45 KVA transformer max amperage?

Status
Not open for further replies.

Alfont1120

Member
Location
Dighton, MA
Occupation
Electrical Project Manager/Journeyman Electrician
We are trying to feed a new 200A panel at 120/208 and myself and one of the other electricians are in disagreement of the transformer to install. I wanted to put in a 75 kva, but he is saying we can use a 45 kva because we aren't using 3 phase loads in the panel. (although we will be using two 3phase circuits). I was under the impression that when feeding a transformer with 3 phases, you had to use the 1.73 factor in your current equation, but he says we don't have to because of the lack of three phase circuits. I'd appreciate any input you guys may have, and also your recommendation on a different transformer if neither of us are going about this efficiently, thank you.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
A 45kVA, 208/120v secondary transformer has 125A rated secondary current. It makes no difference if the loads are all single phase or all three phase.

I'd be inclined to use a 75kVA transformer with a 200A panel (or maybe make it a 225A panel.)
 

Alfont1120

Member
Location
Dighton, MA
Occupation
Electrical Project Manager/Journeyman Electrician
A 45kVA, 208/120v secondary transformer has 125A rated secondary current. It makes no difference if the loads are all single phase or all three phase.

I'd be inclined to use a 75kVA transformer with a 200A panel (or maybe make it a 225A panel.)

Thank you David for your response. Is there a code article that could be cited for this or something definitive i could use to show him?
 

Smart $

Esteemed Member
Location
Ohio
Little 3? load has no bearing on transformer sizing if you are using a 3? transformer (contrasting a bank of three 1? transformers). And it usually isn't a concern when using a wye-configured secondary anyway. Your 1? circuits should be balanced on all three legs.

Determine the calculated line current for each of the three legs. Multiply largest by 120V, and again by 3 [windings]... and that'll give you the minimum transformer kVA rating. In some cases where there is a large percentage of motor, that figure would have to be upsized to handle inrush issues, but the figure is a good for basic determination.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I was under the impression that when feeding a transformer with 3 phases, you had to use the 1.73 factor in your current equation, but he says we don't have to because of the lack of three phase circuits.
This isn't a code issue, but rather a basic physics issue. When you deal with three phase systems, the factor of 1.732 is going to come into the equation somewhere. You are dealing with the sizing of a transformer, so the nature of the loads is no relevant. What matters is only the total KVA of the loads. If your mission is to match a transformer size to the rating of a 200 amp panel, you get there by multiplying 200 amps times 208 volts times 1.732. That result is 72 KVA. Even if you allow for the short time overloading of a 45 kva transformer to a level of 125%, it would still only give you 56 KVA. That is well short of the rating of the 200 amp panel. So you should call for the 75 KVA transformer.

 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Determine the calculated line current for each of the three legs. Multiply largest by 120V, and again by 3 [windings]... and that'll give you the minimum transformer kVA rating.
That is correct, in that it gives the right answer. But it is not an approach that I favor. I prefer to always deal with the line-to-line voltage, in this case 208 volts, and not the line-to-neutral voltage of 120. Thus, you multiply the largest calculated current by 208 and then by 1.732, and you get exactly the same answer that your approach would give. There is no difference in the math, but there is a difference in how math is set up to model a physical situation. It's just a personal preference, I know.

 

Smart $

Esteemed Member
Location
Ohio
That is correct, in that it gives the right answer. But it is not an approach that I favor. I prefer to always deal with the line-to-line voltage, in this case 208 volts, and not the line-to-neutral voltage of 120. Thus, you multiply the largest calculated current by 208 and then by 1.732, and you get exactly the same answer that your approach would give. There is no difference in the math, but there is a difference in how math is set up to model a physical situation. It's just a personal preference, I know.

The reason I use 120V x 3 rather than 208V ? 1.732 is to reduce rounding error. The 208V is actually a rounded value derived from 120V ? sqrt(3).
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
141118-1309 EST

What is the total cost of two 25 kVA single phase vs one 75 kVA three phase transformer?

What is the core loss comparison of the above two possibilities?

.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
The reason I use 120V x 3 rather than 208V ? 1.732 is to reduce rounding error. The 208V is actually a rounded value derived from 120V ? sqrt(3).
I looked in vain for your tongue-in-cheek icon - no joy.

So you want to reduce rounding error

Let's see:
120 x 3 = 360

208 x 1.732 = 360.256

360.256/360 = 1.0007111... (repeating)

That's 7/100 of 1%, or 7/10,000 of the total. But both of those are still round-offs.

So, in rational numbers the error is: 90064/90000 (no round-off here)

I'm thinking if this shows on your profit/loss statement - you're screwed

Mathematicians know that sqrt(3) approaches 2 for large values of 3.
:)
This works ;)

ice
 

Besoeker

Senior Member
Location
UK
The reason I use 120V x 3 rather than 208V ? 1.732 is to reduce rounding error. The 208V is actually a rounded value derived from 120V ? sqrt(3).
The error is about 0.15V or 0.074%.

Insignificant for this application I would suggest.
 

Smart $

Esteemed Member
Location
Ohio
You can justify it anyway you want...

My point is I have no rounding error. I have nothing to justify.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
My point is I have no rounding error.
Of course you do. :happyyes: The square root of 3 is a non-terminating, non-repeating decimal number. Anything short of using an infinite number of digits will give you a rounding error. :happyyes:
I have nothing to justify.
And I agree. Your method works as well as mine, as I said already. I am simply not in favor of promoting your method, for the reason I gave earlier.


 

Besoeker

Senior Member
Location
UK
I think that there is a "not" missing somewhere. :)
You are correct of course......my apologies...

So what if all the load is 125A, 120V single-phase and all it on the same phase to neutral?
An unlikely scenario I agree, but it demonstrates that total kVA is NOT only what matters.
 

Smart $

Esteemed Member
Location
Ohio
Of course you do. :happyyes: The square root of 3 is a non-terminating, non-repeating decimal number. Anything short of using an infinite number of digits will give you a rounding error. :happyyes:
No, I do not have any rounding error.

I'm not using the "square root of 3".

I'm just using "3".

I agree we essentially get the same result. Convention has a way of messing with one's rationale (specifically mine at times ;)). Consider that convention started by one person or group making their way the only 'proper' way. And that is fine if you subscribe to that convention. I do not have to. :happyno:

Absolute does not convention make (Yoda speak).
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Load should always be determined in KVA. Amps means nothing until you size the conductors and select the breaker size.

If you put all the load on a single phase of a three phase panel, add up the total KVA of that phase, multiply by 3, you will get the minimum size of your 3-phase transformer. Then calculate amps of the transformer to get your breaker size.

KVA is KVA is KVA!
 
Status
Not open for further replies.
Top