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UPS Cable sizing

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  • 05-10-2013 9:36pm
    #1
    BrianDug
    Registered Users Posts: 163 ✭✭


    Hi,

    Has anyone got experience working with UPS systems? I am wondering about sizing cables for UPS units. Can not find any good information online.

    Say we have a 3-phase 20kVA UPS that operated at 0.99pf. Would we just essentially be sizing a cable based on the 20kVA rating? i.e kVA/V*srt3*pf .. 20kVA/(400*1.732*0.99) = 29.15A and size cable based on this information and keeping the output cable size of the UPS the same as the input?

    I would appreciate any information.

    Thanks


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    #2
    whizbang
    Registered Users Posts: 1,935 ✭✭✭whizbang


    Is it not the case that the 20K rating applies only is if the load is .99pf or better. Derate the UPS at lower factors.?

    So then no need to upsize. I think!


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    #3
    Steve
    Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    As with any load, the supply cable should be sized based on the protection device, you should consult the manual for the UPS for the recommended supply fuse / breaker size.

    At a guess, this will be at least 20A, possibly 32A for a UPS this size.

    You can de-rate the output based on what it's supplying (if it's a fixed load) but the same rule applies re cable size.

    For short runs (less than 5m), you can use EN60204 recommendations for minimum sizes:

    General sizing guidelines for tri-rated cable used in power circuits.
    Rating of CPD (A) Min cable size (mm²)(*)
    Up to 16 1.5
    20 2.5
    25 4
    32,40 6
    50 10
    63 16
    80,100 25
    125,160 35
    (*)Cable derated to 75% of value for single conductor in free air @ 35 deg C


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    #4
    cast_iron
    Registered Users Posts: 2,454 ✭✭✭cast_iron


    BrianDug wrote: »
    Say we have a 3-phase 20kVA UPS that operated at 0.99pf. Would we just essentially be sizing a cable based on the 20kVA rating?
    I think your question has been answered above, but just on this, I don't really see the relevance of the PF of the actual UPS itself here. The PF of the load it's supplying would be more relevant. Your calculation has the PF the wrong side of the sum in that case from what I can see.

    But as has been said, the cable sizing now must be done based on the size of the protective device.


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    #5
    BrianDug
    Registered Users Posts: 163 ✭✭BrianDug


    The UPS itself operates at at 0.99 according to the manufacturers data thats why I included it and was asking about only the UPS not the load itself. Of course Iz = In/Cg*Ca*Ci..


    Thanks for the info


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    #6
    Steve
    Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    BrianDug wrote: »
    The UPS itself operates at at 0.99 according to the manufacturers data thats why I included it and was asking about only the UPS not the load itself. Of course Iz = In/Cg*Ca*Ci..


    Thanks for the info

    The power factor is irrelevant in this case, you only need that to calculate the VA from the actual power rating.
    Given you have the VA, you can simply use VA/V/3 to estimate I per phase.

    In reality, a UPS is like an inverter so it will probably draw some weird out of phase current and generate harmonics so, to repeat myself, refer to the manufacturers instructions for the correct CPD. :)


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    #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,594 Mod ✭✭✭✭2011


    Steve wrote: »
    In reality, a UPS is like an inverter so it will probably draw some weird out of phase current and generate harmonics so, to repeat myself, refer to the manufacturers instructions for the correct CPD. :)

    Yes, and this can cause problems.
    I worked on a project where a power factor panel within an MCC overheated and went on fire. Following an investigation it was found that over the years a large number of DOL starters fed from an MCC where swapped out for VSD's. In addition several UPS units were added to the load of the same MCC. These non-linear loads were not suited to the existing power factor correction. We had to change it out for a "detuned" power factor correction panel. It has been fine since.


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