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  1. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    Hi
    A job that I'm currently looking at requires a 300A 3ph supply to a unit. The cable will be 100m long so the calc comes out at 240mm2. This cable is to be pulled down a duct, the main issue is it's weight and size will be a battle. I'm thinking of running two 95mm2 in parallel that would make this job possibly easier. What are everyone's thoughts?
     
  2. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    If they where singles you where pulling in I'd say parallel is a good choice but 2 x 95mm can you get them down the duct?
    Plus it's two glands to make off and terminate at each end.
    Aluminium conductors maybe a better choice.
    Or stick with the one cable.
    240 mm at 70 degrees operating temp 3 phase is less than 300 amps tho ref method D only gives ambient temp at 20 degrees and that's with out a correction factor of 0.9 to bring it to 30 degree ambient temperature
     
    Last edited: Apr 25, 2017
  3. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Parallel cables will have a reduced overall csa compared to one cable and are often used for large submains. If your calculations are okay it is the best option.
     
  4. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Two 95.0 better than pulling one 240.0.
     
  5. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Depends on the duct that's in I suppose and making sure the cables don't cross and get jammed.
    Would rather terminate the one cable even though it's a pig to do.
     
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  6. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Think one cable will need more washing up liquid.
     
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  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    I probably wouldn't duct them just bed them in the ground.
     
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  8. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    240mm 3 or 4 core , 3 phase is only 280 amps then using the maximum operating temperature at 70 degrees without applying correction factors
     
  9. Andy-1960
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    Andy-1960 Electrician's Arms

    Location:
    Wolverhampton
    And the amount you have to derate by when you run them in a duct can be staggering...

    Oh.. and buy a winch, makes life much easier!
     
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  10. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    The duct is 12" so quite large and is completely straight. I calc it with 0.8-pf, ambient temp 20 degrees and on a xlpe at 240mm2. Do you get a different answer?
     
  11. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Think you've used xlpe at 90 degrees operating temperature and not 70 degrees.
    There is a note in the tables making you aware of connecting 90 degree to 70 degree accessories and switch gear.
    You need to apply a 0.9 correction factor straight away If cable in a duct or buried as tables give ccc at 20 degrees and not 30 degrees reference method D
     
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  12. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    Ha you just have read my mind Ian! Yes I have done that! I the panel board it is connecting to is quite small so I was thinking the two 95mm2 would be easier to terminate. I was always under the impression that washing up liquid makes the pvc deteriorate?
     
  13. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Given the choice I would rather terminate two smaller than one large.
     
  14. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    W
    well never poured it on cables so I don't know haha.
     
  15. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    Dpes anyone know what factoring is applied for parallel supplies? I think the CSA normally works out less that with one cable
     
  16. darkwood
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    darkwood It's all about Gmes Staff Member Electrician's Arms

    Location:
    West Yorkshire
    Regardless of this been 1 or 2 cables, the task of trying to pull through an 100m duct even at 12" is going to be a massive challange, the weight alone plus the friction are going to make this a hard job, I would have trenched and buried such a cable(s) run ...good luck on it!
     
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  17. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    You may have to apply grouping factors as well for cables in parallel
     
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  18. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    Yea we were asked to have a look at the job after they put all the ducts in roads down etc. The joy!
     
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  19. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Nightmare!!
     
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  20. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Have you tried sizing it with three cables. Another thought does the armour have to support main protective bonds upstream.
     
  21. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Was going to say are you installing a separate cpc (half the line conductor using table 54.7) or using armour as cpc?
     
  22. bigspark17
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    bigspark17 Electrician's Arms

    Location:
    wales
    What is this supplying? With the info above id lean twoards 2x 95mm 4c
     
  23. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Each cable needs the appropriate correction factors applying first before adding together if using in parallel. volt drop and making sure zs is low enough for the protective device needs considering also.
     
  24. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    There will be a separate cpc installed to achieve the maximum Zs for the mccb. The supply is for a unit that will be effectively rented so no idea what sort of load it will be.
     
  25. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Do you know the mccb max zs from manufacturer, as this may be the big influencing factor to what size cable(s) you need to install as 100meter run and 300 amp supply is rather large.
     
  26. bigspark17
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    bigspark17 Electrician's Arms

    Location:
    wales
    Have you already taken a Ze at mccb panel board.? What is the max zs you need to meet.?
     
  27. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    The Ze was 0.09 I'll have a look when I get in the office buts it's a 400A Square D MCCB set at around 0.8 mulitiplyer to give 300A I have a feeling it was 0.37
     
  28. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    You need to apply a tolerance of +20% using a mccb for max zs giving a multiplier of 1.2
    See beama guide to low voltage circuit breaker standards.
    It's a good download to give good guidance.
     
  29. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    What's the tolerance of 20% for? I'm really thinking the 2x 95mm are going to be a lot easier than a 300mm2
     
  30. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    It's the tripping characteristic of the device.
    Manufacturers may state lower tolerances that's why it's best just to ask the manufacturer
     
  31. Perci
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    Perci Active EF Member

    Location:
    Melton Mowbray
    I have found on their website what they state the maximum Zs at max- 0.968 min- 0.052
     
  32. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Is it a nsx 400 micro logic 2.3?
    I have zs times from Schneider which is square D now anyway.
    You need the isd short time setting which will be on the device.
    For example an (isd)Im short time setting of 1.5 is o.33 ohms.
    Isd of 10 is 0.050 ohms.
    If you've got your info from an old Square D site it maybe more accurate tho as mine is Schneider.
     
    Last edited: Apr 26, 2017
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