3 Phase Sub Board off 4 Core 6mm SWA

[kenai36]

Hi all,

Company I work for have moved into a new factory and asked me to wire up the work benches, properly this time as before they just used a selection of extension leads dragged across the floor.

Anyway. On the wall there is a 4 core 6mm SWA cable terminated into a rotary isolator fed from a 3 phase 32a MCB.

Am I right in thinking that I can fit a TPN sub board in place of the isolator and then run 3 radials out to supply the benches?

One guy I work with said to just take 3 circuits off the existing cable in a junction box and share the neutral between them but I don't like the sound of that. Segregation of circuits and all.

Basically I want to use the 6mm 4 core as a distribution circuit to feed 3 x 20 radials, 3 x 32 rings if possible. I'm just unsure as the loads will be slightly unbalanced depending on what the guys are working on. I'm trying to save running 3 x 2.5 / 4 mm 3 core SWAs from the main distribution board because it's a fair distance away. Load on each phase would realistically only ever hit 10amps and am I correct in saying that the 6mm cable is good for approximately 32 amps per phase?

Does this sound acceptable to you guys? I rang the NICEIC tech helpline and the guy wasn't really sure what I was getting at.

Thanks in advance.

Rich

 

[Leesparkykent]

Hi all,

Company I work for have moved into a new factory and asked me to wire up the work benches, properly this time as before they just used a selection of extension leads dragged across the floor.

Anyway. On the wall there is a 4 core 6mm SWA cable terminated into a rotary isolator fed from a 3 phase 32a MCB.

Am I right in thinking that I can fit a TPN sub board in place of the isolator and then run 3 radials out to supply the benches?

One guy I work with said to just take 3 circuits off the existing cable in a junction box and share the neutral between them but I don't like the sound of that. Segregation of circuits and all.

Basically I want to use the 6mm 4 core as a distribution circuit to feed 3 x 20 radials, 3 x 32 rings if possible. I'm just unsure as the loads will be slightly unbalanced depending on what the guys are working on. I'm trying to save running 3 x 2.5 / 4 mm 3 core SWAs from the main distribution board because it's a fair distance away. Load on each phase would realistically only ever hit 10amps and am I correct in saying that the 6mm cable is good for approximately 32 amps per phase?

Does this sound acceptable to you guys? I rang the NICEIC tech helpline and the guy wasn't really sure what I was getting at.

Thanks in advance.

Rich

Hi rich, How far?

 

[kenai36]

Sorry should have said.

20m from the main board. (so not that far if I'm honest)

Would still like to do it this way as the RCBO'sI would install would be easier for the guys to get at if there was a fault.

 

[Leesparkykent]

The 6mm SWA at that length is fine for the 32amp OCPD.

 

[kenai36]

Thanks mate. And it doesn't matter that they are unbalanced as it will flow down the neutral?

 

[Darkwood]

Problem is discrimination in a short circuit... if you don't mind the sub 32amp tripping losing all the sub supply then fine.. it would all depend on your supply to the building size and whether your customer is willing to pay for a dearer install that will afford discrimination - if the DNO supply size allows it.

 

[kenai36]

That's why I was leaning more towards running the final circuits from the sub board on 20a RBCOs so that a short / over-current on one circuit on one of them should take out that instead of the 32a back in the main board.

 

[Leesparkykent]

Thanks mate. And it doesn't matter that they are unbalanced as it will flow down the neutral?

Obviously you want to balance the load as much as possible. And like darkwood has said the 32amp mcb feeding the sub main will not discriminate. So if a fault occurs on one of the sub circuits there is as much chance of it taken out the 32amp sub main mcb as the RCBO feeding one of the sub circuits.

 

[Darkwood]

That's why I was leaning more towards running the final circuits from the sub board on 20a RBCOs so that a short / over-current on one circuit on one of them should take out that instead of the 32a back in the main board.

It will not achieve any discrimation - a short circuit will be seen by both the 20a rcbo and the 32amp mcb and is just as likely to take out either and sometimes both... you cannot achieve discrimination with this design and like I said earlier, its down to your supply and a much more costly install to either achieve full or a better partial discrimination than you have at the moment. You will only achieve overload discrimination between the 20amp rcbo and the 32amp mcb but that still relies on the combined load of the final circuits been small to start with.

To have a good design here you want to be looking at having upfront BS88 of about 125amp or MCCB to get a full discrimination to the 20amp rcbo if I remember correctly that is, this is how any submain should be designed - its considered poor design to put 2 mcb's/ rcbo's in series as you can only really achieve full between a 6amp and 63amp with the other values only giving partial, but with the good earth value of a tncs/pme and even a tns then the partial can really be forgotten about and it just down to pot luck which one trips.