Open Delta Connection

[SDENKI]

I have an application where I am providing a transformer to step down 480v to 120/240v. The loads on the 120/240v are all single phase except for one air conditioning unit which is 240v three phase. The approximate single phase load is 45kw, the three phase 240v air conditioner is approximatly 7kva.
I would like to use a 50KVA 120/240V single phase transformer and a 10kva 120/240V single phase transformer connected in an open delta on the secondary. All single phase loads would be connected to the 50kva transformer. The primary of course would be 480V three phase.
I'm not sure what size overcurrent protection would be needed in each phase to meet NEC requirements since these are different size transformers. Can anyone enlighten me on this?

 

[infinity]

Re: Open Delta Connection

I'm only guessing but wouldn't a single larger transformer (say 75Kva) wired with a 3 phase, 4 wire Delta secondary be a more economical choice?

 

[augie47]

Re: Open Delta Connection

infinity: my guess is he has a large 120v load.
Three phase delta (center tap) transformers have a horrible restriction on the kva capacity of the 120v loads. Often overlooked on conversion jobs, but there, nonetheless.

 

[jim dungar]

Re: Open Delta Connection

infinity,
I know of no commercially available single three phase 3ph 4w delta transformer that is not limited to 5% single phase loading. This is a problem of physics and circulating currents.

SEDNKI
I don't have a good answer other than different size fuses. This is why I always prefer two seperate transformers, one for three phase and one for single phase.

 

[john m. caloggero]

Re: Open Delta Connection

Wow, I really had to jog my 72 yr old memory banks. I believe that the 45 Kva single phase load and the 7Kva 3-phase load supplied by an open delta configuration will require the following: A single phase 50Kva, 240/120volt transformer and a single phase 10kva transformer.

 

[hurk27]

Re: Open Delta Connection

Wow John your preaty sharp for that age. I read many of your post and learn much from them, At least as much as this thick hard head of mine will let me. :roll: