Transformer calculation

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shortcircuit1

Senior Member
Location
USA
Hello,

If i have a 3phase 480-208v 75kva step down transformer supplying a panel 208v,3phase which carries a load of 100kva. How do you relate that kVA to 480v side or what kva would it be on 480v side?

Thanks,
shortcircuit.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Hello,

If i have a 3phase 480-208v 75kva step down transformer supplying a panel 208v,3phase which carries a load of 100kva. How do you relate that kVA to 480v side or what kva would it be on 480v side?

Thanks,
shortcircuit.

Well if your load is TRULY 100kVA in the load side of a 75kVA transformer, you are missing the elephant in the room...

But to your question, kVA is kVA. It doesn’t change with voltage, that’s why you use that instead of amps.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Why would it matter? Are you thinking a calculated load could be too high?
The total connected load might be 100 KVA. But some of that may be non-coincidental loads, and there may be demand factors that come into play. So the total demand load might be lower than the transformer's 75 KVA rating. If so, then there is no problem with using a 75 KVA transformer for this application.

 

jumper

Senior Member
The total connected load might be 100 KVA. But some of that may be non-coincidental loads, and there may be demand factors that come into play. So the total demand load might be lower than the transformer's 75 KVA rating. If so, then there is no problem with using a 75 KVA transformer for this application.


Okay cool, that I understand. I was thinking something else.
 

shortcircuit1

Senior Member
Location
USA
The total connected load might be 100 KVA. But some of that may be non-coincidental loads, and there may be demand factors that come into play. So the total demand load might be lower than the transformer's 75 KVA rating. If so, then there is no problem with using a 75 KVA transformer for this application.


My bad i picked a poor example.
 

Besoeker

Senior Member
Location
UK
The total connected load might be 100 KVA. But some of that may be non-coincidental loads, and there may be demand factors that come into play. So the total demand load might be lower than the transformer's 75 KVA rating. If so, then there is no problem with using a 75 KVA transformer for this application.


I was thinking maybe some of the loads don't run continuously and/or at lower than their nameplate rating.
Not enough information. It needs a detailed study.
 

Besoeker

Senior Member
Location
UK
Sparkies always have to go by nameplate...
I would always advocate that for cabling individual pieces of equipment.
But adding together the total name of all the nameplate to come up with required supply capacity could be overkill in some cases.
 

topgone

Senior Member
Utilities are notorious characters for this when sizing transformers.

Nah. That's not true. It just that utilities factor in the life of the transformer as compared to investing on a bigger transformer when the smaller could do the job at the edge of the limits.
 
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