75 KVA feed

[TheElevatorGuy]

I would like to know if a 75 KVA 3 phase 208/480 step up transformer can be fed with a 200 Amp Gap switch fused with transformer fuses at 200 Amps? Thank you in advance. The secondary of this transformer will be feeding a frequency drive for elevator motor control.

 

[don_resqcapt19]

You can use the 200 amp fuses, but it is possible they will open when you energize the transformer... the FLA is 208 amps and I would be looking at 250 or 300 amp fuses for that installation.

 

[BillyMac59]

FLA on primary: 90A ; FLA on secondary: 208A . Choking the secondary might cause nuisance - and expensive - blown fuses.

 

[TheElevatorGuy]

Even if I use TR fuses? Or Slow blow? And obviously I can not put fuses in rated more than 200 Amps in that Gap switch? Thanks

 

[infinity]

FLA on primary: 90A ; FLA on secondary: 208A . Choking the secondary might cause nuisance - and expensive - blown fuses.

Are those numbers correct for a step-up transformer?

 

[TheElevatorGuy]

The numbers are reversed. The primary would have 208 Amps and the secondary would be 90 Amps.

 

[infinity]

The numbers are reversed. The primary would have 208 Amps and the secondary would be 90 Amps.

As Don stated maybe the 200 amp primary fuses would not blow when the transformer is energized but with a maximum primary current of 208 amps it might be close. Worst part of it is the jump from 200 to 400 amp equipment is pricey.

 

[TheElevatorGuy]

So the right thing to do is put a 400 Amp gap switch in and use the fuses recommended. I have 4/O wire feeding the line side of the existing 200 Amp gap disconnect. That wire is rated for 230 Amps. Can I still use 250 Amp fuses? I know for sure that the transformer will not be close to the full load current. I really appreciate the help.

 

[jap]

I would think it would depend on what the demand load is of the elevator equipment.

A transformer is a dumb animal.

Although there may be inrush current to energize it, and a small coil load at idle, it's not going to pull on the primary what's not needed on the secondary.

JAP>

 

[infinity]

So the right thing to do is put a 400 Amp gap switch in and use the fuses recommended. I have 4/O wire feeding the line side of the existing 200 Amp gap disconnect. That wire is rated for 230 Amps. Can I still use 250 Amp fuses? I know for sure that the transformer will not be close to the full load current. I really appreciate the help.

Yes the next standard size rule would apply to the primary so a 230 amp conductor can be protected next size up at 250 amps. IMO that is probably enough to not trip on inrush with the right fuses.

 

[BillyMac59]

Sorry for the oops. I missed the fact that it was stup-up xfmr. As others noted, the numbers were correct but backwards. Sorry for any confusion.

 

[TheElevatorGuy]

Sorry for the oops. I missed the fact that it was stup-up xfmr. As others noted, the numbers were correct but backwards. Sorry for any confusion.

No problem. Thank you very much for your help.

 

[TheElevatorGuy]

Yes the next standard size rule would apply to the primary so a 230 amp conductor can be protected next size up at 250 amps. IMO that is probably enough to not trip on inrush with the right fuses.

Thank you. I am never disappointed by this chat room. Have a good day.

 

[infinity]

Thank you. I am never disappointed by this chat room. Have a good day.

You're welcome.