Design 32a 4mm radial

[Big daddy B]

Good morning I wonder if I could have some confirmation in regards to a radial circuit.

I have been showing apprentices different methods if wiring ring and radial circuits.

I wish to continue this next week.
I can not see any regulation not permitting the drawing enclosed, I would just like to confirm if it is installed on a 32a mcb with rcd protection and uses 4mm throughout, in my mind this would operate the same as if you ran a cable from main consumer unit to secondary (e.g garage) using an adequate power supply and cable and then allocating circuits.

Your assistance would be appreciated!
Kind regards
Paul

 

[pc1966]

My first question here is how are all the 4mm cables joined?

Many MCB and terminal blocks have a limit on the number/size of cable they can accommodate.

Beyond that your 32A MCB should be fine for 4mm under most installation conditions, so it should be fine. You ought to check the supply Ze and cable length just in case, but for most reasonable situations the only big question is the cable splitting arrangement.

While you can legitimately use an RCD to achieve disconnection with high Ze (e.g. TT system) I would usually suggest you plan for disconnection being met on the OCPD aspect first, and only if no sane route to meeting it should you depend on the RCD.

 

[westward10]

Cable type and installation method?

 

[Big daddy B]

Thankyou totally agreed the terminals would obviously be rated to 32amp and 4mm size (maybe bus bar style or bank of wago) to meet the requirements.

 

[Mikegh]

I think they use a method like this on the continent for sockets

 

[Big daddy B]

Cable type and installation method?

4mm t&e individual conduit no grouping bo insulation

 

[westward10]

4.0 T&E contained in conduit would not be rated for 32A.

 

[Mikegh]

I think they use a method like this on the continent for sockets

Similar afaik but the branching isn't done at the MCB but at a junction box

 

[Big daddy B]

I think they use a method like this on the continent for socketd

Agreed and as so many of our regula

Similar afaik but the branching isn't done at the MCB but at a junction box

Thay was initially how I was thinking as obviously mcb should not ideally have multiple cables thaboy8u everyone first time on here!!!

 

[Pretty Mouth]

4.0 T&E contained in conduit would not be rated for 32A.

Singles would work though

 

[Mikegh]

I can't think of any issues with that type system

The junction needs to be accessible

 

[timhoward]

I don't have the book handy - but I think this is only ok if the 4mm T+E is surface clipped direct.
On a completely different topic what I don't like about it is extra joints that have to be accessible for maintenance, but that is more of a practical application matter.

 

[DefyG]

Looks effectively like a 'lollipop' arrangement!
Which not everyone likes but is deemed acceptable these days providing usual design criteria is met!

 

[westward10]

Looks effectively like a 'lollipop' arrangement!
Which not everyone likes but is deemed acceptable these days providing usual design criteria is met!

Not to my eyes it doesn't.

 

[Mikegh]

The wires are bunched at the MCB which isn't the way you'd do it

 

[DefyG]

The wires are bunched at the MCB which isn't the way you'd do it

Looks to me like only one cable emerging from the MCB then split later on?

 

[Mikegh]

Looks to me like only one cable emerging from the MCB then split later on?

Sorry ya you're right

 

[Pete999]

Not a standard circuit, no description of this method in BS 7671, S personally I wouldn't install it this way.

 

[pc1966]

Looks effectively like a 'lollipop' arrangement!
Which not everyone likes but is deemed acceptable these days providing usual design criteria is met!

I though the lollipop was a RFC fed from a sort of sub-main cable like an existing cooker supply that was repurposed? (Or sometimes a deliberate feed & isolator for things like school laboratories, etc)

 

[pc1966]

It is more of a tree circuit, with a feed "trunk" and branches to each of the sockets.

Personally I would prefer a multi-socket radial to be all in-line as then a check on the last socket verifies all continuity (rather like end-end on RFC).

A quick check in OSG Table 7.1(ii) as CCC met for clipped direct for feeder but not otherwise, and marginal for other cases on the branches (e.g. max demand for double sockets would be fine for all but 1010/103 method)

 

[telectrix]

horrible way to do it. more joints than Sainsbury's. and 2 lots of 7 conductors in a terminal. wagos only go upto 5. better direct to first socket then daisy chain 1-2-3-4-5-6.

 

[Zerax]

Generally... I'd always stick to a standard circuit and only depart from that if there are VERY good reasons not to. You may have very good reasons... but you haven't told us yet.

 

[telectrix]

Generally... I'd always stick to a standard circuit and only depart from that if there are VERY good reasons not to. You may have very good reasons... but you haven't told us yet.

maybe he's ROMAN....... What have the Romans ever done for us?

 

[telectrix]

or do i mean Romulans. easily confused when you get to my age.

 

[DefyG]

I don't think Big Daddy's drawing has helped as it implies terminations are outside the sockets but could easily be inside as per my drawing but personally I would take the feed cable to one end ( as dotted line) for a more conventional radial circuit.
Of course as we all know, in practice it is not always possible to wire it as shown on drawings.

 

[timhoward]

Of course as we all know, in practice it is not always possible to wire it as shown on drawings.

Agreed, the initial post saying this was a demo for apprentices is actually my main concern as I’d have thought they would be better off starting out with clear thoughts about more typical circuits, and built on that with understanding of overload and fault protection (encompassing selecting ocpd, installation methods, ccc etc) so they can assess anything quirky from first principles.
If this was a classroom exercise to discuss the regs then fair enough.

 

[Zerax]

Agreed, the initial post saying this was a demo for apprentices is actually my main concern as I’d have thought they would be better off starting out with clear thoughts about more typical circuits, and built on that with understanding of overload and fault protection (encompassing selecting ocpd, installation methods, ccc etc) so they can assess anything quirky from first principles.

Oh... I'd assumed that they were kitchen fitting apprentices ?

 

[timhoward]

Oh... I'd assumed that they were kitchen fitting apprentices ?

You may well be right, in which case even more reason to stick to Appendix 15 circuits!