# Transformer Sizing Help



## Magnettica (Jan 23, 2007)

An electrician would know all of this information. 

Does this look like the electrical isle at the Home Depot?

Sent from my iPad using ET Forum


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## Mr. Sparkle (Jan 27, 2009)

You should dig out the NEC and post your math first.


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## bobelectric (Feb 24, 2007)

I would feed a75 k.v.a. xfmr. with 90 amps to a 225 3ø panel.


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## Island Electric (May 9, 2011)

Amps = (KVA x 1000) divided by (volts x 1.732)

75 x 1000 = 75000

208 x 1.732 = 360

75000 divided by 360 = 208 amps step down side 

Transformer size 75 KVA


480 breaker=

480 x 1.732 = 831



75000 divided by 831= 90 amps

90 amp breaker on 480 side.

Transformer size 75 KVA


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## chicken steve (Mar 22, 2011)

Ugly's.....

~CS~


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## newspark80 (Feb 20, 2011)

Thank you to those that sent some help! It is humbling to say the least getting on here posting questions that I know I should know. Part do to my own laziness and part to "getting the dumb end of the measuring stick my entire apprenticeship" I have had to work much harder to get this stuff engraved in my head. I will in future posts do some bookwork. Just got this job given to me and got a little overwhelmed. Know how to do the physical work, just need to brush up on my books.


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## micromind (Aug 11, 2007)

A few posts here recommended a 75KVA transformer. 

What if the load is 100KVA?

Or 10KVA?

It's simply not possible to size a transformer without knowing what the load will be.


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## dronai (Apr 11, 2011)

Mr. Sparkle said:


> You should dig out the NEC and post your math first.


 *I Agree !!!*



Magnettica said:


> *An electrician would know all of this information. *
> 
> Does this look like the electrical isle at the Home Depot?
> 
> Sent from my iPad using ET Forum





newspark80 said:


> Thank you to those that sent some help! It is humbling to say the least getting on here posting questions that I know I should know. Part do to my own* laziness* and part to "getting the dumb end of the measuring stick my entire apprenticeship" I have had to work much harder to get this stuff engraved in my head.* I will in future posts do some bookwork.* Just got this job given to me and got a little overwhelmed. Know how to do the physical work, just need to brush up on my books.


*I think people should at least show some knowledge first, and post some of their calculations, before just being given the answer.*


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## Island Electric (May 9, 2011)

*


micromind said:



A few posts here recommended a 75KVA transformer. 

What if the load is 100KVA?

Or 10KVA?

It's simply not possible to size a transformer without knowing what the load will be.

Click to expand...

*He said he is installing a 200 A Panel.


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## Magnettica (Jan 23, 2007)

dronai said:


> *I think people should at least show some knowledge first, and post some of their calculations, before just being given the answer.*



Totally agree...


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## Island Electric (May 9, 2011)

*


dronai said:



I think people should at least show some knowledge first, and post some of their calculations, before just being given the answer.

Click to expand...


What do you think this is Mike Holt:no: 


This for the OP... What wire size are you going to use on the 480 side? What wire size are you going to use on the 208 side? What size Ground will you use? Where will you Ground it to. What will the Neutral Lug be Labeled on the 208 side of the transformer? Will you bond it to the frame of the Transformer?
Yup this is a test:laughing:*


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## BBQ (Nov 16, 2010)

bobelectric said:


> I would feed a75 k.v.a. xfmr. with 90 amps to a 225 3ø panel.


While that is certainly a code compliant installation it could also be hard to energize the the transformer with a 90 amp breaker, the inrush could be a problem and as transformers are made more efficient a by product of that is higher inrush currents so just because it worked on another 75 KVA is no proof that it will work on another.

Usually I see engineers specify at least a 125 amp breaker with a 480 volt primary 75 KVA.


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## newspark80 (Feb 20, 2011)

Island Electric said:


> What do you think this is Mike Holt:no:
> 
> 
> This for the OP... What wire size are you going to use on the 480 side? What wire size are you going to use on the 208 side? What size Ground will you use? Where will you Ground it to. What will the Neutral Lug be Labeled on the 208 side of the transformer? Will you bond it to the frame of the Transformer?
> Yup this is a test:laughing:


Thanks for the help and Questions. Dusted of my Code Calc book and and found this info. Would you check it out.

I = kVA x 1000 / E x 1.73
Where I got lost is where you started out your previous post with the already known 75kVA transformer. Is this just a known common size? I get the math after that....but apparently I should know the different sizes available.

Size wire on the primary side 480 for 90 amps I am using 2 Al from table 310.16. My OCP is going to be a 100 amp breaker. Was thinking of uping my wire size to 1 Al. Using 1 1/2 flex from Table C.3

Size of wire on 208 secondary side for the 200 amps I am using 3/0 Cu. Do I count my neutral as a current carrying conductor coming from the Transformer? This means I have to go with 4/0 because my conduit is over 24 inches?

Yes I am bonding the transformer.

I am running a #4 ground to the MDP ground bussing.

Then neutral lug labled? I have seen that the neutral lug is usually labled XO but I will be landing my neutral on the grounding us bar. There is no XO lug on my transformer.

Do I need grounding bushings because I am using flex? I have done this before in the past but I can not find why.

Thanks


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