Need help with transformer calculations

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carnes51214

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Was asked this question and need some help to answer.

Primary side is 480v
Secondary side is 240v
Secondary panel is 200amp 3 phase

I need to determine the kVA rating of transformer and the conductor size for both the primary side and secondary side.

Here is what I did.

Since we have a 200 amp panel I took the 240*1.732*200 which gave me 84 kva which next size transformer is 100kva.

Then on primary side I took 100kva divided by 480*1.732 which gave me 120 amps. 120*125% is 150 amps 1/0 cu conductor.

On secondary side I divided 100kva by 240*1.732 which gave me 240.8 amps. 240.8 *125% is 301 amps 350 kcmil conductor.

350 kcmil conductor seems to large for this application. So It seems I'm not calculating something correctly.

Also does the 200 amp panel need to be sized down 80%. Making it 160 amps full load.

Thanks for any help.
 
H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
carnes51214 said:
Was asked this question and need some help to answer.

Primary side is 480v
Secondary side is 240v
Secondary panel is 200amp 3 phase

I need to determine the kVA rating of transformer and the conductor size for both the primary side and secondary side.

Here is what I did.

Since we have a 200 amp panel I took the 240*1.732*200 which gave me 84 kva which next size transformer is 100kva.

Then on primary side I took 100kva divided by 480*1.732 which gave me 120 amps. 120*125% is 150 amps 1/0 cu conductor.

On secondary side I divided 100kva by 240*1.732 which gave me 240.8 amps. 240.8 *125% is 301 amps 350 kcmil conductor.

350 kcmil conductor seems to large for this application. So It seems I'm not calculating something correctly.

Also does the 200 amp panel need to be sized down 80%. Making it 160 amps full load.

Thanks for any help.
Click to expand...

I think you over added a few times:happyyes:

240*200*1.732 = 83.1kva
83.1/480/1.732 = 100amps*1.25=125 amps
you have already set your 240 volt panel amps so there is no other calculation to figure, you know it's 200 amps which will require a 3/0cu conductor.

Now your needing to figure the size of the transformer, from above we know we need 83.1kva of availability to feed the 200 amp panel at full capacity, so we just divide the 83.1/480/1.732 which gives us 100 amps, this is what you size the wire to, #3cu awg nothing requires us to size a transformer @125% but 450.4 allows us to size our breaker feeding it up to 125% for 600 volts and below, this is for inrush current, but pay attention to 240.4(F) as your post is describing a delta to delta configuration if your using the primary protection for the secondary conductors.

I might be wrong but I don't know of anything that says we have to resize our conductors if we up-size the transformer to the next available size, as long as we maintain the primary to secondary ratio?

I could be wrong on the above, but I'm here to learn also
 
T

T.M.Haja Sahib

Guest
hurk27 said:
..... I don't know of anything that says we have to resize our conductors if we up-size the transformer to the next available size..........
Click to expand...

If 200A is the ultimate load,there might be no need to re-size the conductors, unless the chosen size is unsuitable from the fault current level point of view.

If 200A is not the ultimate load,it is better to re-size the conductors accordingly.
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
The inrush is based on the transformer size, so using a larger transformer will mean a higher inrush. The protection will need to be coordinated to handle the inrush, whatever it is.
 
T

T.M.Haja Sahib

Guest
kingpb said:
The inrush is based on the transformer size, so using a larger transformer will mean a higher inrush. The protection will need to be coordinated to handle the inrush, whatever it is.
Click to expand...

Unfortunately,the conductor size based on inrush is lower than that on short circuit fault current and should not be attempted.
 
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