Rewiring - new alongside old

[Lobster]

Having just come across my latest nasty I've decided I really have to bring forward my rewiring. I was knocking the plaster off the hall wall at the weekend and unearthed the incoming rising main, which travels horizontally along the wall, just buried beneath the plaster. Beautiful. Then I find a live cable travelling diagonally down the wall, again beneath the plaster, with a marked kink where it crossed over the water pipe. With the insulation damaged at the kink, exposing live wires about 1 mm from the pipe (and, up until last week when I installed following advice received here, no earth bonding).

The previous week I was levering off a 3x2 timber stud, from an old partition wall, when I found a live cable (apparently from an old wall light) under the stud, with bared, uninsulated wires; only luck had prevented me from touching them with my wrecking bar.

Anyway - query follows... When I rewire I obviously want to make use of any holes conduits etc that I can from the present installation, rather than make new ones in parallel; however that would mean cutting off the power to the old wiring. I'd really rather not do that until I'm fairly well on with the new wiring, which will take me some time as I'll be doing it very much part time (and it will be my first go at doing a complete rewire).

Maybe given the appalling standard of the old installation I should just bite the bullet and pull the plug on it completely! but does anyone have any tips on how to rewire while not killing the old wiring? Should I just install a small temporary circuit off the old CU, to provide a couple of sockets at the bottom of the stairs, to cover the whole house? (it's vacant at least, but I do need to use lights, power tools etc).

By the way it's only a small terraced place so I wasn't envisaging more than one ring main for the whole place (especially as it has solid ground floors so upstairs and downstairs sockets will both be fed from under the floor upstairs.

Thanks David

[Christian McArdle]

Put the new consumer unit in. Transfer the tails to the new consumer unit. Run the old consumer unit off a 32A RCBO in the new unit, which you can reuse for a socket circuit when the old stuff is disconnected.

Better (given vacant possession) is to install the new consumer unit, transfer the tails and quickly wire up a new socket circuit, surface mounted for your power tools and lighting (1 double socket per floor might be enough) which can be removed when you've finished.

Christian.

[Mike Harrison]

What I'd do would be to pull the plug on all the old wiring, and temporarily hardwire a 4-way block or two into the CU & run extension leads as necessary.

If the old wiring is in conduit, you may be able to use it to pull new cables through.

[Lurch]

Best solution would be to take out the old CU, chop off all the old circuits and fit the new CU, with new tails. Then just fit a couple of double sockets just below it on a circuit with a 32A RCBO or 32A MCB off the RCD side. That's what I've been known to do when rewiring old properties. ..

SJW A.C.S. Ltd.

[Martin Angove]

As others have said, put power to the new CU and either run a minimal number of temporary sockets from this for your tools, or feed the old system from an RCD-protected MCB (or RCBO) through a bit of 6mmsq. Personally I'd prefer the latter as you'll then still have lights! If any of the old circuits are particularly bad then you can remove them anyway.

So long as you've ensured the total floor area is under 100sqm this should be ok, but there are serious advantages to two or more rings:

1: Not everything dies if you trip the MCB on one ring.

2: You can work on one ring powering your tools from the other (handy even after the rewire).

3: One ring for a modern house is pushing it a bit if the kitchen is included in that ring, especially if you're in the habit of using the kettle (2.5kW) and toaster (500W) at the same time as doing a boil-wash for the dirty nappies (2kw) and tumble-drying (2kW) last night's washing. It's been known in our house... yes, ok, most of that is short-term, but if you have a long ring, and it's all in the kitchen (we don't yet have a dishwasher, but I'm sure we could manage to turn that on at the same time), that could unbalance the poor circuit.

4: Ummm... there must be more...

Of course, a second or third sockets circuit needn't be a ring - a common suggestion is to install a 16A or 20A radial without RCD protection solely for powering the fridge and freezer. Another is to install a 32A (4mmsq) radial just for the washing machine and tumble dryer.

At the least I'd consider installing a separate ring for the kitchen - even if the whole of the rest of the house is on one ring - simply due to the number of heaters and motors it is possible to use in a modern kitchen (see above).

For a larger house, the standard split would be upstairs / downstairs / kitchen, but I have to say (having just finished rewiring a 1900s terrace) that in hindsight it might have made more sense to do a front of house / back of house (/ kitchen) split - this would have simplified cable runs, though it would have meant that the knocked-through living room had sockets at one end on one circuit and at the other on another.

I'm about to start rewiring a 1930s council-built semi, and there's a very easy East / West split here. No faffing about running cables around the landing - one circuit for master bedroom and living room, one circuit for bedrooms 2, 3 and the dining room, one for the kitchen and utility area.

You're going to have the floor up anyway, it only means an extra hole or two through joists (or just making a larger hole in the first place) and two or more sockets circuits could be so much more convenient.

Just my 2p.

Hwyl!

M.