# 3phase transformer calc



## jclarmo (Oct 18, 2012)

When calculating the secondary side amps of a 120/208 Y which voltage do you use in the calculation? 120 or 208?


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## Tom45acp (Sep 6, 2011)

208 volts.


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## erics37 (May 7, 2009)

jclarmo said:


> When calculating the secondary side amps of a 120/208 Y which voltage do you use in the calculation? 120 or 208?


Transformer VA / (208 x 1.7320) = secondary amps.


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## 123electric (Jun 3, 2012)

Save steps. Faster to always remember(360) for 208 volts.
208 x 1.732 =(360)
For 480 volts use (831) 
480 x 1.732 = (831)


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## erics37 (May 7, 2009)

123electric said:


> Save steps. Faster to always remember(360) for 208 volts.
> 208 x 1.732 =(360)
> For 480 volts use (831)
> 480 x 1.732 = (831)


Yeah, no thanks. I find it a lot easier to simply remember one number - the square root of three - than a batch of oddball numbers like 360 and 831, or whatever voltage you come across.

Standard nominal voltages are already indelibly impressed on my brain, as is square root of three, no sense memorizing an additional wad of numbers. Besides, how much time does that really save you? Four seconds per transformer calculation? :laughing:


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## ponyboy (Nov 18, 2012)

I know every common transformer calc by heart after putting together so many of them. And by common I mean common to me- 15, 30, 45, 75, 150, 300


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## l0sts0ul (May 7, 2011)

I use the 360, 832, 1036 all the time for all 3phase calcs. Just easier. 

only time I use 208, 480, or 600 is if Im pulling single phases.

The reason I don't use root 3 is because if you forget it somewhere, you're screwed. if I use those numbers, I know I haven't and can't forget.


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## BBQ (Nov 16, 2010)

ponyboy said:


> I know every common transformer calc by heart after putting together so many of them. And by common I mean common to me- 15, 30, 45, 75, 150, 300


Wow, you are amazing. :notworthy:


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## ponyboy (Nov 18, 2012)

BBQ said:


> Wow, you are amazing. :notworthy:


thank you. i was afraid my awesomeness was going to go unnoticed.


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## Magnettica (Jan 23, 2007)

erics37 said:


> Yeah, no thanks. I find it a lot easier to simply remember one number - the square root of three - than a batch of oddball numbers like 360 and 831, or whatever voltage you come across.
> 
> Standard nominal voltages are already indelibly impressed on my brain, as is square root of three, no sense memorizing an additional wad of numbers. Besides, how much time does that really save you? Four seconds per transformer calculation? :laughing:


I disagree. The numbers 360 and 830 are engrained in my memory.


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## micromind (Aug 11, 2007)

If it's a wye, one way is the line to neutral voltage X 3 gives you the 3ø VA per amp. 

The result is the same as line to line X 1.732


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## BBQ (Nov 16, 2010)

Magnettica said:


> I disagree. The numbers 360 and 830 are engrained in my memory.


Great, now here is a 480 delta - 400Y/231 volt transformer. What is the secondary current?


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## Chris Kennedy (Nov 19, 2007)

kva?


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## BBQ (Nov 16, 2010)

Chris Kennedy said:


> kva?


Not really my point.

The point was that not knowing how to use 1.73 to figure transformers limits you just to the commonly used voltages.


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## l0sts0ul (May 7, 2011)

If you know the 3 used voltages in your area, that ones not mean you don't know the root 3 factor, it just means you understand it at a higher level that you can choose to apply it what you run into a different voltage like 380/50hz or 240/30hz


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## BBQ (Nov 16, 2010)

l0sts0ul said:


> If you know the 3 used voltages in your area, that ones not mean you don't know the root 3 factor, it just means you understand it at a higher level that you can choose to apply it what you run into a different voltage like 380/50hz or 240/30hz


True, but if I was teaching a class or being taught in a class I would want to be taught *how to do it* first and then given the short cuts later.

Short cuts are great as long as you know why they work.


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