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Thread: Sizing Transformers

  1. #1
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    Sizing Transformers

    Just wondering what the easiest way is to size a transformer. For example, in a commercial/ educational environment, if I needed to install a 3 phase 120/208v panel with 225 amps worth of service would I just use the formula for calculating load side current, I= kVA/(208 x1.73) or for my case, kVA= 208 x 225a x 1.73 ? I came up with about 81 kVA. What am I missing?

  2. #2
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    If your supplying a 225 amp 208Y/120 panel from a 480 feeder the typical transformer size is 75 kVA.

    The primary side breaker is usually between 100 to 150 amps.

    The rated secondary current is 208 amps at 208 volts

    The rated primary current is 90 amps @ 480

  3. #3
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    Quote Originally Posted by iwire View Post
    If your supplying a 225 amp 208Y/120 panel from a 480 feeder the typical transformer size is 75 kVA.

    The primary side breaker is usually between 100 to 150 amps.

    The rated secondary current is 208 amps at 208 volts

    The rated primary current is 90 amps @ 480
    Thanks for the feedback. I left out the fact that I already knew what size transformer I needed because, as you stated, this is a fairly typical application. What I'm trying to figure out is, from a design standpoint, how do they calculate this rating. For example, we have 3 phase/208-120v/ 100a load centers all over the school district that have 22.5 kVA xfmrs feeding them. If you do the math, a 22.5 should only be good for 62 amps. I have yet to get anyone from our engineering firm to tell me how you can put a 100 amp main in a panel that has a transformer that is only good for 62 amps? I'm sure I'm missing something...

  4. #4
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    It's because the panel rating isn't the concern, only the actual load. The panel could be rated for a bazillion amps, but you could feed it with a 22.5 kVA transformer if you know the actual load is only 60 amps. The only requirement for the panel at that point is that is has to be rated for at least 60 amps. It can be larger.

  5. #5
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    Two things come to my mind.

    1) The transformer must have a higher rating then the calculated load.

    2) Table 450.3(B) provides the requirements for transformer over current protection.

    With over current protection on both sides of your typical delta Wye transformers the primary side device can be 250% above the rated load current and the secondary side device can be 125% above the transformer rating and can be rolled up to the next standard size over current device.

  6. #6
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    Keep them coming. All of this is good advice. In response to drbond24, there's nothing to keep maint personel from adding breakers, and therefore load, other than running out of spaces. Sounds like I need to get with our design/ eng firm to request that if they are going to spec 100 amp load centers then they need to be sure the xfmrs will support future expansion right?

  7. #7
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    Quote Originally Posted by ssandoval View Post
    there's nothing to keep maint personel from adding breakers, and therefore load, other than running out of spaces.
    The NEC expects qualified personal to be working on the systems.

    See Article 100 "Qualified Person"

    No one should be adding load without knowing that the supply is adequate.




    Sounds like I need to get with our design/ eng firm to request that if they are going to spec 100 amp load centers then they need to be sure the xfmrs will support future expansion right?
    That is certainly an option if your paying the bill. :smile:

  8. #8
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    Quote Originally Posted by ssandoval View Post
    Keep them coming. All of this is good advice. In response to drbond24, there's nothing to keep maint personel from adding breakers, and therefore load, other than running out of spaces. Sounds like I need to get with our design/ eng firm to request that if they are going to spec 100 amp load centers then they need to be sure the xfmrs will support future expansion right?
    It would be better, and more cost effective, to train your personnel not to add load without confirming the system can handle it.
    Just because you can, doesn't mean you should.

  9. #9
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    A couple of things here, unfortunately in our system "maintenance" and "qualified" are mutually exclusive, even though they are state licensed. The other thing is that there are many times that circuits have to be added for whatever reasons, so we are trying to utilize the spares that are there. The problem seems to be that our engineers are not sizing the xfmr large enough to facilitate future expansion. If I have a 30 ckt load center then I darn sure ought to be able to have thirty circuits without overloading the xfmr, I would think.

  10. #10
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    Quote Originally Posted by ssandoval View Post
    A couple of things here, unfortunately in our system "maintenance" and "qualified" are mutually exclusive, even though they are state licensed.
    I understand the real world but I was giving you NEC reasons. :smile:


    The problem seems to be that our engineers are not sizing the xfmr large enough to facilitate future expansion.
    I am sure the engineers would be more than happy to provide more capacity, I will even say it is likely the first round of drawings had more capacity but when put out to bid and prices came back some 'value engineering' had to be done and not providing capacity for the future is an easy place to save money.

    Not to mention having 'over sized' transformers everywhere wastes power.


    If I have a 30 ckt load center then I darn sure ought to be able to have thirty circuits without overloading the xfmr, I would think.
    You can never assume that.

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