Sub Consumer Unit

[JohnW2]

...Stacked CUs have special arrangements to extend the tails from master CU to the slave CU - ...

Yes, they often do - and if one is designing a dedicated 'stacked' system, then that is the 'obvious' and reasonable thing to do.

However, it's not the only approach. As I have told you repeatedly (and illustrated), what you describe as an 'unprofessional' method is the most common way in which mainstream manufacturers of 'high integrity' CUs do it - and how they come in their ex-factory state. One can but presume that these manufacturers don't share your view that what they are doing is 'unprofessional'.

Even non-high-integrity dual-RCD CUs usually have at least one conductor (in addition to the bus bar) 'stuffed' into a main switch terminal - do you also regard that as 'unprofessional'?

 

[hard-work]

What a nice person you are.

Thank you.

 

[hard-work]

Yes, they often do - and if one is designing a dedicated 'stacked' system, then that is the 'obvious' and reasonable thing to do.

However, it's not the only approach. As I have told you repeatedly (and illustrated), what you describe as an 'unprofessional' method is the most common way in which mainstream manufacturers of 'high integrity' CUs do it - and how they come in their ex-factory state. One can but presume that these manufacturers don't share your view that what they are doing is 'unprofessional'.

Even non-high-integrity dual-RCD CUs usually have at least one conductor (in addition to the bus bar) 'stuffed' into a main switch terminal - do you also regard that as 'unprofessional'?

The two terminals of the 25mm cable are usually crimped into one lug that is inserted into the mainswitch terminal.
I would rather drill the bus bar, crimp on a lug and bolt the cable to the bus bar, rather than stuff in two 25mm cables into a terminal. But that means having a suitability large crimping tool. The terminals Sunray pointed to solve all that a professional easy way.

What you proposed was electrically sound though, but I do not like that sort of thing. Lots of things are electrically sound, but frowned upon.

 

[JohnW2]

The two terminals of the 25mm cable are usually crimped into one lug that is inserted into the mainswitch terminal.

Not in ones I have here (I have several, all Wylex, ones).

I would rather drill the bus bar, crimp on a lug and bolt the cable to the bus bar, rather than stuff in two 25mm cables into a terminal. But that means having a suitability large crimper.

There's nothing wrong with that, provided one has the right tools and can do so without impairing placement of the safety cover over the bus bar (and without any of what one has done being 'exposed').

What you proposed was electrically sound though, but I do not like that sort of thing. Lots of things are electrically sound, but frowned upon.

As I've said, I can but presume that those manufacturers who use that method (Wylex in my case) do not feel that it should be frowned upon.

 

[hard-work]

Also a 25mm cable and a bus bar lug into the terminal of an RCBO? mmmmm No. Not comfortable with that at all.
It is either drilling the bus bar or the dedicated terminals Sunray linked to.

 

[SUNRAY]

I would rather drill the bus bar, crimp on a lug and bolt the cable to the bus bar,

There's nothing wrong with that,

NO NO NO never never drill a CU busbar.

Measure them and they are usually around 10x2mm [20mm²] drill a 6mm hole into it and its down to 8mm²... do the sums.

I've heard some try to say that adding bolt and nut will increase the CSA, my personal opinion is not to botch or take chances.

Also be aware of the size of the branches, there is normally one large but the rest are usually too small to use for a main feed, can be seen in this: https://www.tlc-direct.co.uk/Produc...BWGcpzIjxEzbuNAksuMdRUWHUPce00CRoCd-oQAvD_BwE.

These sort of 'conversions' are not to be done lightly and is one area where I'd usually advocate employing someone experienced in the matter and certainly using the proper parts. Above all this is not the place to randomly drill a hole.

 

[JohnW2]

NO NO NO never never drill a CU busbar. ... Measure them and they are usually around 10x2mm [20mm²] drill a 6mm hole into it and its down to 8mm²... do the sums.

Good point. I can't say that I've ever done it, or even thought of doing it - but if I had, I suspect that I might well have subscribed to ...

... I've heard some try to say that adding bolt and nut will increase the CSA ...

Also be aware of the size of the branches, there is normally one large but the rest are usually too small to use for a main feed, can be seen in this:...

Whilst that's true, in the OP's case he's not really talking about it's use as a 'main feed', but rather just a feed to the few things which won't fit into his main CU - so I don't think that issue, per se, would concern me too much. Let's face it, I don't think that anyone would worry about using one of those 'smaller bus bar lugs' to feed a 50A MCB/RCBO (or, theoretically, even a 63A one).

Kind Regards, John

 

[hard-work]

You can always double up the bus bar, having two sandwiched together, then drill though the two of them then put the bolt in.

