15 KVA Transformer

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jonny1982

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What's the biggest panel (amps) I can put on a 15 kva transformer (277/480 in, 120/208 out)?

Thanks
 

Smart $

Esteemed Member
Location
Ohio
There is no maximum.

In short, what's important is the transformer secondary conductors and panel bus, and in many cases the transformer secondary itself, are protected by proper rated ocpd and conductor ampacity, and the calculated load does not exceed the lowest of those ratings.
 
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charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I think the question you are trying to ask can be answered in this way. Take 15,000 VA, divide it by 208 volts, and divide it again by 1.732 (i.e., the square root of 3). The result is 125 amps. We are allowed to go up to 125% of that value, which brings us to 156 amps. So I would select a 150 amp panel.

Smart$ was right, in that you can power an 800 amp panel (or whatever) from this transformer, if you wish. But it would be a waste of money, because the highest overcurrent protection device would be 175 amps (i.e., the next standard size above 156 amps).
 

kwired

Electron manager
Location
NE Nebraska
I think the question you are trying to ask can be answered in this way. Take 15,000 VA, divide it by 208 volts, and divide it again by 1.732 (i.e., the square root of 3). The result is 125 amps. We are allowed to go up to 125% of that value, which brings us to 156 amps. So I would select a 150 amp panel.

Smart$ was right, in that you can power an 800 amp panel (or whatever) from this transformer, if you wish. But it would be a waste of money, because the highest overcurrent protection device would be 175 amps (i.e., the next standard size above 156 amps).
Maybe it is because we didn't go to the same school or at the same time but when I take 15000/208 and divide again by 1.732 I get 41.64;)
 

Smart $

Esteemed Member
Location
Ohio
I think...
I agree*... except in some scenarios a higher rated panel may provide the desired number of spaces not available in a lesser rated panel. As such, a downsized main ocpd will be required.

(*And except for calculation error as pointed out by kwired ;))
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Maybe it is because we didn't go to the same school or at the same time but when I take 15000/208 and divide again by 1.732 I get 41.64
I plead "inadquate coffee." :( My best guess is that I hit the "4" on my calculator instead of the "1," and wasn't awake enough to realize the result was obviously too high. Mea culpa.

So please change my design to a 60 amp panel and a 60 amp main breaker.

 

kwired

Electron manager
Location
NE Nebraska
I plead "inadquate coffee." :( My best guess is that I hit the "4" on my calculator instead of the "1," and wasn't awake enough to realize the result was obviously too high. Mea culpa.

So please change my design to a 60 amp panel and a 60 amp main breaker.

I think you inadvertently multiplied by 1.732 instead of dividing, that does yield the result you gave.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Well, if he meant it in Latin instead of Spanish then he needs two more cups of coffee!:D
No, that part I had right. It comes from a point in a particular religion's (that shall not be named, so as to avoid discussions of religion) service, at which the speaker takes responsibility for his or her failures. The services were in Latin in my early youth. The entire quote would have been, "mea culpa, mea culpa, mea maxima culpa," or "through my fault, through my fault, through my most grievous fault."

 

Canton

Senior Member
Location
Virginia
Occupation
Electrician
No, that part I had right. It comes from a point in a particular religion's (that shall not be named, so as to avoid discussions of religion) service, at which the speaker takes responsibility for his or her failures. The services were in Latin in my early youth. The entire quote would have been, "mea culpa, mea culpa, mea maxima culpa," or "through my fault, through my fault, through my most grievous fault."


Just teasing Charlie, I need you to stay on your "A" game....I rely on your posts and comments a lot.:D
 
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