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Thread: 3 phase electrical distribution board wiring

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    3 phase electrical distribution board wiring

    Hello all, my first post, so forgive me if this cannot be posted here and might have been covered before.

    I'd like your opinion on the wiring of a DB board. I am reviewing the wiring in a house that I bought, that has been added on to many times, but the original DB has never been upgraded. The installation is 3 phase. The current DB can accomodate the amount of circuit breakers required to seperate the various plug and lighting circuits, but is too small to accomodate the amount of cables needing to be pulled in and we would not like to replace the board. So the question to you all is as follows:

    Could one terminate all cables into connectors mounted in a customised galvanised connection box, placed in the celing above the DB, then pull a common neutral and earth (16mm˛) to the DB together with a live for each circuit, thus eliminating 66% of individual neutral and earth wiring? I have read the latest SANS10142-1_2017 Edition2 and am still not sure whether the above would be allowed.

    Your comments would be appreciated.

    For the record, I'm not an electrician, I do have an electrical engineering light current qualification and background and I do know that a qualified electrician should complete the work. This is an investigative stage to determine the best way forward.

    Thank you and regards
    Stephen

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    Site Caretaker Dave A's Avatar
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    I suggest combining the neutrals across separate circuits outside of the distribution board would be a contravention of 6.1.4 -

    All the conductors of any circuit shall originate at the same distribution board.

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    Thanks Dave, I actually misssed that.

    I suppose another way of getting around this would have been to create a sub DB where everything can fit, with the current main DB feeding it, this to eliminate pulling in or extending the main DB supply feeder. cable But then the sub DB would need to be mounted somewhere else in the house, which mean extending all circuits.

    The problem with replacing the current DB is that I have a space limitation. The board is recessed into a brick piller between 2 doors and only a narrow board can be fitted. The amount of conduits needed to bring down all the various circuits is limited. I can't insert a deeper board either to accomodate more conduits as there is a cupboard directly behind the piller. The previous owner got away with this by joining multiple plug circuits together and only feeding from one circuit breaker in the board.

    Any suggestions?

    Regards

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    New board

    You don't have to bring conduits down. I would bring a suitable trunking down plastic or other.
    If you going to do the job then bring the circuits down and protect them properly and that applies to the neutral as well. In the long run you won't have problems. I f you take short cuts now later on there might be need for a coc, maybe a sale, then the contractor will have to pull the place to pieces.

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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by Stephenvr View Post
    Any suggestions?
    Photos that give an idea of the extent of the challenge might inspire some suggestions.
    The trouble with opportunity is it normally comes dressed up as work.

    Alcocks Electrical Services | Alcocks Pest Control & Entomological Services

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Stephenvr View Post
    ....Could one terminate all cables into connectors mounted in a customised galvanised connection box, placed in the celing above the DB, then pull a common neutral and earth (16mm˛) to the DB together with a live for each circuit, thus eliminating 66% of individual neutral and earth wiring? I have read the latest SANS10142-1_2017 Edition2 and am still not sure whether the above would be allowed.....
    I'm not sure 6.1.4 is directly applicable TBH although I can see good arguments that it might be.

    What you're suggesting in essence is relocating the neutral and earth bars outside of the space of the existing DB which would effectively make the area in the wall that encloses the new 16mm earth and neutral conductors, plus any additional enclosure above that houses the new earth and neutral bars (or connector arrangements), become a part of the DB. In effect you're installing a DB extension albeit an unorthodox one.

    Regardless of how you view the idea topographically it's a bad idea. There's a very high likleyhood it's going to be in violation of other parts of the regs and (no offense intended) it reeks of convoluted and unorthodox design that's been arrived at because you're trying to avoid doing things you're unsure about such as simply extending the main supply wiring (which an experienced electrician will have no problem doing) and specifying and installing a new and suitably sized or shaped DB enclosure. If there's no DB available off the shelf that's suitable for the space you have available then there's nothing preventing a qualified person from sourcing a suitable naked enclosure and populating it with DIN rail and other components to make it into a DB.

    I'm interested why you think the existing DB can't handle the number of conduits you need for your circuits. Is it because there's insufficient 20mm knock-out points or is there insufficient area in the walls of the DB to squeeze them in? As mentioned above, alternative containment can be used such as trunking or even a much larger diameter conduit(s) such as 50mm for example. Even cable tray may be another possibility or, depending on what cable is used, it might be possible to to have uncontained cables entering the DB via a gland or a brass bush. The possibilities are many and I'd suggest you'd probably benefit most and save yourself considerable money and headaches by having some input from someone with good electrical installation experience at this stage of the design.
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    The problem i see with this, the poor sucker who has to do fault finding when the e/l unit trips and its a neutral fault.

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    Sounds to me like the wall is too narrow Andy. Depending on the amount of circuits and their load what I might consider would be a junction box in the roof in which some lighter circuits are combined thereby reducing the number of circuits needing to come down into the DB. For example, if there are 6 lighting circuits it may be reduced to 3. I have given a house 16 plug circuits but, there was space, it was being newly built. It was much more than usual but, at the owner's request for minimum future disruption of power. Obviously this is not cut and dry, there are factors to consider.

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    Hi Andy

    The house has been added onto a few times. In all, there is now a main DB, and 3 sub DBs. The amount of 20mm and 25mm knock outs are insufficient for the amount of cables to be pulled down to the main DB. I have however solved my problem. I have re-located the position of the DB to a suitable position in the house. This was made possible by re-routing all cables and installing a few new ones. I have installed a 3 level DB to accomodate the 3 phases, capable of taking 50 circuit breakers together with earth leakage. Let the re-wiring begin.

    Regards

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    Good afternoon all

    Apologies for the delay in responding. Thank you to all who made comments.

    I have however solved my problem. I have re-located the position of the DB to a suitable position in the house. This was made possible by re-routing all cables and installing a few new ones. I have installed a 3 level DB to accomodate the 3 phases, capable of taking 50 circuit breakers together with earth leakage. Let the re-wiring begin.

    Regards

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