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Thread: Inrush Calculation

  1. #1
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    Marlboro, NJ, USA
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    Inrush Calculation

    How can I calculate/determine if the 100A circuit breaker protecting a transformer won't trip due to inrush current?

    I have a transformer rated for 25KVA
    Primary: Operating on 208V single phase using 2 wires
    Secondary: Operating on 480V single phase using 2 wires
    The connected load on the secondary is 13300 VA operating at 480V single phase.

    Per Per NEC Table 450.3 B Primary protection only: the maximum primary protection is 120 * 1.25 = 150A.
    However, the existing panel has a label stating to use 100 A circuit breakers maximum.

    How can I guarantee a 2-pole 100A breaker on the primary side won't trip due to inrush current?
    Are their any calculations I can perform?
    Should I use an HACR breaker?

  2. #2
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    Quote Originally Posted by Malteser View Post
    How can I calculate/determine if the 100A circuit breaker protecting a transformer won't trip due to inrush current?

    I have a transformer rated for 25KVA
    Primary: Operating on 208V single phase using 2 wires
    Secondary: Operating on 480V single phase using 2 wires
    The connected load on the secondary is 13300 VA operating at 480V single phase.

    Per Per NEC Table 450.3 B Primary protection only: the maximum primary protection is 120 * 1.25 = 150A.
    However, the existing panel has a label stating to use 100 A circuit breakers maximum.


    How can I guarantee a 2-pole 100A breaker on the primary side won't trip due to inrush current?
    Are their any calculations I can perform?
    Should I use an HACR breaker?

    The snarky answer would be hook it up, but seriously... It takes a current approximately 10 times the breaker size to trip in the time that the field is being generated in a transformer, So, it is not likely that a 100A breaker is going to trip from the inrush. I just wouldn't worry about it. But (and I am just an electrician, get a sensitive meter, and read the resistance of the primary coil. The maximum current is going to be that I=E/R and it will go down immediately. A breaker is generally a plus or minus 10% and it isn't even within 1-2% for repeats, so you can't really calculate exactly what amperage would trip the breaker.


    I know what I don't know, and I know where to go to find it!

  3. #3
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    Marlboro, NJ, USA
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    Thanks. Your response helped.

    I'm an electrical designer so I have very little hands on experience. I'd look really bad to our client if I designed something and said we need to test it after it's built to confirm if it'll work properly. In this case, I didn't even do the design and I'm just doing construction support.

  4. #4
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    Quote Originally Posted by Malteser View Post
    Should I use an HACR breaker?
    Warning-side track
    The HACR rating does not have anything to do with the ability to handle inrush currents. It may have at one time (like back in th 50's) but it is an all but meaningless rating at this time. Presently, and for maybe 20 years, the old HACR requirements have been a part of the UL standard for all breakers. So, if you have a breaker listed to UL489, you automatically have an HACR breaker (an actual HACR label is optional, per UL, even though some inspectors still ask for it).


    Now back to the problem.
    Is your transformer designed as a step-up transformer? If so, then the 10 times rule of thumb is probably reasonable, although the standard 'power study' method is usually to check the Time-Current curves at 12 times for 1 sec.

    If your transformer is being run in reverse (a step-down being used as a step-up), then the rule of thumb should be at least 14 times and maybe as high as 20 times.

    Transformer inrush can not be found on nameplates or even in most catalogs, you need to contact the manufacturer.
    Just because you can, doesn't mean you should.

  5. #5
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    Adding to jim's comments:
    Quote Originally Posted by Malteser View Post
    How can I calculate/determine if the 100A circuit breaker protecting a transformer won't trip due to inrush current?

    ...Per Per NEC Table 450.3 B Primary protection only: the maximum primary protection is 120 * 1.25 = 150A.

    ... How can I guarantee a 2-pole 100A breaker on the primary side won't trip due to inrush current?
    Are their any calculations I can perform?
    Should I use an HACR breaker?
    Quote Originally Posted by Malteser View Post
    .... I'd look really bad to our client if I designed something and said we need to test it after it's built to confirm if it'll work properly. In this case, I didn't even do the design and I'm just doing construction support.
    Disclaimer: I'm a field engineer, mostly construction support, and commissioning. Most of the design work I do is submit design changes to get the installation to meet the client' expectations.

    First: You are absolutely correct - if there are nuisance trips you will look like a bum.

    With the information you have, there is no way you can guarentee no nuisance trips.

    Quote Originally Posted by Strathead View Post
    ... it is not likely that a 100A breaker is going to trip from the inrush. I just wouldn't worry about it. ...
    Inrush data on small single phase xfm is minimal. Inrush models for small single phase xfm is minimal. I have not seen any papers nor have i seen it covered in any standards.

    The 100A CB available is 83% of the xfm fla. This would not leave me warm and fuzzy.

    strat -
    With all due respect, without MFG data, or your personal test data. I don't know how you could say that.

    So, what to do?
    Depends on how important it is that there are no nuisance trips. Maybe a 100A feeder CB has to be reset occasionaly after a power outage. Most of my customers would have a fit if that happened often enough that they could still remember the last one - say more than once every 7 years. Unless it was mission critical - in which case if it happened ever that would be too often. But, if nobody particularly cares, maybe you are fine with multiple energization attempts. You get to figure this part out.

    So, assuming you have one of my customers:
    I'd call the mfg and see if the tech staff could give me any guidance. Maybe you will get lucky as they have some data.

    If not, go find another panel that will take 150A CBs. Either replace the existing panel or find another with space/capacity.

    No way would I ever guess. But likely that is because most of my works comes from design engineers that did guess


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    Last edited by iceworm; 03-05-13 at 02:19 PM. Reason: slow poster
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