So how would the garage or shower circuits be protected?
Sub Consumer Unit
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I'm not sure I understand the question, since the only answer I can give is 'in the usual way'.
The cable feeding the slave CU will have (downstream) overload protection from the same MCBs/RCBOs (or fuses!) as are providing (upstream) overload protection to the garage and shower circuits. Fault protection is unlikely to be an issue.
Kind Regards, John
D
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I follow that logic but surely no one would install a CU with 10 ways using a cable and tecniques which can't handle the full load of the supply, regardless of the predicted load. It would simply be madness.
It would indeed.
However, since it seemed to be a response to what I had written in the immediately preceding post, your comment/question was presumably in response to what I wrote about the common situation in which the 'slave/sub CU' had 1 or 2 (NOT 10), ways - and that was the basis of my point (i.e. that 'it depends' on the circumstances).
Kind Regards, John
That is why I quote what I'm responding to, it saves a lot of confusion.
That was my point - that, although you made it very clear (by quoting) that you were responding to my comments which were specifically about sub/slave CUs with 1 or 2 ways, you then started talking about one with 10 ways - which obviously was in a totally different ball-park from what I had been talking about!
Kind Regards, John
OK the way the thread has meandered has made it a little difficult to keep track.
My summary:
All cables need adequate OCP, ideally by some form of upstream OCD.
Downstream protection is OK with very stringent caveats.
We can certainly agree about that!
I guess that depends upon what one means by "very stringent caveats".
I would say that what it really comes down to (i.e. the only real 'caveat') is that one needs to be fairly confident that the protection of the cable is unlikely to ever decrease to an unacceptable extent. With upstream protection, that's more or less a no-brainer, and one cannot really do anything about some idiot who might subsequently come along and change he upstream OPD to one with too high a rating for the cable.
With downstream protection, as we've been discussing, 'it depends' - and I think that we really agree, at least in principle, about the nature of the caveats. If what is on the end of the cable is a fully populated small CU then, as with upstream protection, I don't think one has to consider the possibility of some idiot subsequently up-rating the MCBs in it - so I would personally be happy if the cable was adequately protected by the 'total rating' of the downstream MCBs at the time of installation. However, if it is a larger CU with some spare 'ways', then I agree that one has to 'assume the worst' in terms of what might subsequently be done with those spare ways in the future.
Can we agree on that basis?
Kind Regards, John
What Sunray appears to be saying is that if Over Current Protection is adequate up steam, then there is little chance of matters getting out of hand in the future via incompetence.
An example is installing a new CU with 16mm meter tails as the house will not consume more than 50A, even though the main fuse is 100A. Upstream protection is inadequate for the 16mm cables, it should be a 63A fuse. But downstream mbs's collectively protect the 16mm tails. Perfectly safe and legal.
But!! We are going over to EV cars fast with currently a number of companies engaged only in installing on-drive EV charging points. More will follow when EV sales take off even more, maybe an avalanche of installations. I read that some new homes are having them fitted as standard. These need their own mcb of 32A or 40A at the CU. No doubt the odd jump-on-the-bandwagon throw-them-in cowboy company will get involved as fast bucks are there. No doubt the odd one will be leaving in 16mm tails which would not be suitable for the expanded consumption, being underprotected by the downstream mcb's. If 25mm tails are installed from the outset, as it is a 100A supply, then the homeowner is covered for anything in the future.
As an aside, I can see many poor installation problems coming up with EV charging point installations. They need the earth from an earth rod, not from the home's earthing system.
An example is installing a new CU with 16mm meter tails as the house will not consume more than 50A, even though the main fuse is 100A. Upstream protection is inadequate for the 16mm cables, it should be a 63A fuse. But downstream mbs's collectively protect the 16mm tails. Perfectly safe and legal.
But!! We are going over to EV cars fast with currently a number of companies engaged only in installing on-drive EV charging points. More will follow when EV sales take off even more, maybe an avalanche of installations. I read that some new homes are having them fitted as standard. These need their own mcb of 32A or 40A at the CU. No doubt the odd jump-on-the-bandwagon throw-them-in cowboy company will get involved as fast bucks are there. No doubt the odd one will be leaving in 16mm tails which would not be suitable for the expanded consumption, being underprotected by the downstream mcb's. If 25mm tails are installed from the outset, as it is a 100A supply, then the homeowner is covered for anything in the future.
As an aside, I can see many poor installation problems coming up with EV charging point installations. They need the earth from an earth rod, not from the home's earthing system.
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Indeed, and I have agreed with that. However, where I am perhaps a bit less cautious than him is that I believe that there are certain situations (like the 1-way and 2-way sub-CUs I've been talking about) where there is also little chance of future incompetence upsetting things even when the protection is downstream.
Some idiot might subsequently decide to re-purpose a 'garage CU' (previously supplying just a couple of lights and a couple of occasionally used sockets) to include circuits to feed a 10 kW shower, a large cooker and a kiln or welder - but there's nothing one can really do about such possible idiocy!
Yes, when there no spare slots. If there is future circuit expansion then a new sub CU needs to be fitted then a reassessment of the whole supply.
An example of circuits not being well designed is the new house I once had. The garage was fed from the downstairs ring with a 13A unswitched FSU on the outside wall to a 2.5mm armoured cable through the outside wall (rated well over 13A), down outside, under the ground to the garage. The garage CU had a 16A 'main' fuse, then a few mcbs for lights and sockets. If there is a major problem in the garage the 13A fuse in the house would blow, not the 16A fuse in the garage CU. The 16A fuse was near useless. Bad design for sure.
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