Results 1 to 6 of 6

Thread: 3 core cable running 2 phases and neutral is OK?

  1. #1
    Silver Member
    Join Date
    Mar 2008
    Posts
    201
    Thanks
    4
    Thanked 37 Times in 31 Posts

    3 core cable running 2 phases and neutral is OK?

    Hi there
    This isn't the way I'd usually do it but there is and existing 3 core 16mm SWA cable. It is currently running a DB board(located above a bath believe it or not which is why it is being moved) which supply two shops. I want to redirect this cable to a new DB which will supply these 2 shops and 2 other shops. The loads on the shops is very minimal, the biggest shop is drawing 30 amps with everything switched on. The other 3 shops draw no more than 25 amps each, again with everything on. I want to run 6mm cables from this new board to each new shop. These 6mm will be covered by a 40 amp breakers, the longest run of the 6mm cables is 18 meters, shortest run 3 meters.
    Can anyone see a problem with just running 2 phases and a neutral to the new board. At the main intake position where the armoured is fed from I'm going to put it on a phase 63 amp breaker, using this protect and isolate the cable. I know your going to say that the load isn't equally spread but at present another 2 shops, not fed from this armoured are on the other phase coming from the main intake position.
    Both these shops carry very little current (max 25 amps if everything is on) I can't see a problem as the neutral current, using the formula on page 305 for calculating neutral current comes well within limits of the 16mm, which can carry 77 amps.
    Even if I ran 63 amps through the cable the neutral current would be 63 amps if I have done my sums right.
    Volt drop seem to be fine as well, well within, even if you use 63 amps(It will never go close to that high) as the load for the purpose of calculation.
    I'm going to run a separate 10mm earth for good measure as teh earth is currently provided only by the armouring of the cable.
    So anyone see a problem with that????

    I'm just used to running a single phase supply of a 3 phase an neutral, never tried just using 2 phases and neutral, only doing it because that is what is existing and doing the maths, I can't see a problem.

    Thoughts?
    Thanks

  2. #2
    Silver Member
    Join Date
    Sep 2010
    Location
    cape town
    Posts
    298
    Thanks
    3
    Thanked 42 Times in 36 Posts
    i cant see anything wrong with it,as long as your supply breaker breaks both phases with overload protection. double pole circuit breaker not single pole+neutral.(or use a 3 pole c/b cutting the neutral as well?)the neutral wire wont use more current than any of the phases. i would label it as 2 phase. the phases will be a bit unbalanced at the supply db.
    using the blue as neutral,you will have to sleeve it black as far as you can.

  3. #3
    Silver Member
    Join Date
    Mar 2008
    Posts
    201
    Thanks
    4
    Thanked 37 Times in 31 Posts
    I couldn't think of anything wrong with either, just a bit of an odd way of doing it that all. Going to use a 63 amp 3 phase breaker to protect the 2 phases and run the neutral through that as well then use a 3 phase 63amp isolator to break the 2 phases and neutral in the new board with the 40 amp breakers.
    Thanks for the reply

  4. #4
    Gold Member Sparks's Avatar
    Join Date
    Dec 2009
    Location
    Port Elizabeth
    Posts
    854
    Thanks
    15
    Thanked 117 Times in 93 Posts
    According to the regs all phase conductors including neutral must be isolated, that means for 2ph+ N you need to use a 3pole isolator.

  5. #5
    Silver Member
    Join Date
    Sep 2010
    Location
    cape town
    Posts
    298
    Thanks
    3
    Thanked 42 Times in 36 Posts
    you are correct. i just wonder why in 99% of 3 phase d.b's the neutral isnt switched.

  6. #6
    Gold Member Sparks's Avatar
    Join Date
    Dec 2009
    Location
    Port Elizabeth
    Posts
    854
    Thanks
    15
    Thanked 117 Times in 93 Posts
    It's all about the money.

Similar Threads

  1. [Question] netsurfer dual core touch 7
    By AmithS in forum Technology Forum
    Replies: 2
    Last Post: 01-Dec-11, 04:49 PM
  2. Geyser running.
    By Greg in forum Electrical Contracting Industry Forum
    Replies: 9
    Last Post: 21-Jul-11, 10:41 AM
  3. cable sizes
    By bergie in forum Electrical Contracting Industry Forum
    Replies: 2
    Last Post: 20-Mar-11, 08:50 PM
  4. [Article] Exe cable glands for braided cable
    By Master Electrician in forum Electrical Contracting Industry Forum
    Replies: 0
    Last Post: 26-Aug-10, 02:47 PM
  5. The Core i5 (interesting Tech)
    By tec0 in forum Technology Forum
    Replies: 7
    Last Post: 25-Aug-10, 11:28 PM

Did you like this article? Share it with your favourite social network.

Did you like this article? Share it with your favourite social network.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •