Hello-
Have not done much design work with 15KV equipment.
Voltage is /V
Have a primary service coming from a vacuum circuit recloser (supplied by power company) to a metal enclosed switchgear with fused switches. Feeding a liquid filled 500KVA transformer with one of the switches. My question is regarding Neutral and ground. Do I need a neutral wire? Instinct tells me I only need (3) 15 KV MV-90 EPR cables. #2 is the minimum size cable at this voltage per table 300.106(A) which works because fuse size is only 50A, I use 200% for primary side protection, and according to 310.60(C)(77) #2 is good for 145A. I guess I do not understand the V. Is that the line-to-line voltage, which does not make sense to me?
Finally, if no neutral is required (?) a ground certainly would be. How is the size of the ground determined? Circular mills of the #2 cable? Ampacity of the fuse? I would think circular mils of the cable otherwise the ground size would only be a #10 which does not seem correct to me at all.
Any articles I can read to help me understand are always appreciated.
Thanks for your reply
Hello-
Have not done much design work with 15KV equipment.
Voltage is /V
Have a primary service coming from a vacuum circuit recloser (supplied by power company) to a metal enclosed switchgear with fused switches. Feeding a liquid filled 500KVA transformer with one of the switches. My question is regarding Neutral and ground. Do I need a neutral wire? Instinct tells me I only need (3) 15 KV MV-90 EPR cables. #2 is the minimum size cable at this voltage per table 300.106(A) which works because fuse size is only 50A, I use 200% for primary side protection, and according to 310.60(C)(77) #2 is good for 145A. I guess I do not understand the V. Is that the line-to-line voltage, which does not make sense to me?
Finally, if no neutral is required (?) a ground certainly would be. How is the size of the ground determined? Circular mills of the #2 cable? Ampacity of the fuse? I would think circular mils of the cable otherwise the ground size would only be a #10 which does not seem correct to me at all.
Any articles I can read to help me understand are always appreciated.
Thanks for your reply
Use MV-105. Same price, higher ampacity. It’s not MV-105 EPR by the way it’s just MV-105. The insulation is part of the spec. You MUST use termination kits here. If you have no experience installing them get someone who is. At those voltages even accidentally slicing something or crimping the lug with say an indent crimper or not filing edges can cause major issues (failures). Lots of little things not in the 2-3 pages of instructions. Call a larger local motor shop if you don’t know someone. They do these all the time.
No neutral assuming transformer is delta primary. If not delta-wye for most customers that’s mistake #1, 2, and 3. If you go wye-wye every ground fault on the utility side is also a ground fault on the secondary side. The utility generally doesn’t supply a neutral.
Size ground sane as all service entrances. No change there. The only thing new here are the shield drains. Tie back to ground. They aren’t very big and just drain capacitive charges. Currents are under 1 A. They have to be there but size is no critical.
Use MV-105. Same price, higher ampacity. It’s not MV-105 EPR by the way it’s just MV-105. The insulation is part of the spec. You MUST use termination kits here. If you have no experience installing them get someone who is. At those voltages even accidentally slicing something or crimping the lug with say an indent crimper or not filing edges can cause major issues (failures). Lots of little things not in the 2-3 pages of instructions. Call a larger local motor shop if you don’t know someone. They do these all the time.
No neutral assuming transformer is delta primary. If not delta-wye for most customers that’s mistake #1, 2, and 3. If you go wye-wye every ground fault on the utility side is also a ground fault on the secondary side. The utility generally doesn’t supply a neutral.
Size ground sane as all service entrances. No change there. The only thing new here are the shield drains. Tie back to ground. They aren’t very big and just drain capacitive charges. Currents are under 1 A. They have to be there but size is no critical.
Thank You for your reply. I am an electrical designer so Ill leave the installation details up to the professional installers for sure. However, I am the type of designer who likes to fully understand what I place on my drawings as if I had to install it myself. I've always said I would like every contractor to spend 1 year working as an engineer and every engineer to spend 1 year working as a contractor. Imagine the harmony in the world of construction & design.
Yes - Delta Primary - Wye secondary on the LFPM transformer.
Makes sense on the ground size as it is a service entrance , 250.66 it is.