3 Phase transformer sizing

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jake21

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I am new to this industry, so my question may seem stupid to some of you, but I will still ask :)

I have three loads of 240/3/60, 35.4 A each. I need to select a 3-phase, 480/240V transformer for these loads. Can somebody suggest me the correct transformer size.

Thanks.
 
For a 3-phase load, the VA is found by multiplying the phase-to-phase voltage by the current, and multiplying the result by the square root of 3. For your example,
240 x (3 x 35.4) x 1.732 = 44,145 VA. The next higher standard size transformer is 45 KVA. But if you select that one, you will have no capacity for other existing loads or for any possible future load growth. So I would suggest considering a higher size, depending on what you know (or can learn) about other loads this transformer might have to serve.

Welcome to the forum.
 
jake21 said:
I am new to this industry, so my question may seem stupid to some of you, but I will still ask :)

I have three loads of 240/3/60, 35.4 A each. I need to select a 3-phase, 480/240V transformer for these loads. Can somebody suggest me the correct transformer size.

Thanks.
Click to expand...
Not sure just what I should tell you

your profile says you are training to be an engineer - this should be pretty basic stuff or you are literally just starting your electrical educating process. If just starting your education you should not be the one making such decisions yet.

If you were tasked to figure this out by a supervisor or instructor as a sort of test, you need to do your own research, asking us for a direct answer is the easy out.
 
charlie b said:
For a 3-phase load, the VA is found by multiplying the phase-to-phase voltage by the current, and multiplying the result by the square root of 3. For your example,
240 x (3 x 35.4) x 1.732 = 44,145 VA. The next higher standard size transformer is 45 KVA. But if you select that one, you will have no capacity for other existing loads or for any possible future load growth. So I would suggest considering a higher size, depending on what you know (or can learn) about other loads this transformer might have to serve.

Welcome to the forum.
Click to expand...

especially if these are motor loads.
 
kwired, even a newly graduated electrical engineer would not necessarily know how to do this type of calculation. The study of power systems is not a requirement in many EE degree programs.
 
charlie b said:
kwired, even a newly graduated electrical engineer would not necessarily know how to do this type of calculation. The study of power systems is not a requirement in many EE degree programs.
Click to expand...
Why? They spend more time and money on education then the typical "electrician" and can not even do load calculations when all is done:?

I likely could have answered this question 3-4 months into a one year degree at a community college about 25 years ago.

Any way I still feel he needs to do some real studying and learn how to do this. Just handing him the answer doesn't teach him anything, and this is basic stuff for the position he claims to be in. Throw a few wrinkles into the mix (maybe power factor issues, motor starting concerns, harmonics issues... and maybe the question becomes more valid from someone in said position.

Sorry don't intend to be mean, but I am kind of going along with the lines of forum rules here as well.

Had he been an apprentice instead of an engineer - I may not have been as hard on him. I don't expect an apprentice to know this, but an engineer should, or if at point where they haven't learned this yet should not be in a position to be making such decisions. That leaves us with it possibly being school work and he is learning how to do it - then we can give him tips but shouldn't just be doing it for him.
 
kwired said:
Why? They spend more time and money on education then the typical "electrician" and can not even do load calculations when all is done:?

I likely could have answered this question 3-4 months into a one year degree at a community college about 25 years ago.

Any way I still feel he needs to do some real studying and learn how to do this. Just handing him the answer doesn't teach him anything, and this is basic stuff for the position he claims to be in. Throw a few wrinkles into the mix (maybe power factor issues, motor starting concerns, harmonics issues... and maybe the question becomes more valid from someone in said position.

Sorry don't intend to be mean, but I am kind of going along with the lines of forum rules here as well.

Had he been an apprentice instead of an engineer - I may not have been as hard on him. I don't expect an apprentice to know this, but an engineer should, or if at point where they haven't learned this yet should not be in a position to be making such decisions. That leaves us with it possibly being school work and he is learning how to do it - then we can give him tips but shouldn't just be doing it for him.
Click to expand...

I appreciate your answer and suggestions. I know KVA= root 3 x V x I, so I reached to 43KVA, but my question was whether I should be going to next higher size or stick to 45KVA as I understand the industry standards of loading a transformer to a 80% max. But I am sure I am not going to add any more load to this transformer in future, so in that case does NEC allows me to load it to 97% or not was my concern. Sorry to throw you off with my question.
 
jake21 said:
I appreciate your answer and suggestions. I know KVA= root 3 x V x I, so I reached to 43KVA, but my question was whether I should be going to next higher size or stick to 45KVA as I understand the industry standards of loading a transformer to a 80% max. But I am sure I am not going to add any more load to this transformer in future, so in that case does NEC allows me to load it to 97% or not was my concern. Sorry to throw you off with my question.
Click to expand...

there is no industry standard to load a transformer to 80%. if it is mostly motor loads, 50% (or less) might be more appropriate. these are design choices, not code requirements. in fact, as best I can tell there is no minimum size that a transformer has to be per code. as long as it is properly protected you can use a 15 kvA transformer in this case. it might not work real well, but it is not a code violation.
 
petersonra said:
there is no industry standard to load a transformer to 80%. if it is mostly motor loads, 50% (or less) might be more appropriate. these are design choices, not code requirements. in fact, as best I can tell there is no minimum size that a transformer has to be per code. as long as it is properly protected you can use a 15 kvA transformer in this case. it might not work real well, but it is not a code violation.
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Thanks, this is what i wanted to know if there is any NEC code which stops me to load a transformer to certain extent. In my case, there is no motor load, it is more of combination of heaters and some other control loads. So, I guess 45KVA should be OK. I was worried of going to higher size because it increases the available fault current and SCCR of my load is just 5KA.
 
jake21 said:
I appreciate your answer and suggestions. I know KVA= root 3 x V x I, so I reached to 43KVA, but my question was whether I should be going to next higher size or stick to 45KVA as I understand the industry standards of loading a transformer to a 80% max. But I am sure I am not going to add any more load to this transformer in future, so in that case does NEC allows me to load it to 97% or not was my concern. Sorry to throw you off with my question.
Click to expand...
If you would have asked that question in the OP, I would have never replied with what I did. All I saw was how do I size a transformer to supply three loads that are 35.4 amps each, and that they likely are 240 volt three phase loads.
 
kwired said:
If you would have asked that question in the OP, I would have never replied with what I did. All I saw was how do I size a transformer to supply three loads that are 35.4 amps each, and that they likely are 240 volt three phase loads.
Click to expand...

It was my bad to not frame the question correctly. Learning!! :)
 
kwired said:
Not sure just what I should tell you

your profile says you are training to be an engineer - this should be pretty basic stuff or you are literally just starting your electrical educating process. If just starting your education you should not be the one making such decisions yet.

If you were tasked to figure this out by a supervisor or instructor as a sort of test, you need to do your own research, asking us for a direct answer is the easy out.
Click to expand...

...
 
charlie b said:
kwired, even a newly graduated electrical engineer would not necessarily know how to do this type of calculation. The study of power systems is not a requirement in many EE degree programs.
Click to expand...

Thanks for the answer and backing me up here. I guess I framed the question incorrectly. So, do you also agree with the fact that there is no NEC code which restricts me from loading a transformer to 100%? Just making sure since I did not get any such code in NEC. Please do reply.
 
No NEC violation, may even be able to overload it some and still be ok with NEC in some situations.

Is really more of a design issue, and whether you have any concerns of the load and/or possibly ambient temp effecting the life of the transformer.
 
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