DIY Doctor

Mixing Red Black With Brown Blue and Cooker on Kitchen Ring

Postby templeofseven » Sun Aug 16, 2015 11:00 am

Hello all,
Just bought a new house all seemed ok. Trouble is my kitchen ring main has tripped a few times while using garden tools on an extension lead connected into the kitchen ring.

Transpires that the kitchen ring has the cooker attached, AHHH This believe should be a separate 32A or higher. So when the wife uses the oven, the freezer compressor starts, and I start using the rotervator the breaker trips.
So, to run a new 4 or 6mm radial directly back to the consumer unit means clearing out a busy room, ripping up my newly fitted carpet and lifting boards.
I have however in my airing cupboard a 4mm 2+E back to the consumer unit which I won’t use. This goes directly back to the consumer unit and is no more than 7 meters in length. The airing cupboard is no more than 5 meters from the kitchen so I could run this from the kitchen back to the airing cupboard making a total length of 12 meters or so.

My next issue is, the consumer unit is an old Wylex installed somewhere in the 90’s. It’s protected overall by a 65Amp 30ms trip RCD and all the currents rings/radials are red/black. I also have a spare 80Amp 30ms RCD should my current requirement ever change.

If I were to join the two cables should I?

Buy 4mm red black and extend the colours as is, joined with an appropriately rated junction box then terminate in the kitchen on a cooker socket, or

Buy 4mm brown/blue and use this to extend the red/black into the kitchen terminate on the appropriate socked. Then sleeve the red/black in the consumer unit to brown/blue?

You help would be very much appreciated
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Simply Build It

Postby ericmark » Mon Aug 17, 2015 12:46 pm

We have what should be done to comply with law and regulations, what is normally done, and what you can get away with. So to start with I will talk about what should be done.

Very early in the regulations it says you should split a supply into circuits, and it suggests this also means circuits formed by using RCD’s. It is considered by many having 2 RCD’s in a house is enough, however others will want to use RCBO’s which is a RCD and MCB combined so any fault be it overload or earth leakage will not effect other circuits.

Before 2008 TT earth systems had 100 ~ 300 mA RCD’s fitted to all circuits but most houses with TN earth systems would only have RCD’s protecting circuits in bathrooms and outside. The 100 mA RCD is not considered sensitive enough to protect life so in 2008 the regulations had a major change and the idea of all circuits protected by 30 mA RCD’s came into being.

The 100 mA hardly ever tripped but the 30 mA can easy be tripped by simply all the small leakages adding up. There are a number ways to reduce tripping splitting in to more circuits and using special electronic anti trip software which is built into the more expensive models like the X-Pole which is a rotten name as internet searches tend to bring up things nothing to do with electrics!

So likely your tripping is down to earth leakage and you have to consider if worth the money to sort it. The most likely method is a consumer unit change. It is unlikely to be less than £300 could be a lot more.

This needs to be addressed first as it will likely have an impact on the cooker supply question.

On to cooker supply first cables can be run outside my mother has a large SWA cable from consumer unit to kitchen and a mini consumer unit in the kitchen which supplies all kitchen items except for lights.

4 mm² cables do not have a single current rating. It varies according to type of cable and how it is run. So that cable could be anything from 17.5 to 37 amp with thermoplastic twin and earth without considering thermosetting versions. 32A would be ample for a cooker but 20A is stretching it a bit many boards don’t have 25A MCB’s so you have to work out rating of existing cable.

Likely reference Method 100 (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) or reference Method 102 (in a stud wall with thermal insulation with cable touching the inner wall surface) will limit the cable to 27 amp but since you can’t get 27A MCB’s in real terms it’s 25A.

Volt drop will limit you to around 42 meters of cable. With a standard Ze of 0.35Ω one would expect the impedance at end to be around 0.98Ω to comply with volt drop regulations. i.e. 42.6 meters of 4 mm² will have a resistance of around 0.635Ω which will cause a volt drop of 11.5 volts which it the maximum permitted based on a 25A load. Increase to a 32A load and then the length reduces to 31 meters.

As you can see 0.635 is a very low figure so to measure it requires special meters a low ohm meter which the regulations say must draw at least 200 mA or a loop impedance meter. Either way they are expensive to buy or hire.

In real terms unlikely to be a problem joining the cables just needs a sticker on the consumer unit to say CAUTION
This installation has wiring colours to two versions of BS 7671.
Great care should be taken before undertaking extension,
alteration or repair that all conductors are correctly identified.

The regulations in the appendix section says, that all fixed equipment, over 2kW, should have its own dedicated supply. However the biggest problem is likely the immersion heater because it stays on so long and then next the tumble drier the oven does not actually cause as much of a load as the tumble drier as the thermostat switches it on and off once temperature set is reached. Since we often run the tumble drier from the ring final compared with that the oven is not really a problem assuming under 3kW.

If the trip is caused by over load then using the other supply will help, however if caused by earth leakage it will do nothing.

A rotorvator may draw over 13A most are petrol driven, It may also have a large in rush on start. It is possible that what ever you do it will not help. As to freezer my old one on start up used 8A but new one uses just 0.8A again there is a huge difference between old capacitor start and new inverter control.

I hope I have given you some pointers. In theory nearly all DIY work does not comply with the regulations simply because the DIY guy does not have the test gear required. So what ever you do is taking some risk. What you have to assess is if the risk is worth it.
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