 

[SUNRAY]

Good point. I can't say that I've ever done it, or even thought of doing it - but if I had, I suspect that I might well have subscribed to ....

Whilst that's true, in the OP's case he's not really talking about it's use as a 'main feed', but rather just a feed to the few things which won't fit into his main CU - so I don't think that issue, per se, would concern me too much. Let's face it, I don't think that anyone would worry about using one of those 'smaller bus bar lugs' to feed a 50A MCB/RCBO (or, theoretically, even a 63A one).

Kind Regards, John

I've been in panel building since 2002 where busbars rule OK... In the pro shops the calculations for maximum current are made [I tend to simply allow for 110% of supply] then the size of the busbars are calculated then the size of the required holes are added, the lumps of copper can get big very quickly. I have done a fair bit of work to clean out the big black marks and replace all the damaged kit where those calculations have not been correctly made.

I hope domestic CU busbars are fully rated for the maximum size of MCB [63A] and an easy way of upping the I/C supply rating is to add a second bar. BUT I still wouldn't drill them.

The big problem of only allowing for the initial installations requirements is at some time in the future a spare way can so easily attract a 10KW shower, So yes I would make sure that the additional board is perfectly capable of taking the whole load of the supply [unless fused down]. I had a comestic job in 2016 or 17 which had an additional enclosure added to house an RCD, 45A & 6A MCB's for a new shower room which had been wired from the CU busbar with 6mm² singles. Perfectly adequate for the new room but IMO that's far too small to be protected by a 100A DNO fuse.

I can't help thinking that CU's are classed as a special location for a very good reason.

 

[EFLImpudence]

Perfectly adequate for the new room but IMO that's far too small to be protected by a 100A DNO fuse.

It doesn't have to be; it is protected from overload by the MCBs and short-circuit is unlikely but 6mm² is adequate.

I can't help thinking that CU's are classed as a special location for a very good reason.

 

[SUNRAY]

It doesn't have to be; it is protected from overload by the MCBs and short-circuit is unlikely but 6mm² is adequate.

Except there were 2 spare ways in the additional board and we used one of them for the feed to garden sockets and hot tub so we were up at 12.6KW plus DSSO and there was still a spare way. Suddenly the 6mm² was too small, we changed it to 16mm²

 

[hard-work]

I had a comestic job in 2016 or 17 which had an additional enclosure added to house an RCD, 45A & 6A MCB's for a new shower room which had been wired from the CU busbar with 6mm² singles. Perfectly adequate for the new room but IMO that's far too small to be protected by a 100A DNO fuse.

What you are saying is that the meter tails, mainswitch, bus bars and any cables off the bus bars must be rated for 100A, as that is the size of the DNO's fuse?

 

[JohnW2]

... In the pro shops the calculations for maximum current are made [I tend to simply allow for 110% of supply] then the size of the busbars are calculated then the size of the required holes are added ....

As a matter of interest, what design working temperature do you calculate for in the case of a bare bus bar in free air (and, also as a matter of interest, where does one find the relevant {'current carrying capacity'} data to use for such calculations)? The 70°/90° conductor temperatures we are used to working with for cables are presumably essentially dictated by the material of the insulation (hence, for example, the difference between 70° and 90° cables).

The big problem of only allowing for the initial installations requirements is at some time in the future a spare way can so easily attract a 10KW shower ...

Whilst that's obviously true, even the "initial installation's requirements" could obviously include a 10kW shower, together with a cooker circuit, two or three sockets circuits etc. etc. One assumes that commercial CUs which come with bus bars have those bus bars designed so as to be able to carry the full rated current of the main switch (or other incomer)?

I can't help thinking that CU's are classed as a special location for a very good reason.

I presume you're talking about notification requirements? If so, it's only "replacing a CU" which is notifiable - 'doing things inside a CU' (even drilling holes in a bus bar!) would not be notifiable (in England) if it did not involve the creation of a 'new circuit' (although, admittedly, most such activity probably would!).

Kind Regards, John

 

[SUNRAY]

What you are saying is that the meter tails, mainswitch, bus bars and any cables off the bus bars must be rated for 100A, as that is the size of the DNO's fuse?

In a CU supplying multiple fuseways? Yes.

 

[JohnW2]

What you are saying is that the meter tails, mainswitch, bus bars and any cables off the bus bars must be rated for 100A, as that is the size of the DNO's fuse?

I would personally say (for a 'typical CU') "Yes" for the meter tails, main swithch and bus bars (since, at least theoretically, they could 'legitimately' serve a total load of 100A) - but that for "any cables off the bus bars" it would depend upon what downstream over-current protection they had (which could be a lot lower than 100A).