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    X-Ray feeder conductor sizing

    Hi,
    we have 135 KVA, X-Ray machine in the Hospital with 380V 3phase supply.

    we do not have any other data about the momentary rating or the long time rating, so we considered the long time rating for sizing, so 135000/380/1.732 = 205 Amps
    so, as per 660.6, the ampacity of the circuit shall not be less than 100% of the long-time rating ( which we do not have) it will be 205A.

    and from table 310.15(B).16 75C column(because more than 100A as per 110.14.(C).1.(b), for terminal Temperature limitations ) we have to use 3/0 conductors and (as per Table 240.6(A)) 200A Circuit Breaker is selected.

    and as per table 250.122 the size of Equipment Grounding Conductor (EGC) is 6 AWG.

    please note that Voltage drop, temperature correction factor, or Conductor Bundling adjustment will not affect my selection as per site condition.

    so as a summary, I will pull 4x3/0 copper conductor (4 cores cable) + 6 AWG green yellow conductor for grounding.

    My questions are:
    1-Is the above sizing is right?
    2-As per my knowledge X-Ray machines is considered as a nonlinear load, so, we can't use reduced neutral as per 310.15.(B).(5).(c), am I right? if the answer is no, how to size the neutral for the x-ray machine?
    but if the answer is yes, so the neutral conductor will be counted as a Current Carrying Conductor (CCC), so Conductor Bundling Adjustment factor shall be applied, because we are going to have 4 CCC, am I right?

    Thanks for reading all this.
    Regards,
    Ammar M. I.


    #2
    The 75C amapacity of 3/0 Cu conductors is 200A, which is less than 205A. I don't believe that you are permitted to 'round up' for this application. Getting the momentary and long term data might really be worth it so that you don't oversize the circuit.

    If you select an OCPD > 200A then you need an EGC larger than 6 AWG.

    The X-ray machine is likely a non-linear load, but is probably _not_ a large line-neutral load. The requirement for counting a neutral as a CCC comes into play when you have a large number of small non-linear L-N loads, such as certain types of lighting fixtures or computer power supplies. L-L loading, even heavily non-linear L-L loading does not create current on the neutral.

    To properly size the neutral for the X-ray machine you need the specifications for the machine. It is very likely that you don't need a neutral at all, or that the neutral will be the minimum size of the EGC. But there is also a chance that you will need a full size neutral.

    -Jon

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      #3
      It would definitely be good to consult with the manufacturer. Like welders, x-ray machines have very short duty cycles. (but much, much shorter -- only a few seconds at a time)

      It is likely that the machine itself has a very short duty-cycle limit, maybe only a few percent, based on the ability of the x-ray tube's anode to dissipate heat. It would be wasteful to install a feeder with a capacity greater than the machine's.

      Is the NEC even applicable in Saudi Arabia?

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        #4
        The 135KVA is probably either the long time rating, or the momentary rating.

        My guess is that it is probably the momentary rating.

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