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2 single phase circuits in 1 SWA cable

Discussion in 'Electrics UK' started by thompa25, 17 May 2021.

  1. thompa25

    thompa25

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    Not sure on the regs for this, if anyone could enlighten me. What are the rules on running 2 single phase circuits in 1 SWA cable. I've got a lenght of 10mm 5 core cable and want to run 2 circuits in it out to the outbuilding. What would this do to current carrying capacity ratings? Just wondering if i would be better running 2 of the cores in parallel and putting it on a bigger supply?

    Cheers,

    David
     
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  3. plugwash

    plugwash

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    As far as i'm aware the standard current rating tables don't cover the case of multiple circuits in the same cable.

    Normally in three phase wiring it is assumed that no more than three out of the four conductors will be carrying full current simultaneously. Whereas with two single phase circuits all four conductors could be carrying full current at the same time.

    Conductor heating is proportional to the square of current.

    So I would argue that you should take the current ratings for three phase and multiply them by √¾ (about 0.86)
     
  4. JohnW2

    JohnW2

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    Although seemingly not referred to in text, Table 4B5 seems to address this issue, albeit only for between 5 and 48 (i.e. not including 4 !) "loaded cores". If I'm interpreting this correctly, with 5 "loaded" cores, the CCC would be 0.72 times that for a 2-core cable of the same type and installation method - so the CCC for the OP's 4 "loaded cores" would presumably be a little higher than that.

    upload_2021-5-17_14-29-5.png

    Kind Regards, John
     
  5. plugwash

    plugwash

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    I notice though that the table you posted only claims to apply to smaller cables, not to the OPs 10mm

    I decided to compare the two methods, I took 10mm² 90C burried SWA as my baseline and used the current rating figures from https://www.rm-electrical.com/wp-content/uploads/2018/10/SWA-Chart-RM-Electrical.pdf

    The current rating is quoted as 85A for single phase and 73A for three phase.

    Multiplying the three phase rating by √¾ and rounding to two significant figures gives 63A

    Multiplying the single phase rating by 0.72 gives and rounding to two significant figures gives 61A.

    So similar ballpark.
     
    Last edited: 17 May 2021
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  7. JohnW2

    JohnW2

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    Ah, I missed that. As I said, there seems nothing in the text which relates to that table, so there appears to be no guidance on the situation with >4mm² conductors.
    As you say, very close (although I presume the equivalent of the Table 4B5 figure would be a bit lower than 0.72 for 4 'loaded cores').

    As a matter of interest (if you have the time) how would figures obtained from an extension of your approach compare with the Table 4B5 ones for CSAs ≤4mm² and increasing numbers of "loaded cores"?

    Kind Regards, John
     
  8. JohnW2

    JohnW2

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    In wondering how well this 'first approximation' (making assumptions about current and heat) works (in comparison with tabulated CCCs) it seems that:

    Multiplying the three phase rating by √(3/2) and rounding to two significant figures gives 89A​

    So not far of the quoted 'single phase' figure of 85A, but a little higher.

    Kind Regards, John
     
  9. plugwash

    plugwash

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    Yeah, total power dissipated in the cable clearly isn't the only factor, distribution of the heat sources and the physical size of the cable presumably also matter.

    Still I consider my calculation to be conservative, it represents the same total power dissipation, the power is dissipated in a more spread out manner and the cable is physically larger.
     
  10. JohnW2

    JohnW2

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    Yep, all agreed.

    Kind Regards, John
     
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