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Ring Final

wessex
With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.
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    The 32A ring and 20A cable is based on someone else's calculations about load bunching, and is a well understood and time-tested design.

    However, with no risk at all,  the same assumptions could be made and scaled for any similar layout,  so a 64A device and 40A cable or a 16A device and 10A cable for example, keeping the ratio of MCB to cable rating.

    So a 25a MCB should allow you to use cable rated at (25/32)*20A ~ 16amps.


    But not all makers do a 25a MCB or RCBO however.

    (but for example Wylex do...)


    Mike.
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  • 0
    The 32A ring and 20A cable is based on someone else's calculations about load bunching, and is a well understood and time-tested design.

    However, with no risk at all,  the same assumptions could be made and scaled for any similar layout,  so a 64A device and 40A cable or a 16A device and 10A cable for example, keeping the ratio of MCB to cable rating.

    So a 25a MCB should allow you to use cable rated at (25/32)*20A ~ 16amps.


    But not all makers do a 25a MCB or RCBO however.

    (but for example Wylex do...)


    Mike.
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