Sizing a 3 phase transformer by Load Current

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KNCTChief

New member
Location
Killeen, TX
I'm sizing a transformer for a generator system for our TV and FM stations. The loads vary, everything from tower lights to HVAC systems. The primary side of the transformer is 480V, the secondary will be 208/120. And with everything running, Phase A is pulling 188 amps, B is pulling 186.2 amps, and C is pulling 203.7 amps. Do you just add the currents to calculate KVA, or is it the average of the currents to calculate KVA?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I agree. I think you will end up around a 75kva transformer.
What is the output of your generator ? No matter what size transformer you install you still need the kw output from the generator. With motor loads (HVAC) you may find the need to upsize.
 

Electric-Light

Senior Member
If computers and broadcast equipment represent a significant portions of load, you need to take harmonics into consideration and de-rate accordingly.
 
I'm sizing a transformer for a generator system for our TV and FM stations. The loads vary, everything from tower lights to HVAC systems. The primary side of the transformer is 480V, the secondary will be 208/120. And with everything running, Phase A is pulling 188 amps, B is pulling 186.2 amps, and C is pulling 203.7 amps. Do you just add the currents to calculate KVA, or is it the average of the currents to calculate KVA?

Averge +125%, if no large(greater than 10% of total load) single motor load. If so double the motor size and add to the base load if the motor is under 50% of total load, otherwise start calcualting actual inrush and duration. That of course will/would also change your generator size.(Presume that the generator feeds the transformer when normal power is loast.)
 

Phil Corso

Senior Member
KNCTChief...

Load-profile is extremely important if economy is the goal. If maximum load-profile is based on 24/7 duty, then I suggest:

kVA = K x SQRT(3) x Vavg x Irms, where,

o Vavg is the average of either the primary or secondary ph-ph voltages.

o Irms is the Root-Mean-Square of the 3 line-currents, associated with Vavg.

o If Load-profile is well known, then K=1, so pick next standard xfmr size!

o If Load-profile is unknown, then K = 1.25, and pick next standard xfmr size!

Regards, Phil
 
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