DIY Doctor

Old House - general advice needed

Postby cmhicks » Mon Apr 18, 2011 1:03 pm

Hi all,

I live in an old (c. 400 years) house which I believe was totally rewired in about 1968, but I think not very well. The previous owners had the old re-wireable fuses replaced with a Wylex MCB-based CU, I guess about 10 years ago.

I'm not sure what the correct term is, but I have a general certificate of 'good health' from about 5 years ago (in order to issue that he noted on it that he had to do one or two additional things, such as disconnect the attic light).

There are some inadequacies which I want to address.

1) The Wylex CU does not have any RCD protection and I have two children under two. Nuff said.

2) The 2.8kW electric oven is plugged into an ordinary socket in the kitchen (hob is gas, so no problem) on one of the socket circuits.

3) The 3kW immersion heater (rarely used) and boiler are spurred off (fused spur) a different socket circuit.

Although it has been tested as basically safe about 5 years ago, much of the existing wiring is clearly not to modern standards. In particular I am not at all sure that the socket circuits are wired appropriately.

I know that the ideal would be a top-to-bottom re-wire, but that is not an option for many reasons. The compromise I have in mind is as follows, and I wonder if it sounds reasonable.

1) Install a new, larger CU. Install new dedicated circuits for oven and immersion heater (and also one for power to detached garage, but that's a separate issue).

2) Use one RCD-protected 32A feed from the new CU to supply the old CU, thus limiting the total load on all old wiring to 32A and adding RCD protection.

3) In the old CU, replace the 32A MCBs on the socket circuits with 20A (or even 16A ones), thus protecting the old wiring further. Obviously this will reduce what I can draw from the existing sockets, but I can live with that.

4) As I restore the house, room by room over the next 5 years or so, take the opportunity to install new wiring to modern standards, and gradually move this over to the new CU.

5) Eventually, all wiring will be new and on the new CU. The old CU will then be redundant and can be removed.

This plan will be expensive to implement in total, but it avoids the unacceptable disruption of an immediate total re-wire. Are there obvious pitfalls, or could it be simpler? One slight issue I see is that between step 3 and 4, all the sockets and lighting will be protected by a single RCD. If this trips then we lose the lot. I'd be prepared to install some temporary battery-backed emergency lighting to get around this.

Secondly, I believe that the regs state that there should usually be no more spurs off a ring main than there are sockets actually on the ring, or something close to that. Why is this? I ask because it looks to me as if some effort has been made to convert the socket wiring to rings at some point, but this has resulted in a lot of junction boxes on the ring, from each of which just one single socket is spurred, and there are very few sockets on the actual ring. This would appear to contravene the regs. However, if I cut the ring in a few places and install some sockets on the ring itself, then it would comply. This seems daft...

Sorry for such a long post, and thanks for reading this far!

Christopher Hicks
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Simply Build It

Postby stoneyboy » Mon Apr 18, 2011 6:43 pm

Get the wylex CU replaced with a modern split CU with spare ways and correct circuits as you can.
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Postby ericmark » Mon Apr 18, 2011 11:02 pm

In general as long as circuits are protected with an automatic disconnection device that will prevent damage due to overload then you are reasonably safe.

There are other factors of course. The automatic disconnection device should be able to disconnect within the prescribed time.

Although today bathrooms, outside, sockets under 20A, and wires buried at less then 50mm need protecting with a 30ma @ 40ms RCD it is not as fool proof as people seem to think. 40ms is still enough time to get a nasty belt.

Renewing the CU with one which will take RCBO's is an advantage if the one fitted will not accept them. But having just 2 RCD's in an old house is asking for problems.

Although the oven and immersion heater should be changed if the trips are the right size then the only real problem is loosing power when it trips. There is no real danger.

Set one is to have the installation tested. The guy testing will code the faults and there are basically two things.
1. Dangerous
2. Will need modifying before modifications can be made.

The old system had 4 codes.
1 = Dangerous
2 = Will need modifying before anything can be added
3 = Not tested for example incoming fuse size as it is sealed.
4 = Really same as 2 means complies with previous edition and so if you want to add then needs correcting first.

The 2 code includes things like no earth on the lighting circuit and of course this will have an impact so only fittings you can use will need to be double insulated but in its self it is not a danger. It is only a danger when used with wrong fittings.

Of course you have to ask what is dangerous. To me wires not RCD protected are only a danger when some one drills the wall without first checking where the cables are likely to run. But forewarned is forearmed and if you know there is no RCD you are very careful drilling holes.
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Postby cmhicks » Wed Apr 20, 2011 10:56 am

Thank you for the advice.

[quote]Renewing the CU with one which will take RCBO's is an advantage if the one fitted will not accept them. But having just 2 RCD's in an old house is asking for problems.[/quote]

Yes, that's exactly what I think the situation is. Also the old CU has not got space for more circuits, so a bigger one gives the opportunity to segregate large loads such as the oven and immersion heater. I'm just hoping that I can come up with a plan that enables me to do the rewiring gradually - which stoneyboy seems to think is reasonable.

Anyhow, I've got someone coming to take a look today or tomorrow with any luck.

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