# Simple calculation?



## kbsparky (Sep 20, 2007)

On one of my projects is a 277/480 Volt 3-phase buss duct bank. It is fed directly from of a bank consisting of (3)500 kVA transformers (one per phase).

Does this mean I have a 600 Amp source, or a 1800 Amp (500 vs 1500 kVA)?


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## Dennis Alwon (May 9, 2009)

Each transformer just boost one phase? If so then you have a 500kva set up


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## hardworkingstiff (Jan 22, 2007)

kbsparky said:


> On one of my projects is a 277/480 Volt 3-phase buss duct bank. It is fed directly from of a bank consisting of (3)500 kVA transformers (one per phase).
> 
> Does this mean I have a 600 Amp source, or a 1800 Amp (500 vs 1500 kVA)?


I would say 1800 A

Edit: 
500KVA/277 = 1805

1500KVA/(480*1.732) = 1805


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## wildleg (Apr 12, 2009)

ea phase has 602 amps, so you have a 600 amp service.


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> ea phase has 602 amps, so you have a 600 amp service.


Will you show the math? How did you come up with a 500kVA transformer only supplying 602 amps at 277 volts?


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## macmikeman (Jan 23, 2007)

kbsparky said:


> On one of my projects is a 277/480 Volt 3-phase buss duct bank. It is fed directly from of a bank consisting of (3)500 kVA transformers (one per phase).
> 
> Does this mean I have a 600 Amp source, or a 1800 Amp (500 vs 1500 kVA)?


What does the EE who has the stamp for the job say it is?


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## wildleg (Apr 12, 2009)

hardworkingstiff said:


> Will you show the math? How did you come up with a 500kVA transformer only supplying 602 amps at 277 volts?


he said 277/480. you calculate it from the higher voltage for the service rating, no ? 

500000/(480*1.73)=602


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> he said 277/480. you calculate it from the higher voltage for the service rating.
> 
> 500000/(480*1.73)=602


He also said there are 3 500kVA transformers. That means the 3-phase transformer rating is 1500kVA, not 500kVA.


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## wildleg (Apr 12, 2009)

hardworkingstiff said:


> He also said there are 3 500kVA transformers. That means the 3-phase transformer rating is 1500kVA, not 500kVA.


three single pole transformers. that makes one three phase Y or delta


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> three single pole transformers. that makes one three phase Y or delta


Yes, but the overall rating of the 3-phase is the sum of the individual windings.


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## wildleg (Apr 12, 2009)

ok, let me put it in the way I meant to say it.

If he is going to order a panel for his main, it will be a 600A 480V rated panel. Maybe you are talking about something else and I am not understanding ?


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> ok, let me put it in the way I meant to say it.
> 
> If he is going to order a panel for his main, it will be a 600A 480V rated panel. Maybe you are talking about something else and I am not understanding ?


In his setup, that transformer configuration will be able to supply an 1800-amp panelboard.

Let's look at this a little differently. Let's take one of the 500kVA transformers, just one (ignore the other 2). What is the secondary voltage output of this transformer (assuming it was 1/3 of a 480/277 configuration)? I say it is putting out 277-volts. (Once we bring in the other transformers we will be able to get the 480 because of the phase difference).

Do you agree or disagree?


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> In his setup, that transformer configuration will be able to supply an 1800-amp panelboard.
> 
> Let's look at this a little differently. Let's take one of the 500kVA transformers, just one (ignore the other 2). What is the secondary voltage output of this transformer (assuming it was 1/3 of a 480/277 configuration)? I say it is putting out 277-volts. (Once we bring in the other transformers we will be able to get the 480 because of the phase difference).
> 
> Do you agree or disagree?


You are correct my good sir. 500000/277=~ 1800a. Three phase at 1800a is an 1800 amp 3 phase setup. Thank you


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## wildleg (Apr 12, 2009)

maybe I'm all wet on this, but I see a 600A 480v service. I guess the question is, if this is not feeding a service, but just three disconnects at 277 volts, then sure, you could install 3 single pole discos rated 1800A 277V. I guess I'm not sure what this doing.


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> maybe I'm all wet on this, but I see a 600A 480v service. I guess the question is, if this is not feeding a service, but just three disconnects at 277 volts, then sure, you could install 3 single pole discos rated 1800A 277V. I guess I'm not sure what this doing.


A 3-phase service can be supplied by 1 3-phase transformer which is made with 3 windings (of equal kVA value) on one core. The overall transformer rating is the sum of the ratings of the 3 individual windings.

You can also supply a 3-phase service with three separate single-phase transformers (usually of equal kVA value) fed from different phases of the distributions system. The 3-phase rating is equal to the sum of the 3 individual transformer ratings.

So, the OP has 3 500kVA transformers supplying his service for a total 3-phase service of 1500kVA.

I believe you are of the understanding that a 500kVA 3-phase transformer would be constructed with 3 windings rated 500kVA each, and that is a misunderstanding.


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## wildleg (Apr 12, 2009)

hardworkingstiff said:


> A 3-phase service can be supplied by 1 3-phase transformer which is made with 3 windings (of equal kVA value) on one core. The overall transformer rating is the sum of the ratings of the 3 individual windings.
> 
> You can also supply a 3-phase service with three separate single-phase transformers (usually of equal kVA value) fed from different phases of the distributions system. The 3-phase rating is equal to the sum of the 3 individual transformer ratings.
> 
> ...


right, and a 600A 480 V service would supply 3 legs, each with 500KVA, for a total of 1500kVA on the service. but it is not a 480V 1800A service.


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> right, and a 600A 480 V service would supply 3 legs, each with 500KVA, for a total of 1500kVA on the service. but it is not a 480V 1800A service.


OK, so do you agree that each leg supplies 277V?


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## wildleg (Apr 12, 2009)

ok, so doing it backwards, if you were supplying 1500kva, then you have an 1800A service. makes perfect sense. for some reason I am totally stuck the other way. don't know why.


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## hardworkingstiff (Jan 22, 2007)

wildleg said:


> ... for some reason I am totally stuck the other way. don't know why.


Man that happens to me all the time. But, I have an excuse, I'm getting old. :laughing:


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## kbsparky (Sep 20, 2007)

Wow! What a discussion!

I originally calculated the service capacity at just over 600 Amps, based on a 500 kVA transformer.

But then it occurred to me that we have three transformers in a bank, 500 kVA each, which aggregates into a total capacity of 1500 kVA.

Let's look at this another way: IF I were to purchase a "3 phase transformer," rated at 45 kVA, would its guts consist of 3 separate 15 kVA windings? 

If that is the case, then I have an 1800 Amp service here, not a 600 Amp....

Thanks to all who participated in this, I have learned something.


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## hardworkingstiff (Jan 22, 2007)

kbsparky said:


> IF I were to purchase a "3 phase transformer," rated at 45 kVA, would its guts consist of 3 separate 15 kVA windings?


It is my understanding yes, wound on one core.


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## Tsmil (Jul 17, 2011)

hardworkingstiff said:


> It is my understanding yes, wound on one core.


Diagrams always help me.


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