old circuits, new consumer unit

[David Robinson]

I propose to do the following:

Re-wire upstairs power circuit Re-wire downstair power circuit (because they're both totally inadequate!)

Leave kitchen power circuit because it's perfectly adequate (the kitchen is a 1970s extension) Leave the lighting circuits alone (again, the wiring is fine, and the circuits are adequate) Leave the cooker circuit (no electric cooker, cable seems very recent)

Completely re-wire the garage

Replace the 6-fuse consumer unit with a split load 13 or 20 way consumer unit. RCD everything except lighting and garage (which will have it's own CU with RCD).

My question is: is it OK to connect the old wiring to the new consumer unit?

I know I need to test it to check that it won't trip the RCD (is

20Mohm+ from the earth to phase and neutral, measured on a cheap multimeter, good enough?), and I'm happy that the wiring is in good condition, but I can't be sure that the installation of all this wiring meets new regs. (e.g. The earth bonding in the bathroom is non-existant, but I will fix that).

Does all the wiring connected to a new consumer unit have to meet current regs, or is it OK to transfer older wiring (late 1970s, possibly not quite meeting current regs in all respects) to the consumer unit?

Cheers, David.

[David Robinson]

I can understand that. We're talking about £200, aren't we? That might make it cheaper to get someone else to do the job.

I'll check all that on the kitchen ring main.

I know the kitchen was only built in 1978, but I'm not certain when the lighting in the rest of the house was wired. (The power circuits in the rest of the house don't matter, because I'm replacing them).

From the way almost all the wiring and sockets/switches match throughout, I think the whole house was re-wired when the extension went up, with the electric cooker circuit upgraded (and shower circuit added) later. But I could be wrong. If it was re-wired all at once, they chose sensible routes and positions for the sockets in the extension, but stuck with the existing stupid positions for the sockets in the rest of the house.

Whatever - if I test it, I'll know if it's good enough. Dread to think of the mess I'll cause if it isn't!

Which bit of the on-site guide deals with this? The tables of allowed cable sizes, currents, and MCBs?

Does this give me better protection than one RCD overall?

Yes, that'll be in the garage. That all needs re-doing because there's a single FCU running everything.

Thanks for your help.

Cheers, David.

[Christian McArdle]

You may be able to borrow or hire one.

It is actually quite difficult to make a 6A lighting circuit run in 1mm (or

1.5mm) cable non compliant. The cable is so overrated and carrying so little current, that there are rarely issues with cable lengths. However, you should check that the circuits have an earth and are run in PVC insulated cable. Also, you should check inside all the accessories and pendants for any bodging gems.

As you don't know the length of the installed cable, it will be easier to measure the cable resistance directly and calculate what the maximum earth loop impedence and voltage drop will be. Then just compare with what is allowed. You must have an earth loop impedence that allows 160A to flow to be sure of tripping a B32A MCB.

Not as such. It reduces both the likelihood and consequences of nuisance trips.

Christian.

[Owain]

"David Robinson" wrote | From the way almost all the wiring and sockets/switches match | throughout, I think the whole house was re-wired when the extension | went up, with the electric cooker circuit upgraded (and shower circuit | added) later. But I could be wrong. If it was re-wired all at once, | they chose sensible routes and positions for the sockets in the | extension, but stuck with the existing stupid positions for the | sockets in the rest of the house.

Perhaps they only put new faceplaces on the old wiring? One house I lived in had lovely lead-covered cable and iron-clad DP fuseboards but the brown bakelite light switches were the modern square style.

Owain

[David Robinson]

Thanks for the warning Owain. Luckily I've already changed two light fittings, and been in the loft, and all the wiring is fine. If it was an old house, I'd rather change the wire and keep the nice old fittings, but I guess that's not advisable, and it's not applicable here anyway.

The only thing I could fault is that the earth posts in the metal boxes behind the sockets are not used - the earthing just loops into the earth on the socket, with no lead to the metal wall box (inset into the plaster) itself. Should I add a lead, or is it enough that the sockets themselves connect the earth to the 2 screw holes? I was going to go around and add a wire to them all.

Cheers, David.

[Christian McArdle]

You're right. The problem with old fittings is that the insulation in them will normally have broken down by now and be unsafe, just like the the rubber insulated cable originally used to connect them. I intend to get some reproduction ones for my house with modern internals. In particular, some round Bakelite light switches for the hallways and a couple of dome brass ones for the principal rooms.

Christian.