Transformer OA/FA/FOA secondary feeder size amps

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JEFF MILLAR

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Given Transformer size rated OA/FA/FOA .Do you size the secondary feeder cable ? Do you size for the FOA rated amps, or the base OA KVA AMPS, I have searched far and wide and can find no technical reference for sizing the Ampacity of the secondary feeders. Can you offer experienced help. Thank you.
 
It is how the liquid filled transformer is cooled. OA is oil air which is the base Rated KVA. FA is the fan cooling which adds 12% to the Rated KVA and FOA is fan and oil cooling which increases Rated KVA by an additional 25 %.
 
Old terminology:
OA = Oil and Air cooled - standard liquid filled construction now called ONAN
FA = Forced Air - fans added to standard construction.
FOA = Forced Oil and Forced Air - fans added to standard construction and pumps added to move oil through the radiators faster than normal circulation.
 
ONAN IS OIL NATURAL, AIR NATURAL. There is no fan cooling. Can you please answer my question as written. Your answer to size the cables to meet the load is normally how a transformer is sized. What do you do if the transformer is OA/FA/FOA. Given OA is the base Rated KVA.
 
JEFF MILLAR said:
Can you please answer my question as written.
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If you intend to feed a continuous load that is 1000kVA, you need sufficiently sized cables, regardless of the source (transformer) size
Likewise, if the intent is to feed a continuous load of 100kVA with a short time (17 minutes out of every 2 hours) load of 900kVA, you need suitably sized cables.

So, if you intend to feed a load equal to the maximum capacity of your transformer, you would need cables sufficient for that load.
I believe you said that would be 125% rated kVA based on the construction you have chosen and ignoring and environmental or operating considerations.
 
JEFF MILLAR said:
ONAN IS OIL NATURAL, AIR NATURAL. There is no fan cooling. Can you please answer my question as written. Your answer to size the cables to meet the load is normally how a transformer is sized. What do you do if the transformer is OA/FA/FOA. Given OA is the base Rated KVA.
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Though I agree with Jim in that the load supplied is somewhat more important to conductor size and overcurrent protection, isn't KVA rating on the nameplate going to be somewhat dependent on the cooling method? Same unit but with an external radiator and/or cooling fan should have more KVA rating than one that is otherwise same but without any those features.
 
kwired said:
Same unit but with an external radiator and/or cooling fan should have more KVA rating than one that is otherwise same but without any those features.
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But the question should be; how much load are you intending to pull? 100KVA pulled from a 10MVA source needs the same conductors as 100kVA pulled from a 50kVA transformer. We size service conductors based on the main overcurrent device, not the size of the transformer actually installed by the utility.

Now, what I think the OP wants to know is how much kVA can be pulled from a transformer which was designed with increased capacity due to additional cooling.

The OP said the transformer was rated for 100%, 115%, and 125% capacity based on its accessories.

What I do not know is if there are loading restrictions based on project specifications or environmental considerations. The transformer may have been selected with additional cooling in order to allow 100% load during high temperature conditions. Transformer losses go up with the square of the loading, so a transformer at 125% rated KVA could have losses of 156%, so it may never have been intended to be loaded this high on a permanent basis.
 
Thank you very much for your replies. I now understand that the LOAD on the Transformer is usually kept within the OA Rated KVA and the FA and FOA KVA ratings are for emergency conditions. If I understand this correctly the Transformer would have a nameplate rating that would include the FOA Rated KVA. The secondary feeder conductors would be sized for max. KVA rated amps. Given OA = 878.3 A, FA = 878 x 1.33 = 1168 A. and FOA = 878 x 1.66 = 1457 A. Question can the FA KVA rating be used to calculate the largest motor size calculation or the OA only KVA rating ?
 
FA and FOA ratings are not just for emergency. They can be used for continuous loading.

The transformer also has a temperature basis for its ratings which must also be considered, the same as you would consider temperature ratings for wire ampacity. That is what Jim was referencing, not that FA and FOA is an emergency only rating.
 
Understood. That is then the correct answer to my original question. Understanding the FOA Rated KVA on the nameplate would be used to size the secondary feeders for max possible continuous load. Much appreciate your kind technical clarification. Thank you.
 
Refer to Jim's post about feeder size. He was spot on.

The motor size can be based on OA, FA, and FOA rating. You must consider both the running amps and starting amps. The source impedance will determine the volt drop during start for motor start and flicker analysis.
 
