Addional load to existing transfomer

[BrianMuir]

Hello,
I have a project requiring an additional 25KVA of 120/208V loads to an existing building, and am trying to decide if I can get away without increasing available transformer capacity.

Incoming feed is 600V three phase on a 300A breaker from the facility main service location.
This feeds a 150KVA, 120/208V transformer via a 200A breaker.
This in turn feeds four subpanels via 2x 4#250MCM
Panel B (Kitchen) 53 KVA
Panel C (Level One) 30 KVA
Panel D (Level Two) 25 KVA
Panel E (Level Three) 34 KVA
Total 142KVA

On paper the additional 25KVA pushes the transformer into overload, but it seems incredibly unlikely that the actual load would be anywhere near maximum (this is three story a college building including a kitchen)

I welcome advice from other engineers regarding best practices in this type of situation.

thank you

 

[templdl]

You haven't said anything about the rating of the transformer such as if it is a liquid of dry, the temperature rise and what the ambient temperature is.

 

[skeshesh]

While templdl brings up the correct concerns as far as proper engineering, I think that unless you're facing extreme conditions it would be allright to "overload" the transformer. I'd bet you are right about the demand being unlikely to peak often (if at all), but even if the peak demand exceeds the kVA rating of the xfmr once in a while, you should be allright. Of course the lifetime of the equipment may be affected, but I think after explaining this to client you should be able to get away with avoiding a transformer upgrade.

 

[templdl]

skeshesh,
I agree.
But to justify the application should it be a dry type trasnsformer the KVA is based upon 150deC rise at 40deg C ambient. With a lower ambient which you normally have you have the opportunity to increase the KVA without exceeding the insulation class of 220degC.
SHould you have a 115degC rise transformer you have an overload capacity or 15% without exceeding the 220degC class insulation at 40degC (104degF) ambient.
As such the 40degC ambent rating by itself should cover his requirements.
Dry type transformer ratings are based upon the insulation class temperature rating.
40deg ambient + 150gedegC rise+30degC hot spot allowance=220degC. By lowering the ambient temperature of ratong the transformer a 115deg C or even 80degC can provide extra capacity woth our exceeding that 220degC insulation class.

 

[dahualin]

Hello,
I have a project requiring an additional 25KVA of 120/208V loads to an existing building, and am trying to decide if I can get away without increasing available transformer capacity.

Incoming feed is 600V three phase on a 300A breaker from the facility main service location.
This feeds a 150KVA, 120/208V transformer via a 200A breaker.
This in turn feeds four subpanels via 2x 4#250MCM
Panel B (Kitchen) 53 KVA
Panel C (Level One) 30 KVA
Panel D (Level Two) 25 KVA
Panel E (Level Three) 34 KVA
Total 142KVA

On paper the additional 25KVA pushes the transformer into overload, but it seems incredibly unlikely that the actual load would be anywhere near maximum (this is three story a college building including a kitchen)

I welcome advice from other engineers regarding best practices in this type of situation.

thank you

Are those existing load for the four panels connected load or measured load? The key is to find the existing peak load. One way is to do a field survey to find all connected existing loads and do a load calculation with all demand factor you can apply to find out if the existing transformer is big enough to handle new load. Another way is to ask somebody who has the special measurement meter to tap the feeder of the transformer for 30days to find the existing peak load, then apply NEC 220.87 to determine if the existing transformer has enough spare capacity.

 

[BrianMuir]

Ambient temp is about 25C.
Transformer is a dry type three phase isolation transformer.
The loads I have provided are calculated loads as specified in the electrical plans.

However,, in the process of checking the shop drawings supplied by the electrical contractor I discovered that they have the model as BC112JM, which is 112.5 KVA! The electrical plans show a 150KVA unit so today I will go over there & put eyes on the target.

Thank you for the very helpful replies. I'll post again with an update.

 

[skeshesh]

skeshesh,
I agree.
But to justify the application should it be a dry type trasnsformer the KVA is based upon 150deC rise at 40deg C ambient. With a lower ambient which you normally have you have the opportunity to increase the KVA without exceeding the insulation class of 220degC.
SHould you have a 115degC rise transformer you have an overload capacity or 15% without exceeding the 220degC class insulation at 40degC (104degF) ambient.
As such the 40degC ambent rating by itself should cover his requirements.
Dry type transformer ratings are based upon the insulation class temperature rating.
40deg ambient + 150gedegC rise+30degC hot spot allowance=220degC. By lowering the ambient temperature of ratong the transformer a 115deg C or even 80degC can provide extra capacity woth our exceeding that 220degC insulation class.

All I was saying is in a typical application with a dry-type having a calculated connected load of 167kVA would likely be acceptable for a 150kVA rated transformer for reasons stated earlier. I do agree with you completely though. If thermal limits are met transformers can be loaded further than their nameplate ratings for prolonged periods without damage to the equipment: http://img148.imageshack.us/i/transformern.jpg/

 

[BrianMuir]

Turns out that a 150KVA unit is installed, whew.

 

[templdl]

skeshesh,

I consider these factors to see if overloading is a possibility, which is if it may be feasible. If it appears to be then I look into it farther and maybe run it past a transformer design engineer to see what extra capacity is available. Basically you may be able to run it over the name plate rating without actually ?overloading? the transformer.
With the 25degC it looks as if he has some based upon just that but he also alluded to a diversity factor which he wasn't very specific about.