3 Transformers

[Hoggedy]

I have a 30 amp single phase (A and B) 480 volt breaker, that feeds 3 seperate transformers that step it down to 120/208 volt secondary 100A. Can anyone explain how I have a 30 amp breaker going through 3 seperate transformers before feeding the 100A load. I know the transformers are paralleled, I just want some clarification.

 

[iwire]

I have a 30 amp single phase (A and B) 480 volt breaker, that feeds 3 seperate transformers that step it down to 120/208 volt secondary 100A. Can anyone explain how I have a 30 amp breaker going through 3 seperate transformers before feeding the 100A load. I know the transformers are paralleled, I just want some clarification.

With 480 volt 30 amp in you can only get about 69 amps out at 208 volt.


480*30=14,400 watts

14,400/208=69.2 amps

Are you sure it is 208 volt output not 240?

 

[electrofelon]

I have a 30 amp single phase (A and B) 480 volt breaker, that feeds 3 seperate transformers that step it down to 120/208 volt secondary 100A. Can anyone explain how I have a 30 amp breaker going through 3 seperate transformers before feeding the 100A load. I know the transformers are paralleled, I just want some clarification.

First, Something is a bit off: IF it is single phase, you would not have 120/208. When you say that are paralleled, is it three single phase transformers connected to form a three phase bank?
As far as the original question, it really depends on what the actual load is. Note that 450.3(B) states that in some cases your OCPD could be 250% of the transformer rated current, so it might not be a horrible design to run three transformers off a single breaker, however tripping on energizing may be an issue.

 

[Hoggedy]

I'll double check the load voltage, the part I'm wondering about is how they have a 30A feeder, then they have 3 paralleled transformers to feed a 100A load (panel).

 

[Hoggedy]

Yea I apologize for the 120/208. It's 120/240. I only briefly looked at the job yesterday. Reason for this post is, one of the 3 transformers caught fire and burnt up. I'm trying to become more familiar with the feed setup to help troubleshoot.

 

[iwire]

I'll double check the load voltage, the part I'm wondering about is how they have a 30A feeder, then they have 3 paralleled transformers to feed a 100A load (panel).

The load cannot be 100 amps, the breaker or fuse on the load side could be 100 amps but the load at 208 volts cannot be more than 69 amps if the primary is single phase 480 volt 30 amps.

 

[ptonsparky]

I have a 30 amp single phase (A and B) 480 volt breaker, that feeds 3 seperate transformers that step it down to 120/208 volt secondary 100A. Can anyone explain how I have a 30 amp breaker going through 3 seperate transformers before feeding the 100A load. I know the transformers are paralleled, I just want some clarification.

Look at your configuration again. What you describe is not normal.

Are you sure that the 30 amp breaker is not a 3 pole?
Do you have a grounded Delta 480v primary?
Is the load 3 phase?
Are the transformers 1 PH? What size?

 

[iwire]

Yea I apologize for the 120/208. It's 120/240. I only briefly looked at the job yesterday. Reason for this post is, one of the 3 transformers caught fire and burnt up. I'm trying to become more familiar with the feed setup to help troubleshoot.

OK, assuming you have three single phase transformers wired in parallel supplied 480 volt at 30 amps the output load at 240 cannot be more than 60 amps.

If I was to repair this I would go with a one - single phase transformer of at least 15 kva.

 

[Hoggedy]

Like I said some of the info is probably off because I looked at everything briefly. Yes you are right, I ment to put 3 phase 30 am breaker feeding single phase 100A panel. My apologies.
Would an ideal situation be to replace the three paralleled existing transformers, with one individual higher KVA transformer?

 

[ptonsparky]

Like I said some of the info is probably off because I looked at everything briefly. Yes you are right, I ment to put 3 phase 30 am breaker feeding single phase 100A panel. My apologies.
Would an ideal situation be to replace the three paralleled existing transformers, with one individual higher KVA transformer?

Dang details get in the way of getting things done.

Yes, as stated by others the single larger transformer would be better.

How long did this setup work? Years, months, or for a few minutes after the switch was thrown?

 

[Hoggedy]

That is what me and my coworker were trying to decide. Thank you, I believe the load is in fact much less than 100A the OCPD in the panel is 100A. Since one of the transformers caught fire, I was unable to clamp amp the panels actual load.

