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Cable table which column to use

Baldyhugh
Hi


Im currently doing a bit of circuit design and am a bit rusty. I’m planning to use a 3 core swa for a single phase circuit. Do I choose the 2core cable single phase column in The regs book current capacity tables even though im using a three core cable or the three or four core cable column? Some of the tables show a little asterix denoting ‘with or without a protective conductor’ which would help in my instance, however table 4d4a doesn’t show this. 

Thanks in advance

Peter
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    Does this apply to all the tables, not just for SWA?

    I'd say so. As a general principle it's the current carrying cores that generates the heat - generally calculated on the number of fully loaded cores - hence a 4-core 3-phase cable has the same rating as a 3-core 3-phase cable - as the N current only carries the differences from the phases - so even in the worst case it's the same as three fully loaded cores.


    But in all cases I'd say it's an approximation. The heat dissipation from say the centre core of a 3C+E flat cable won't be quite the same as from either core of a 2C+E flat cable - as it'll have the extra plastic surrounding the other outer core to contend with, but then once the heat's past that it'll have a larger surface area to dissipate from. Likewise the geometry of a 3-core SWA with two loaded cores isn't quite the same as a 2-core. But as long as you're got a few percent of the cable's rating as a safety margin, I doubt there's anything to worry about in practice.


       - Andy.
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  • 0
    Does this apply to all the tables, not just for SWA?

    I'd say so. As a general principle it's the current carrying cores that generates the heat - generally calculated on the number of fully loaded cores - hence a 4-core 3-phase cable has the same rating as a 3-core 3-phase cable - as the N current only carries the differences from the phases - so even in the worst case it's the same as three fully loaded cores.


    But in all cases I'd say it's an approximation. The heat dissipation from say the centre core of a 3C+E flat cable won't be quite the same as from either core of a 2C+E flat cable - as it'll have the extra plastic surrounding the other outer core to contend with, but then once the heat's past that it'll have a larger surface area to dissipate from. Likewise the geometry of a 3-core SWA with two loaded cores isn't quite the same as a 2-core. But as long as you're got a few percent of the cable's rating as a safety margin, I doubt there's anything to worry about in practice.


       - Andy.
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