step up transformer question

Kevin C

Member
Hi guys,
I ran into this situation in a job site. Need to hookup a existing 3 phase 460v RTU with min circuit 21.7 amp and max fuse 30 amp, the space is supplied by 200 amp 120/208 3 phase. There is a existing 75kva step down transformer, was used to step down from 480 delta to 120-208 wye. I confirmed with the manufactor that this transformer could use in reverse to step up. The question is would the 200 amp service we have can handle this transformer, the only load would be the RTU at 21.7 amp. If so, can I use a 30 amp fused disconnect on the secondary.

Thanks.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
75kVA xfmr would be capable of 98A on the 480V side, so it's overkill for a 21.7A load, but yes, it could do that. The transformer will pull about 50A on the 208V side at 21.7A on the 480 side, but the inrush when energized on the 208V side will be around 2000A for a cycle or two, so if you fuse it too low on the 208V side you might find yourself replacing fuses a lot.

I would opt for a smaller transformer, all you need is 22.5kVA.
 

texie

Senior Member
Yes, you can use this in reverse but there a number of good reasons not to. The biggest one is your secondary will need to be ungrounded or corner grounded, both of which present issues. The best way is to just get the correct 208 delta to 480Y transformer.
 

Kevin C

Member
75kVA xfmr would be capable of 98A on the 480V side, so it's overkill for a 21.7A load, but yes, it could do that. The transformer will pull about 50A on the 208V side at 21.7A on the 480 side, but the inrush when energized on the 208V side will be around 2000A for a cycle or two, so if you fuse it too low on the 208V side you might find yourself replacing fuses a lot.

I would opt for a smaller transformer, all you need is 22.5kVA.
Thanks. That's one of my concern as the primary will be on the 200 amp breaker.
 

Kevin C

Member
Yes, you can use this in reverse but there a number of good reasons not to. The biggest one is your secondary will need to be ungrounded or corner grounded, both of which present issues. The best way is to just get the correct 208 delta to 480Y transformer.
The only load is the RTU which is only require 3 phase at 460
 

texie

Senior Member
The only load is the RTU which is only require 3 phase at 460
That may be but a delta secondary will present the issue of being ungrounded or corner grounded both of which bring issues to deal with. Corner grounding will require equipment to to be rated for use as corner grounded. Ungrounded also has a number of negative issues not the least of which is you will then need ground detection.
While not as common as 480 delta to 208Y, 208 delta to 480Y are readily available.
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Hi, first post here and thought to chime in. I have purchased (2) step up transformers (34 kVA) in the past month to do exactly what you are trying to do. Each was delivered in about a week, so I agree they are readily available. If you have the time, that would be your best solution I think.


In our shop we have also reverse fed step down transformers to get 480V when that was the only option available, but as pointed out here in the previous posts the Delta output is problematic – especially if any loads are electronic (like VFDs). These typically want to be sourced with a Wye supply as they have components (MOVs and filter caps) that expect connections to a Wye ground – and there won’t be one on your delta secondary.


One thing not mentioned yet is that, if you do choose to reverse feed your existing 75 kVA unit, do not connect your incoming 208V neutral (or anything else) to the XO terminal. If there is a bond to the chassis on the X0 lug, disconnect it. Also, any tap adjustments will work in reverse because they are now on the secondary side.
 

Kevin C

Member
Hi, first post here and thought to chime in. I have purchased (2) step up transformers (34 kVA) in the past month to do exactly what you are trying to do. Each was delivered in about a week, so I agree they are readily available. If you have the time, that would be your best solution I think.


In our shop we have also reverse fed step down transformers to get 480V when that was the only option available, but as pointed out here in the previous posts the Delta output is problematic – especially if any loads are electronic (like VFDs). These typically want to be sourced with a Wye supply as they have components (MOVs and filter caps) that expect connections to a Wye ground – and there won’t be one on your delta secondary.


One thing not mentioned yet is that, if you do choose to reverse feed your existing 75 kVA unit, do not connect your incoming 208V neutral (or anything else) to the XO terminal. If there is a bond to the chassis on the X0 lug, disconnect it. Also, any tap adjustments will work in reverse because they are now on the secondary side.
Hi
My supplier doesn't carry/make 208 wye to 480 wye unit. They propose a Acem Electric T3030k0013B, which is a 30kva primary 480delta and secondary 208 wye. They told me to use this in reverse and pull a seperat ground. Is there a problem with doing this way?
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Hi Kevin,

You are not looking for a Wye to Wye transformer. You are looking for a Delta to Wye unit, built for a step UP application with a 208D primary and a 480Y secondary. These absolutely exist and are not hard to get.

