Need a4 way metal CU with 2 KOs

[Ben Blaukopf]

Shouldn't be hard to find, I hear you say....

Briefly, I need a metal CU with 4 ways (i.e. RCD plus 2 MCBs), and with

2 20mm KOs on the bottom.

So far I've managed to buy

has 1 KO on the bottom. Also an enclosure isn't quite what I thought it is, and it has only an earth bar, so I can't even figure out how to wire it up even if I drilled the hole myself. Guess I'll have to return it.

I've got 2 SWA cables which used to attach to my old fusebox - one for garage, one for shed. I replaced the fusebox with a plastic CU - carefully chosen to fit the space available - but with no space to get the SWA cables in. Also, the SWA is 2 core, and I couldn't figure out how to attach an earth wire to the banjo tags.

So the plan is: Run 6mm from main CU to the metal one on a 32A MCB, non-RCD side. (Yes, I could use 4mm, but I have loads of 6mm) Put 2*20AMCBs in the metal CU, plus an RCD. 2.5mm SWA will carry 20A. In the garage, load is a 15A circuit for sockets, and a 15A circuit for lights - yes, they wired the lights circuit in 2.5mm. So max load of

16A. Therefore 2.5mm SWA is OK. Shed is identical to garage, neither have any need for bonding (so SWA armour as cpc is okay). Neither cable run is >5m, so no problems with voltage drop.

Ben

[andrew]

I suggest you get a 20mm hole punch and make another hole.

I suspect you really wanted a consumer unit or distribution board, rather than a bare enclosure.

I solder an earth tail to them. Even with a metal case, I would still use them, as paint/corrosion may not provide a good electrical contact to the case.

You won't get any discrimination between the chain of MCBs during a fault -- i.e. it is likely several/all of them will trip, also cutting power to the other circuit. If that matters to you, I would suggest fitting a 32A BS1361 HRC fuse carrier instead of the

32A MCB.

Actually, if you changed the lighting MCB's to 6A, you would not require the 20A MCB's at all. The 16A and 6A MCB's would protect the 2.5mm circuits against overload (I think -- don't have the tables on me to check), and the single 32A MCB would protect the 2.5mm circuits against fault currents.

[Ben Blaukopf]

Ta! Now discovered the magic known as a Q-max punch.

Having done it, the contact is excellent.

In the event of a fault (involving earth), I expect the RCD to trip, which is in the house. A live-neutral short would trip everything, I agree, but I don't have a problem with that. It's vastly better than the old system I'm replacing, which had no RCD at all. The main goal at the moment is to discriminate between the house and outdoor circuits. Is complete discrimination a requirement of BS7671?

When I replace the shed/garage CUs, I'll put just the sockets on RCD, and remove the upstream RCD protection, for complete earth fault discrimination, so if I cut a flex, the lights stay on.

The fuse carrier for the main CU sounds sensible, except that I don't think the manufacturers of the CU make one. Is it generally acceptable to use a different manufacturer in that case?

That'll actually be fusewire (hence the 15A rather than 16) in the shed/garage at the moment, but good idea.

Ben