an SDS and 250.30.
It has a wye secondary so the neutral must be brought to your 3000 amp gear. Since your secondary conductors are not in a metallic raceway, Exception 2 to 250.30(A)(1) applies and you don't need a SSBJ (250.30(A)(2) exception.

#2 I can't answer #2

#3 The minimum size conductor allowed when paralleling is 1/0

 

[Dennis Alwon]

My understanding is that since there is no disconnect outside for the service then

The utility company is providing a delta-wye transformer. I can't just pull off the three phases of the wye transformer to feed a Delta-Wye Pad-Mounted transformer? Or would I have to do like a Wye-Wye transformer or Wye-Delta transformer? We only need 3P, 480V at the switchboard. I think there's higher ground fault risks associated with a delta system on the 480V secondary?

If the utility is giving you a Delta- Wye then why would you go back to delta and to wye again. I assume you can get a 4160v wye to a 480V Wye.

 

[johnj0745]

My understanding is that since there is no disconnect outside for the service then



If the utility is giving you a Delta- Wye then why would you go back to delta and to wye again. I assume you can get a 4160v wye to a 480V Wye.

Are these more expensive? Also, wouldn't this require me to bring the neutral therefore costing more money for materials? I feel like theres pros and cons of each type of transformer when it comes to cost, faults, etc.

 

[johnj0745]

Here is the utility transformer .

 

[johnj0745]

Here's the substation disconnect point. They will install a fused cutout somewhere right here.

 

[Dennis Alwon]

Sorry with sub stations you are way over my head. Hopefully someone can help

 

[ron]

I am confused by the first sentence. You say the util is providing a trany f4om 46kv Delta- 4160V Wye. Then somehow you have a 4160V Delta - 480 V Wye. I assume that should be a 4160v Wye and not a Delta

Unless it was a utility requirement, I don't see why he would need anything else but a delta-wye downstream. Some utilities like wye-wye transformers, but that doesn't make it so that a customer needs one (unless dictated by a utility regulation)

 

[synchro]

I believe utilities sometimes choose Yg-Yg transformers because they are less susceptible to ferroresonance. Ferroresonance is more likely to occur when there is a relatively long underground feeder to the transformer, particularly when a distant cutout or fuse is opened on one of the phases, leaving just the capacitance of the open-ended line. Ferroresonance is also more likely in systems at higher voltages (say 15kV or more) because their higher impedance level makes them more suseptible to the effects of line capacitance.

 

[Engser18]

I am designing an electrical line-up with a 4160V Delta -480V Wye , 2000kVA Pad-Mounted Transformer that is fed from a utility owned 46kV Delta -4160V Wye, 7000kVA transformer. I am wondering what are the specific Service entrance requirements that I need to follow for this facility that's located in Huntsville, Alabama? Also, on the secondary of the 2000kVA Transformer I am feeding a 3000A, 480V, 3P, 4W switch board with 9 Sets of 4-1/C 750 kcmil cables w/#2/0 Gnd (A-Phase, B-Phase, C-Phase, Ground, Neutral) that are being routed in a 9-4" PVC 3000PSI Concrete duct bank ~20' away.

1. Do I have to have neutral cable off the secondary of the transformer to the switchboard because it's considered a "service entrance"? The meter is actually on the utility transformer itself not on the actual pad mounted transformers. I read somewhere that I can have the ground serve as the "Neutral". Think this term was "grounded neutral"? Or possibly I could run the neutral to the Main breaker that is 4-Wire breaker and remove the neutral bus since I don't have any 277V loads? Trying to save some money anyway I can.

2. Can I use NEC Table B.310.15(B)(2)(7) to derate the cables in the duct bank from the transformer to the switchboard? It calls for 6 electrical ducts but I have 9. Also, is this the right table to use? I am planning on using single conductor cables and running (A-Phase, B-Phase, C-Phase, Ground, Neutral) in each conduit.

3. Is a #2/0 ground in each conduit sufficient on the secondary size of the transformer? I plan to tie the ground for the transformer and the switchboard to the ground grid utilizing a 4/0 however I just need to confirm the ground sizing in each conduit in the duct bank is sufficient. I heard there might be specific requirements for service entrance grounds. Also, I am not sure if the "full size ground rule" applies here since there's technically no upstream protection other than fuses on the primary.

I believed the 2MVA (4.16kv-0.480V) isn't belong to utility.

1. You are required to have SD (service disconnect) with OCPD (overcurrent protection device) rated ~350A, 4160V. You will required to have neutral to SD from Utility (bond N and G here). From SD you are able run 3W (5kv) with EGC to your 2MVA Xformer. From 2MVA Xformer secondary side (480v) you can run either 3W with G or 4W with G because it is feeder. (Remember to bond N & G at Xformer secondary)

2. Probably not, but you need to look at Annex B, it will help you to derated your ductbank and using its table if applicable.

3. Since they are secondary conductors of xformer. The G isn't called EGC it is called ssbj (250.102), and it is sized base on the size of phase conductor in each parallel set.

 

[augie47]

I agree with Engser18 other than I believe you can omit the SSBJ IF can meet the requirements spelled out in 250.30(A)(2) exception / 250.30(A)(1) Exception 2