 

[Hoggedy]

This configuration has been working for a long time, over 10 years or more. Unexpected transformer fire led to all this. I do apologize for the miscommunication on the details, mixed up a few things while typing.

 

[iwire]

That is what me and my coworker were trying to decide. Thank you, I believe the load is in fact much less than 100A the OCPD in the panel is 100A. Since one of the transformers caught fire, I was unable to clamp amp the panels actual load.

Here are some single phase transformer stock sizes

Single phase: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25,

If the 30 amp breaker has not been tripping the load has to be less than 14,400 watts or 14.4KW.

That being the case a 15 kva transformer should be fine.

 

[Hoggedy]

Update: after further inspection there is three single phase 5 KVA transformers fed by a 30 amp 480v 3 phase breaker. The load OCPD is a 100A 3 phase 120v breaker panel. Not a single phase(my coworker informed me it was single phase but was wrong). The whole job is not up to code because the 480 volt line is ran through the 120v panel. We are going to open everything up and try to figure out why 1 of the transformers caught fire and melted, we are unsure.

 

[Hoggedy]

Here are some single phase transformer stock sizes



If the 30 amp breaker has not been tripping the load has to be less than 14,400 watts or 14.4KW.

That being the case a 15 kva transformer should be fine.

Yes, we are planning to reinstall a 15 KVA transformer to replace the 3 that are there now. However we still are unsure of what cause one of the transformers to catch fire.

 

[Hoggedy]

Update: we found out my coworker misinformed me. The load is a 120/240 panel that has a 3 phase main not a single phase. There are 3 single phase transformers, 5 KVA fed from a 30A, 3 pole breaker. The 480v line passes through the 120v panel which is against code. We are headed back to the job now to open up the transformer that burnt up, we are now trying to come up with the reason why one of the three burnt up after so long of operation.

 

[kwired]

too much imbalance load can overheat one portion of the "bank" if secondary overcurrent protection is well over the rating of the bank.

Rated secondary current of 15 kVA is 41.6 amps if you configure them in a 208/120 wye or 36 amps if you configure them in a 240/120 delta, you needed to reach 100 amps secondary current before even considering having the secondary device open the circuit. And of course full (balanced) rating on primary side would have been 18 amps and you needed to draw 30 amps before even considering opening a primary device.

Might want to investigate what the loads actually are before selecting a 15 kVA replacement, or at least trying to balance those loads if they are under 15 kVA.


Add: and provide proper overcurrent protection - particularly to the secondary side.

 

[iwire]

Update: we found out my coworker misinformed me. The load is a 120/240 panel that has a 3 phase main not a single phase. There are 3 single phase transformers, 5 KVA fed from a 30A, 3 pole breaker. The 480v line passes through the 120v panel which is against code. We are headed back to the job now to open up the transformer that burnt up, we are now trying to come up with the reason why one of the three burnt up after so long of operation.

:slaphead:

 

[dionysius]

Action.........HELP!!!!!!

Action.........HELP!!!!!!

That is what me and my coworker were trying to decide. Thank you, I believe the load is in fact much less than 100A the OCPD in the panel is 100A. Since one of the transformers caught fire, I was unable to clamp amp the panels actual load.

Dear Mr. Hoggedy.......I do not in any way want to sound disrespectful but I do want you to respect us who really want to help. Please take one hour of your time to reword and restate this whole issue with verified facts which form a coherent picture. I am intensely interested in your scenario and I will post a good analysis but I do NOT want to shoot at a moving target.

Please recast this whole thread to date in an accurate summary and ask your question........

Thank you in advance and I do want you to pony up here.

 

[dionysius]

Update: after further inspection there is three single phase 5 KVA transformers fed by a 30 amp 480v 3 phase breaker. The load OCPD is a 100A 3 phase 120v breaker panel. Not a single phase(my coworker informed me it was single phase but was wrong). The whole job is not up to code because the 480 volt line is ran through the 120v panel. We are going to open everything up and try to figure out why 1 of the transformers caught fire and melted, we are unsure.

Further addition to prev post:
To properly allow us to analyze the situation and suggest a re-design if needed please do this: use a pencil and paper to sketch the exact circuitry as it now exists. Use your phone to forward the schematic. We have to see this. Please do not be rushed. If it takes a few days, then so be it. This is a problem that should be fully understood by all.