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DC Motors - Cable sizing

DavyF15
Working on a project where we are replacing an existing cable serving a DC motor. The motor is 690V and has a current of 1440A. The client has advised that the existing cable is 2 x 2c 300mm2 armoured buried below ground for 130m. Now when looking at the tables in BS7671 the maximum load that a 300mm2 can take is 446A (90C) or 379A (70C) so with the parallel cables this would only provide a current-carrying capacity of 892A ( 758A @70c). We are now concerned that the existing cables installed cira 20 years ago are undersized for the full load current of the motor.


A suggestion was made that we could possibly install single-core armoured cables which would increase the current-carrying capacity. When looking at the tables in BS 7671 (Table 4E3A & Table 4D3A) there are is no data provided for when these cables are in reference method D. I have spoken to a cable manufacturer who pointed me to ERA table 6 but these current-carrying capacities would not be in line with BS7671 as they use different data. I am tempted to use the ERA data as there is no equivalent information in BS7671, Why is this not provided and would there be issues with this.


The client has limited data for the existing installation but they have not had any issues with the current cabling even though it appears undersized. Any help would be appreciated.


regards
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  • 0
    Do realize that the guidance in BS7671 is only that, helpful collection of cable ratings in some common combinations. It is not reasonable to expect the committee to have considered all possible situations, and quite often the results are that the next size up is recommended  just to be sure, especially with large grouping factors or extreme sizes. And cable maker's data need not align with it, and may well be more accurate, for their specific cables.

    Has the existing cable type ever failed due to overheating - why is it being replaced ?  If it is still in use Is it practical to put temperature disclosing stickers on the existing cables to see how hot they get in practice ? For DC you do not need to worry about the inductance, or skin effects, so larger cables will be slightly lower impedance than they would on 50Hz, and spacing can be increased.


    How different are the ERA figures for DC ?
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  • 0
    Do realize that the guidance in BS7671 is only that, helpful collection of cable ratings in some common combinations. It is not reasonable to expect the committee to have considered all possible situations, and quite often the results are that the next size up is recommended  just to be sure, especially with large grouping factors or extreme sizes. And cable maker's data need not align with it, and may well be more accurate, for their specific cables.

    Has the existing cable type ever failed due to overheating - why is it being replaced ?  If it is still in use Is it practical to put temperature disclosing stickers on the existing cables to see how hot they get in practice ? For DC you do not need to worry about the inductance, or skin effects, so larger cables will be slightly lower impedance than they would on 50Hz, and spacing can be increased.


    How different are the ERA figures for DC ?
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