A point EFLImpudence brought up. The total value of the mbc's on a downstream sub CU protects the cable supplying the sub CU.
Last edited:
Sub Consumer Unit
- Thread starter hard-work
- Start date
Sponsored Links
Totally correct but in this case the downstream OCP is to consist of a number of fuseways or MCBs, as yet we know not how many but let's make a random guess at 4, potentially they could all end up as 32A OCDs. IMO the minor additional effort and cost of 16 or 25mm² is a must.
I agree. However I was responding to the general question about "any cables off the bus bars" - which, in some situations, could be protected by something of very modest rating.
Of course, one can always argue that once one has "a cable off the busbar", even if today it is connected to just, say, one 16A MCB (or maybe even just an FCU), someone might subsequently come along and use that cable to feed a 10kW shower through a 50A MCB. However, if one were going to try to design in a manner that anticipated any degree of future stupidity, we would probably have to wire everything, including lighting circuits, in at least 6mm² cable
Kind Regards, John
Quite so, I have frequently added circuits to busbars of 1000's of amps with cables as small as 2.5mm²
I understand this comment but no I don't think it to be a necessity as that is what circuit design is all about. In a CU the 'full load' cables are usually burried behing the plethora of circuit cables and frequenly behind covers so it's not always easy to check their size. We then have the added complication that a significant percentage of domestic work is done by non qualified DIYers and with the greatest respect to much of their abilities [frequently better quality work than some qualitied sparks] checking the 'unprotected' cables will rarely be considered. We can, and I have encountered, decent 10 or 16mm² shower circuits in CU's fed by smaller cables [7/0.044" cotton & rubber main tails very recently and a few years back a 3 way sub FB in the loft conversion fed with 4mm² 32A fuse with a 45A fuse feeding a shower]Of course, one can always argue that once one has "a cable off the busbar", even if today it is connected to just, say, one 16A MCB (or maybe even just an FCU), someone might subsequently come along and use that cable to feed a 10kW shower through a 50A MCB. However, if one were going to try to design in a manner that anticipated any degree of future stupidity, we would probably have to wire everything, including lighting circuits, in at least 6mm² cable
Kind Regards, John
Sponsored Links
then electricity should be banned in domestic properties.
I'll go as far as 50:50. Far too much of my recent work has been in rented property where all sorts of botches are to be found, such as:
https://www.diynot.com/diy/threads/why-i-hate-domestics.507103/
That is the extreme DIY at its worse. The tenant was obvious mentally deranged. It happens.
I came across a 2 bed flat with the bedrooms, hall and l'room split into two 32A rings. A kitchen with the worktop sockets and appliances were off a 13A switched spur FSU, which was off one of the rings. A radial off the FSU, so a kettle, microwave, washing machine, dishwasher, etc, off a 13 fuse and one 2.5mm cable. All done by the builder. Yet, in 10, 15 or 20 years since it was installed, it never once blew the 13A fuse with it always working. Lucky really. It was safe as the 13A fuse protected the cables - and all the kitchen. The bedrooms could handle far more powerful appliances simultaneously than the kitchen.
Rings save builders as a 32A mcb and 2.5mm cable is idiot proof in design - it always works and safely. No calculations needed, one upstairs, one down, or the odd builder who has some idea may have a ring in the kitchen because of the heavier appliances. They use 2.5mm for sockets, 2.5mm for the 3kw immersion, 1.5mm for lighting and 6mm for ckr (even a 3kW oven). Just those three cable sizes. These days 1.5mm for lighting in averages domestic homes is overkill with LEDs, but 1.5mm they keep using because they always do. They know 6A mcb for lighting, 32A for rings and 40A for ckr as it will have a 6mm cable (even though a maybe a 3kW oven) and 16A for an immersion.
What else has saved them is RCDs which come packaged in the CU.
I came across a 2 bed flat with the bedrooms, hall and l'room split into two 32A rings. A kitchen with the worktop sockets and appliances were off a 13A switched spur FSU, which was off one of the rings. A radial off the FSU, so a kettle, microwave, washing machine, dishwasher, etc, off a 13 fuse and one 2.5mm cable. All done by the builder. Yet, in 10, 15 or 20 years since it was installed, it never once blew the 13A fuse with it always working. Lucky really. It was safe as the 13A fuse protected the cables - and all the kitchen. The bedrooms could handle far more powerful appliances simultaneously than the kitchen.
Rings save builders as a 32A mcb and 2.5mm cable is idiot proof in design - it always works and safely. No calculations needed, one upstairs, one down, or the odd builder who has some idea may have a ring in the kitchen because of the heavier appliances. They use 2.5mm for sockets, 2.5mm for the 3kw immersion, 1.5mm for lighting and 6mm for ckr (even a 3kW oven). Just those three cable sizes. These days 1.5mm for lighting in averages domestic homes is overkill with LEDs, but 1.5mm they keep using because they always do. They know 6A mcb for lighting, 32A for rings and 40A for ckr as it will have a 6mm cable (even though a maybe a 3kW oven) and 16A for an immersion.
What else has saved them is RCDs which come packaged in the CU.
Agreed. However, I would remind you that my comments were specifically about "a cable off the busbar" and one can but presume that the purpose of such a cable would (as certainly is the case with the OP's situation) virtually always be to feed something external to the CU.
The 'important end' of that cable would therefore not be hidden by anything within the CU, and anyone subsequently contemplating connecting something 'inappropriate' to the downstream end of the cable (outside of the CU) would presumably be guilty of the sort of idiocy which I said a designer cannot sensibly 'design against' by his/her design'.
Kind Regards, John
Yep, I'm happy with the first para. The case of a slave CU is different though, where both ends of the link between busbars may be hard to see, in fact as it's the first wire to be put into the second CU is may be completely obscured by circuit wiring, MCBs & bar cover and in the original it may be totally invisible behind the bar cover. In fact the reference to 2016 earlier the live link was completely obscured by covers, the neutral link between the N bars at the top of the enclosures was easy to see.
This thread starts with `here is my solution, could you advise how to do it`
I would ask why you need a second CU and what circuits will be fed from it?
Fair enough. I would again say that it really depends upon the actual situation and the extent to which one feels obliged to design in a manner that tries to anticipate future stupidity (rather than 'common sense').
To take what are probably the two most common 'slave CU' situations (albeit not ones in which the 'slave CU' would normally be fed 'from the busbar' of a primary CU), if the 'slave CU' were one in a garage, supplying one small sockets circuit plus a lighting circuit, or even if it were supplying just one 40/50A shower circuit, I would personally not think that it would be necessary to feed that 'slave CU' with a cable that would be adequately protected by the DNOs (possibly 100A) fuse.
Kind Regards, John
https://www.diynot.com/diy/threads/sub-consumer-unit.545529/page-4
This was on page 4......
- One Wylex CU, metal with eight single pole RCBs & one mainswitch incomer. Three years old.
- No space either side or on top.
- Only space available is under CU which can just fit another identical sized CU.
- Needs a sub CU under using the mainswitch (incomer) of the top master CU to switch out both. (have only one mainswitch);
- New sub CU will also be RCBOs, but the new DP.
- The sub CU can accommodate ten RCBOs, as no incomer fitted.
Are you adding 10 New circuits? Have yo calculated the new load?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