The Acme unit your supplier wishes to sell you is a typical step DOWN transformer with a 480V Delta primary. When wired in reverse, as pointed out by the members above, that will become your secondary. Having a Delta output can be problematic depending on what you are driving.

When it comes to grounding an SDS (Separately Derived System), there is nothing simpler than a Wye secondary.

I am a big advocate here in getting the right part. Not sure why they are trying to sell you a step down transformer for a step up application? Maybe that’s what they sell a lot of? A quick search on the web shows me Acme sells exactly what you want… part # T3112K0034B. 34 kVA, 208D primary, 480Y secondary. While it may not be “on the shelf” at your supplier, I believe you can order it and have it inside a week or 10 days.

fwiw – I purchased units from Hammond, specifically HPS DM034BK. Not in stock at my supplier, but at the time of purchase there were 6 in the Hammond facility in WI and they arrived at our shop’s dock in about a week. They have units sized in kVA for 20, 27, 34, and so.

Hope this helps, this is only my 2nd post here - I would hope other more experienced members on this board would agree with me that the best solution here for you is to purchase the right step up transformer.
 
Hi
My supplier doesn't carry/make 208 wye to 480 wye unit. They propose a Acem Electric T3030k0013B, which is a 30kva primary 480delta and secondary 208 wye.
You don't need a wye wye transformer you need a 208V delta 480V wye transformer and they are easy to get.
They told me to use this in reverse and pull a seperat ground. Is there a problem with doing this way?
Yes there is a problem doing this, almost every prior response is this thread has mentioned the reasons why it's not a good idea.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
My supplier doesn't carry/make 208 wye to 480 wye unit. They propose a Acem Electric T3030k0013B, which is a 30kva primary 480delta and secondary 208 wye. They told me to use this in reverse and pull a seperat ground. Is there a problem with doing this way?
Nobody carries a wye-wye dry type transformer in stock.

Unless you need it immediately, have your supplier order a standard 208V delta primary - 480Y/277V step up transformer. ACME and other manufacturers have been building these for at least 40 years.

Based on you questions, I suggest staying away from a 480V delta output.
 

Kevin C

Member
Another question
If I can find a 22.5kva, does it have enough juice for this rtu with a mca of 21.7 amp? The max breaker is 30 amp. Due to the job specification, I could not hang any transformer larger than 22.5 Kva, so if I have to do a 30, the client will lost some office space.
 

kwired

Electron manager
Location
NE Nebraska
Another question
If I can find a 22.5kva, does it have enough juice for this rtu with a mca of 21.7 amp? The max breaker is 30 amp. Due to the job specification, I could not hang any transformer larger than 22.5 Kva, so if I have to do a 30, the client will lost some office space.
How was it going to work out with the 75 kVA unit you originally were wanting to use?
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
Kevin,

fyi... in my reply#9 above, I made a cut and paste error from the Acme catalog and incorrectly gave the part number for a 112 kVA xfmr. If you are looking for Acme transformers, the part for you is likely the 30 kVA model - T3030K0034B.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
.... The question is would the 200 amp service we have can handle this transformer, ...
..., but the inrush when energized on the 208V side will be around 2000A for a cycle or two, so if you fuse it too low on the 208V side you might find yourself replacing fuses a lot. ...
Likely too late to help. I plead working for the last four weeks - ugh, disgusting
Just discussing reverse fed 208Y/480D

GE has a pretty good paper on reverse fed xfm. The case they use is exactly this - reverse fed 408D/208Y.
jraef is correct for forward fed XFM, max inrush is 11X FLA
However, for reverse fed, the inrush can be as much as 37X, which is 7700A

A typical molded case TCC curve:
https://download.schneider-electric.com/files?p_enDocType=Data+Bulletin&p_File_Name=0600DB0105.pdf&p_Doc_Ref=0600DB0105
indicates short time 4000A >> 2cycles
instantaneous 6000A, >> 1 cycle

So, 200A, 208V feeder is small, expect nuisance trips
 
Top