consumer unit change for double oven and inspecting all wiring

Postby daffyduck1962 » Sat Apr 02, 2016 5:27 am

I currently have a consumer unit which has either MCBs or RCDs and need to upgrade the supply to accept a double oven.
I have been told that the consumer unit has to be changed and that this will then mean the whole of the house's wiring will have to be inspected.
Is this correct or is someone trying to get me to spend more money than I need to ?
this was installed in 2013 to run an electric shower
This was here when we bought the house at the end of 2007
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Postby ericmark » Sat Apr 02, 2016 12:12 pm

The shower is OK so forget that. The other distribution unit has not RCD protection there is no need to upgrade existing services to include RCD protection but all new sockets will require it and any cables buried in the wall.

So left to right you have.
B32 unknown
B16 unknown
6 Amp type unknown upstairs lights
B6 unknown
32 A type unknown can't read
16 A type unknown water heater
32 A type unknown can't read
32 A type unknown cooker

So for a double oven you will need a dedicated supply i.e. not from the ring of either 16 or 32 amp and getting replacements for that distribution unit will be hard. Most likely would be the cooker supply and to use that you will need a surface double cooker connection unit and as long as all the new cables are run surface and not hidden behind plaster and the oven is hard wired and nothing in the manufacturers requirements says it needs RCD protection then fine.

I have a cooker which has a double oven and four heat areas induction type and the instructions says use a 32A feed so worse case it trips the MCB no danger installing.

However to hid cables means RCD protection over 13A this means a large unit like you have for the shower so could be done but needs a large ugly box. In theroy swapping the consumer unit is best option but in practice there are a few draw backs.

In the 1960's and 70's houses would typical have 4 fuses. The lights were all from one fuse, but then we started to mood lighting and the tiny spot lights are very inefficient at lighting an area so needed more than 6A, since ceiling roses only rated 6A easy method was to split up and down stairs. However many electricians used only two core and earth between the two way switches and borrowed a line from an adjacent switch. This is OK when all from the same MCB but once split it becomes a borrowed neutral. However although dangerous for some one working on the electrical system it still worked. However once split across two RCD's then there is a problem.

There are ways around the problem including RF linked switches but then the price starts to go up. Again any old rubber cables can degrade and there can be a small leakage, with no RCD there is no problem. As soon as you add an RCD you find the problems.

The same with stuff you use. If the de-frost element in a freezer is leaking power as soon as the RCD is added you have to also repair the freezer. Having had RCD protection for some 25 years anything brought into the house new which then results in RCD tripping there is a big pointer showing the suspect item. But have a house with no RCD's then fit them and working out what is causing them to trip is not easy.

So BEFORE a consumer unit is changed a good electrician will test as much as he can. He does not want to have unexpected bills although not really his fault it is hard to convince the home owner that is the case. So better find them all first and let the home owner know BEFORE they start on the job proper. However this does cost and at the end of the day you pay for it.

The other methods is change the consumer unit and then worry about correcting faults, likely cheaper, but the owner does not know up front what it will cost. The likely £400 quoted to change a consumer unit is just the start. After that you have any items found wrong. Using a all RCBO consumer unit although costs more to start with will reduce hassle latter on. Simple maths 2 x 30 mA = 60 mA total which can leak before a problem but 10 x 30 mA = 300 mA before there is a problem so small leaks are less likely to trip the RCD, and when it does trip only one circuit is lost. If you lose one circuit you can likely work around it until you find the fault loosing half the house is very different.

At some point I expect you will want extra sockets once the consumer unit is changed that is a lot easier and cheaper. New RCD socket £32 new non RCD £5 so very quickly it pays for its self.

Of course the house is also safer.

So you don't have to change consumer unit, but that might be best option in the long run.
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