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Protection When Changing CSA

Les Spiteri
I have always went by the rule " when changing (lowering) csa of cable/wire then insert protection device", and further to that the second rule was "if its only a small run perhaps <2M then first rule need not apply". 


What is experience of everybody on this?


This particular application in design phase has an incoming 95mm cable in through a a switch disconnector which requires distribution to many smaller branch circuits. usually this application for me in control panels is simple as small currents (<30A) are in use but this larger requirement means the terminations get tricky having many different wire sizes.


Thanks Folks!33eaed845ccf5afaa1f80feb683c2254-huge-distribution.png
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    The cost of 1.5 M of cable is trivial and I would therefore be inclined to size the smaller cable as the maximum that readily fits the terminals.


    Example, 95 mm cable into DIN rail terminal block or bus bar, then a smaller cable to a 20 amp fuse. Whilst 2.5mm should suffice for 20 amps, but if the terminals will accept 6mm I would use that rather than 2.5mm.

    This increases the chances of the smaller cable surviving a short circuit.


    Or taking the example of the light in the substation, indeed one would not use 185mm into the 5 amp fuse holder. I might use 16mm though if the fuse holder will take that size.
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  • 0
    The cost of 1.5 M of cable is trivial and I would therefore be inclined to size the smaller cable as the maximum that readily fits the terminals.


    Example, 95 mm cable into DIN rail terminal block or bus bar, then a smaller cable to a 20 amp fuse. Whilst 2.5mm should suffice for 20 amps, but if the terminals will accept 6mm I would use that rather than 2.5mm.

    This increases the chances of the smaller cable surviving a short circuit.


    Or taking the example of the light in the substation, indeed one would not use 185mm into the 5 amp fuse holder. I might use 16mm though if the fuse holder will take that size.
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