JEFF MILLAR said:
Understood. That is then the correct answer to my original question. Understanding the FOA Rated KVA on the nameplate would be used to size the secondary feeders for max possible continuous load. Much appreciate your kind technical clarification. Thank you.
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Assuming the environmental conditions agree with the ratings, then the FOA rating is the max continuous loading.

But as Jim pointed out, just looking at a transformer nameplate FOA kVA is not how you size feeders.
 
OA/FA/FOA, 55/65 C. LIQUID FILLED TRANSFORMER, OA KVA = 2500
How do you size the secondary feeders ? for the connected KVA load or the max. available FOA, 55 / 65 C LOAD.? This is were I get confused and would like a clear technical reference that defines this answer. If you size for the OA KVA LOAD, SAY 2000 KVA , then why have fan cooling and oil pumps. Are they for ambient changing conditions. And can the FA KVA be used in max motor load size that can be started ? sorry if my question was not clear. " How do you size the secondary feeders for a liquid filled Transformer with added fan cooling and oil pumps to circulate the liquid cooling. I can find no where any written technical answer. Please be specific with reference if possible.
 
JEFF MILLAR said:
OA/FA/FOA, 55/65 C. LIQUID FILLED TRANSFORMER, OA KVA = 2500
How do you size the secondary feeders ? for the connected KVA load or the max. available FOA, 55 / 65 C LOAD.? This is were I get confused and would like a clear technical reference that defines this answer. If you size for the OA KVA LOAD, SAY 2000 KVA , then why have fan cooling and oil pumps. Are they for ambient changing conditions. And can the FA KVA be used in max motor load size that can be started ? sorry if my question was not clear. " How do you size the secondary feeders for a liquid filled Transformer with added fan cooling and oil pumps to circulate the liquid cooling. I can find no where any written technical answer. Please be specific with reference if possible.
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The KVA rating is whatever the manufacturer says it is. If they say 2000 OA, and 2750 when it has pink flowers painted on it, and you have pink flowers painted on it, then its rated 2750. As Jim said just make sure there is no "fine print" to the values with accessories.

Also, as has been said, usually you start with your load, and then size the conductors, transformer and OCPD. If you have a given transformer, you dont have to size the conductors to its full potential if you dont want/need to (transformer PRIMARY conductors for MV transformers DO need to be sized to minimum of KVA capacity, 215.2(B)(2))

Note conductor size is also determined by the 450.3 transformer protection size, because of 240.21(C).
 
The question is simple as written, How do you size the secondary conductors for OA/ FA / FOA. 55/ 65C TRANSFORMER.
Two possible answers , the OA Rated KVA at 55 C , OR the max possible Rated KVA at FOA, 65 C. And a reference addressing this would be much appreciated. I have searched all over for a simple technical to this problem and can find none, I know how to size a transformer, calculate the short circuit, size the overload protection. Answers how to start the largest motor are all easy to find. But no where is there a simple written answer how to size the secondary conductors. The answer could be to size the conductors for the nameplate largest Rated KVA. Motor starting where is it written that you can use the FA or FOA Rated KVA in the calculation. Please for give my lack of field experience. This must be a simple answer based on daily industrial applications. Thank you
 
The max possible Rated KVA at FOA, 65 C.

For a reference I’m going with good old fashioned common sense.
why build to the transformer, need the max amps it can deliver continuously, only to have the wire burn down...


FWIW, I don’t do that...
I size the wire for the expected load with future growth projections.
The XF shouldn’t be overloaded. Why spend that much money and burn it up..
 
The applicable KVA will be whatever the transformer says for whatever auxiliary cooling equipment it has.

Secondary conductors are determined by the load and the OCPD at the end of the transformer secondary conductors, 240.21(C), and are usually not sized to the transformer KVA on the nameplate.
 
JEFF MILLAR said:
The question is simple as written...
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And the answer is equally simple. Size the conductors to what your load requires.
In your case the transformer can produce 125% of rated load, but what if your load only requires 120%?

Transformer sizing can be quite an 'art and science' rather than a simple look up.
If your transformer is sufficiently large enough, say in the MVA size, and there is a extremely long period between large loads occurring, say 15 minutes out of every 48 hours, its maximum loading can easily exceed its highest nameplate rating, but the conductors still need to be sized for the maximum load.

In my experience most people do not take the time to size their transformers based on the actual loading profile, rather they look at the continuous loading on the nameplate, especially for transformer in the kVA size.
 
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