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SWA cable current carrying capacity

CPC

Hi all, Hope you are all well.  

Looking for a bit of advice.

Small building fed from a main building by a buried submain 3 core 25mm2 swa (Old colours “red, yellow & blue”). Wire armour & yellow core being used as earth. Swa cable length is approximately 75 meters, protected by switch fuse rated at 60amps which is bushed and coupled to a 200amp busbar chamber in the main building.  

Existing loads total to 45amp.

Client is looking to add an additional fixed load rated at 40amps.

Existing load + new load = 85amps.

Using table 4D4A - Max current carrying capacity of a 2 core 25mm2 swa = 99 amps however if using 3 core its 82 amps.     

If using the 3-core current carrying capacity the cable isn't large enough @ 82amps. 

In reality only two cores are being used but technically the cable is a 3 core.

Which value of current carrying capacity should be used in this situation?   

Your thoughts appreciated.

Thank you all.

CPC

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  • 0

    I would apply engineering judgment and consider the existing cable sufficient to for 100 amps, on the grounds that the heaviest load is the shower, which is only used intermittently.

    Or apply engineering judgment and use the "2 core rating" for the existing cable, justified on the grounds that only 2 cores are loaded.

    Or fit an 80 amp fuse in the new 100 amp switch fuse..

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  • 0

    I would apply engineering judgment and consider the existing cable sufficient to for 100 amps, on the grounds that the heaviest load is the shower, which is only used intermittently.

    Or apply engineering judgment and use the "2 core rating" for the existing cable, justified on the grounds that only 2 cores are loaded.

    Or fit an 80 amp fuse in the new 100 amp switch fuse..

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  • 0 in reply to broadgage

    If you can parallel  up 2 cores to give a double neutral, you can exceed the 2core rating, as now 2 of the 3 cores are running cool - because the total dissipation for a given load current, which is after all the heat the cable has to sweat off, is lower.

    Noddy example, consider a very long length of cable such that each core is 1 ohm, utterly impractical due to the VD, but it makes for some easy sums,

    At 10 amps, any one core dissipates 100 watts (I squared R...) spread out over the length.

    So 3 core cable, designed for 3 phase with each of 3  cores carrying 82 amps. 3 lots of 82 squared. == 20182 watts. This is the maximum total power for that length of that cable construction.

    Now, change that to 2 cores each carrying 100A, so twice 100 squared --  20000 watts, so a touch less power, and would run very slightly cooler than the 3* 82A of the 3core max rating.

    Now consider  the parallel neutral case, so 1 core carrying 100A, plus 2 cores each carrying 50A each.

    10000 watts plus 2 lots of 2500 watts = 15000 - so that is actually quite a bit  cooler than the 2 cores at 100A.

    The risk as that the chosen cable rating is not the right one, because it runs through thick insulation or something,.. which in this case,  no it does not, so you are OK.

    Of course, you wont be dropping 82V, or 100V, and with a length of ~  75m,  presumably expect a resistance per core of 50 to 70 milliohms, and corresponding voltage drop, but the situation  all scales - unless the cable is coiled back on itself,  it is the linear heating of the cable (in watts per unit of length) that sets the maximum safe current.

    Mike.


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