TonyThunder
Member
I need to supply power to three separate groups of HID lighting with loads of 50 KVA each. Just about 80% of the power requirement is for 400W light fixtures with 240v and 208v ballasts (accomodates the voltage drop), some other 100W 120 volt fixtures and some 120v outlets. I cannot place large transformers near the lighting so my plan is to send 480v from a 15KV : 480Y/277 transformer to a main distribution panel and then send power to individual single phase transformers that will either be 480 : 120/240 or 277 : 120/240. The distance from the main transformer to the distribution panel is roughly 350' and the distance between each light group is 300' which causes some issues with voltage drop. Is it best to send 480v or 277v to a single phase transformer? I want to use 480v for less % voltage drop and smaller conductors to save money.
If I send single phase 277v (line and neutral), then I surmise I need 180 amps per phase which is easy, but then I must increase the size of the conductor. This would essentially be a WYE at the 15KV transformer and a WYE at the 277 stepdown.
If I step back a bit and sum all three groups of lights, I get 50 KVA x 3 = 150 KVA. With a three phase system I would then get 180 amps per phase for each 50 KVA transformer. With a WYE transformer, the line current = phase current so Power per phase = Volt per phase x Current per phase, 50 KVA = 277v x 180 amps. If I run the single phase 277v to each separate transformer, I get 180 amps on each A, B, C conductor with zero on the neutral if I have all transformers operating equally (balanced).
But what if I wanted to send single phase 480v (line to line), then how would I calculate the load? If I run two hots to the three 480v transformers, A-B, B-C, and C-A, then I have actually created a Delta system and the line current would no longer equal the phase current but would now be 1.73 x the phase current. Also, the line voltage would now be equal to the phase voltage so the phase current would now be 104 amps (50,000 KVA/ 480v = 104amps). That means the line current would be 180 amps. Is that correct?
Also, if I run 480v to the transformer would I also need to run a neutral or just the two line conductors? I'm thinking I need to run just the two hot conductors plus my #6 AWG grounind conductor.
And since there is a good chance that one set of lights may be off, would it not better to use the WYE setup instead of the delta system? I really don't think I can keep the loads balanced. I realize that with the WYE I'll need to keep the neutral full sized for the unbalanced current load.
If I send single phase 277v (line and neutral), then I surmise I need 180 amps per phase which is easy, but then I must increase the size of the conductor. This would essentially be a WYE at the 15KV transformer and a WYE at the 277 stepdown.
If I step back a bit and sum all three groups of lights, I get 50 KVA x 3 = 150 KVA. With a three phase system I would then get 180 amps per phase for each 50 KVA transformer. With a WYE transformer, the line current = phase current so Power per phase = Volt per phase x Current per phase, 50 KVA = 277v x 180 amps. If I run the single phase 277v to each separate transformer, I get 180 amps on each A, B, C conductor with zero on the neutral if I have all transformers operating equally (balanced).
But what if I wanted to send single phase 480v (line to line), then how would I calculate the load? If I run two hots to the three 480v transformers, A-B, B-C, and C-A, then I have actually created a Delta system and the line current would no longer equal the phase current but would now be 1.73 x the phase current. Also, the line voltage would now be equal to the phase voltage so the phase current would now be 104 amps (50,000 KVA/ 480v = 104amps). That means the line current would be 180 amps. Is that correct?
Also, if I run 480v to the transformer would I also need to run a neutral or just the two line conductors? I'm thinking I need to run just the two hot conductors plus my #6 AWG grounind conductor.
And since there is a good chance that one set of lights may be off, would it not better to use the WYE setup instead of the delta system? I really don't think I can keep the loads balanced. I realize that with the WYE I'll need to keep the neutral full sized for the unbalanced current load.
