Forced Air (FA) Transformer Rating

[elec_eng]

I am reviewing a project that has a 2500/3300KVA (AA/FA) transformer 13.8Kv to 480V. The engineer is showing 3000A Switchgear at the 480V based on 2500KVA(AA). Shouldn't the secondary switchgear be 4000A based on 3300KVA rating (FA)?

Or this FA rating is simply for the overload capability for the transformer, not necessarily for the secondary switchgear capacity?

 

[iwire]

I don't see any issue as long as the gear is protected at or below its rating.

Am I missing something?

 

[elec_eng]

I don't see any issue as long as the gear is protected at or below its rating.

Am I missing something?

I understand the gear is protected per code but from the engineering standpoint, what is the point of 3300KVA rating if you are not going to utilize the extra capacity?

 

[infinity]

Sounds like the EE only needed the 3000 amp switchboard and chose to not utilized the higher transformer rating with a more expensive 4000 amp installation.

 

[elec_eng]

Sounds like the EE only needed the 3000 amp switchboard and chose to not utilized the higher transformer rating with a more expensive 4000 amp installation.

Then here goes my question again. Why would he specify the AA/FA transformer if he doesn't want to utilize the higher transformer rating? Why not just go with 2500KVA transformer instead of spending more money on 2500/3300KVA transformer?

 

[big john]

Why not just go with 2500KVA transformer instead of spending more money on 2500/3300KVA transformer?

Apparently the load is greater than 2.5MVA but less than 4MVA?

Also, are you certain the transformer will actually be equipped for forced-air? Seen many installations where they had a dual rated tranny only being used as AA. Sometimes this is done to leave room for future expansion without replacing the transformer.

 

[iwire]

I understand the gear is protected per code but from the engineering standpoint, what is the point of 3300KVA rating if you are not going to utilize the extra capacity?

Is your review directed at code compliance or are you tasked with value engineering?

 

[elec_eng]

Apparently the load is greater than 2.5MVA but less than 4MVA?

Also, are you certain the transformer will actually be equipped for forced-air? Seen many installations where they had a dual rated tranny only being used as AA. Sometimes this is done to leave room for future expansion without replacing the transformer.

I do not have information on the load, yet. yes, the spec is very specific on the FA transformer including the temperature controller.

 

[elec_eng]

Is your review directed at code compliance or are you tasked with value engineering?

All of above. I am a peer reviewer for the owner.

 

[junkhound]

Then here goes my question again. Why would he specify the AA/FA transformer if he doesn't want to utilize the higher transformer rating? Why not just go with 2500KVA transformer instead of spending more money on 2500/3300KVA transformer?

It is called de-rating: The transformer will last longer since it will run cooler:
research Arrhenius equation, de-rating, etc.. for more technical background.

 

[elec_eng]

It is called de-rating: The transformer will last longer since it will run cooler:
research Arrhenius equation, de-rating, etc.. for more technical background.

If that was the intent, wouldn't it be better go with a low temperature rise?

 

[Ingenieur]

Why don't you use some professional courtesy and ask him?
you seem determined to prove that he erred
Otherwise just write it up and let him respond

what is the cost impact on the total project for adding the fan package?
1%, 1/2%, less? My bet MUCH less
what clime will it be used? High temps for extended periods? Desert? Etc

as was noted running it cooler at 2.5 MVA will extend it's life and reliability considerably

are you sure the clg package was ordered?
I've seen cases where the controller/sensor pkg was ordered (sometimes difficult to retrofit while in service) just in case fans where needed at a later date

The cost is probably insignificant to total project cost
it is my understanding the xfmr is the same for 2.5 or 2.5/3.3
the 3.3 rating is achieved with the fan clg pkg

 

[Ingenieur]

What specific clg method
ONAF
OFAF
etc
???

altitude?
outside/interior?
large motor load percentage with frequent starting/stopping?

many variables

 

[elec_eng]

Why don't you use some professional courtesy and ask him?
you seem determined to prove that he erred
Otherwise just write it up and let him respond

what is the cost impact on the total project for adding the fan package?
1%, 1/2%, less? My bet MUCH less
what clime will it be used? High temps for extended periods? Desert? Etc

as was noted running it cooler at 2.5 MVA will extend it's life and reliability considerably

are you sure the clg package was ordered?
I've seen cases where the controller/sensor pkg was ordered (sometimes difficult to retrofit while in service) just in case fans where needed at a later date

The cost is probably insignificant to total project cost
it is my understanding the xfmr is the same for 2.5 or 2.5/3.3
the 3.3 rating is achieved with the fan clg pkg

When we do a peer review, often time we don't even know who the design engineer is..not to mention all the project requirements or histories. Of course, I will put together my comments and send them to the engineer to respond but I just want to understand his logic/design intent based on my experience.

I am also an design engineer so I try to put myself in his shoes and think what I would have done if I were him.

 

[elec_eng]

What specific clg method
ONAF
OFAF
etc
???

altitude?
outside/interior?
large motor load percentage with frequent starting/stopping?

many variables

This is dry type transformers AA/FA, located indoor within a conditioned space. All motor loads are with VFDs.

 

[infinity]

When we do a peer review, often time we don't even know who the design engineer is..not to mention all the project requirements or histories. Of course, I will put together my comments and send them to the engineer to respond but I just want to understand his logic/design intent based on my experience.

I am also an design engineer so I try to put myself in his shoes and think what I would have done if I were him.

That's a compelling reason to come here and ask the question. I for one find the topic interesting and I can see you point. Probably the only one that can answer the question is the guy who designed it. We often get equipment that is purchased based solely on availability and not necessary being the minimum required for the job.

 

[Ingenieur]

When we do a peer review, often time we don't even know who the design engineer is..not to mention all the project requirements or histories. Of course, I will put together my comments and send them to the engineer to respond but I just want to understand his logic/design intent based on my experience.

I am also an design engineer so I try to put myself in his shoes and think what I would have done if I were him.

The drawings are not sealed? No title block? Blank? Weird
I've done more than a few peer reviews
it was always a requirement to meet before issuing my report
saved a lot of back and forth

 

[Ingenieur]

This is dry type transformers AA/FA, located indoor within a conditioned space. All motor loads are with VFDs.

that seems to answer why FA
air cooled, no oil
indoors
VFD's

what PU Z ?
is the room ventilated?
room volume?

at 5% Z and 75% loading
you need 40-50 Tons of cooling (35% duty cycle)
capital cost and long term power

 

[Tony S]

This is dry type transformers AA/FA, located indoor within a conditioned space. All motor loads are with VFDs.

I think you've answered your own question.