Trasnformer question... rating primary

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DigiMan

Member
Location
Scranton/Wilkes-barre, PA
On a 3 phase Y480-208/120 stepdown transformer, if it's rated for 150,000VA, how would I go about calculating how many amps would be on its primary at max load?

208 x sqrt(3) = 360+/- v
120 x 3 phases = 360v

Would I just take 480v / 150kva???
Or do I have to do something with the ratio of the 480/360 or take into account the 480 Y?
 
bob

bob

Senior Member
Location
Alabama
DigiMan said:
On a 3 phase Y480-208/120 stepdown transformer, if it's rated for 150,000VA, how would I go about calculating how many amps would be on its primary at max load?
Click to expand...

150 kva/(0.480 x 1.73) = 181 amps @ 480 volts.
150 kva/(0.208 x 1.73) = 417 amps @ 208 volts
 
R

rcwilson

Senior Member
Location
Redmond, WA
Back to basics:

For three phase power VA = V x I x 1.732. The square root of three factor is used to convert the 480 V phase-to-phase voltage to the 277V phase-to-neutral and simultaneously account for three phases carrying equal current. Rearrange the equation to solve for I:

I= VA / (V x 1.732) = 150,000 / (480 x 1.732) = 180 amps.

A 150 KVA (that's = 150,000 VA) 3-phase transformer can be considered as three 50 kVA single phase transformers. The single phase voltage feeding each single phase winding is 277 volts = 480/ 1.732. (120V on the 208/120 side.)

Amps for one single phase 50 KVA transformer at 277V = 50 kVA x1000 / 277 = 180 Amps, same answer as above. The 180 Amps on each of the three phases delivers 50 kVA for a total of 3 phases x 50 kVA/phase = 150 kVA.

Put this together:

Three phase kVA = 3 phases x line current x line-neutral voltage / 1000. (= 3 x 180A x 277V /1000 = 150 kVA.

Since Line - Neutral Voltage = phase-phase voltage / 1.732,

Three phase kVA = 3 phases x line current x (phase-phase voltage / 1.732)/ 1000. (= 3 x 180A x (480/1.732)V /1000 = 150 kVA.

But 1.732 is the square root of 3 and 3/1.732 = 1.732. So the 1.732 factor in the denominator and the 3 phase factor in the numerator combine to leave just 1.732.

Result is kVA = Line Current x Phase-Phase voltage x 1.732 / 1000.

kVA = V x I x 1.732 / 1000 = VA/1000

This works for any 3-phase balanced load, not just transformers.

With transformers the turns ratio or voltage ratio is not used to calculate amps. KVA in = KVA out. Just use the rated KVA and the phase-phase voltage on the winding you need the current.


I think I just confused you more.
 
T

TonyThunder

Member
I need to send power to three separate groups of HID lighting with loads of 50 KVA each. My plan is to send 480v from a 15KV : 480Y/277 transformer to a main distribution panel and then send power to individual single phase transformers that will either be 480 : 120/240 or 277 : 120/240. The lights in each group have a 240v ballasts but I must also feed 120v outlets. The distance between each light group is 300' which causes some issues with voltage drop. Is it best to send 480v or 277v to a single phase transformer?

If I send single phase 277v (line and neutral), then I surmise I need 180 amps per phase which is easy, but then I must increase the size of the conductor. But what if I wanted to send single phase 480v (line to line), then how would I calculate the load? If I step back a bit and sum all three groups of lights, I get 50 KVA x 3 = 150 KVA. With a three phase system I would then get 180 amps per phase. Now if I run two hots to a 480v transformer, does that mean each line conductor would then be 90 amps, 180 amps/2 conductors?

Also, if I run 480v to the transformer would I also need to run a neutral or just the two line conductors?
 
bob

bob

Senior Member
Location
Alabama
Tony
I'm sure you did not mean to but you, as we say on this forum, hi-jacked
Dig-mans question and post. We can answer your question but I think you should post your own original question.
 
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