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#187389 - 06/25/09 01:54 PM Is this transformer calculation correct?
bigpapa Offline

Registered: 03/18/06
Posts: 75
Loc: canada
600v 3 phase to 120/208v 150kva dry type isolation transformer.
For some reason I need to be reminded of certain things that I have
forgotten over the years. Maybe someone can refresh me on this
transformer calculation.
Transformer is dry 150kva 3ph 600 to 120/208v isolation type.

Rated primary current is 144.5A
Primary breaker chosen is 150A therefore secondary current is 150x turns ratio of
2.88 which is 432A
Secondary conductors are 125% of RATED secondary current and
not actual secondary current which would be 416.6 x 1.25 = 520.8A.

I am thinking that secondary conductors are not calculated on the basis of actual secondary current but the transformers rated current.
Am I correct or have I missed something here?

2014 / 2011 NEC & Related Books and Study Guides
#187395 - 06/25/09 05:52 PM Re: Is this transformer calculation correct? [Re: bigpapa]
Yoopersup Offline

Registered: 03/04/03
Posts: 826
Loc: Michigan
This is a 480 volt 3 phase to 120/208 volt 3 phase right??
Also this is a feeder right???
1.73 x 480 = 830.4
150 x 1000 = 150000
150000 divided 830.4 = 180.63 amps primary
1.73 x 208 = 359.84
150000 divided by 359.84 = 416.85

then go to table 450.3(B)

That 600volts on Primary I think might be more like 480 volts.

#187398 - 06/25/09 06:32 PM Re: Is this transformer calculation correct? [Re: Yoopersup]
bigpapa Offline

Registered: 03/18/06
Posts: 75
Loc: canada

#187401 - 06/25/09 08:29 PM Re: Is this transformer calculation correct? [Re: bigpapa]
twh Offline

Registered: 03/11/04
Posts: 895
Loc: Regina, Sask.
Rule 26-258: 125% of rated secondary current
150,000 / 208 / 1.732 x 1.25

primary conductors are also 125% in the same rule. It's the same calculation except 600 volts is substituted for 208 volts.

The same rule allows you to reduce the size to the demand providing... etc.

Overcurrent rules are in 26-256. Watch the ampacity of panels and splitters fed from transformers. They also must be rated for 125%.

For our Southern friends, 600/347 volts is common in Canada.

#187403 - 06/25/09 10:30 PM Re: Is this transformer calculation correct? [Re: bigpapa]
jay8 Offline

Registered: 07/28/05
Posts: 183
Loc: Vancouver, BC
Lets not turn this into some American bashing thing, his calculations are correct for secondary current. Maybe you forgot to use those little icons above each post.


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