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Electrical

Double oven and induction hob power supply

We are getting a new double oven and induction hob installed as part of a kitchen installation. These two appliances are rated at 5.7kW (oven) and 4.6kW (hob).They will have separate isolation points but be fed from the same supply from the consumer unit.

My questions are:

* Will an existing 6mm^2 power cable (~10m, within concrete ceiling) meet the needs of both these appliances?

Some further context:
* I am not intending to install this myself. Putting in a second supply is not going to be trivial and we need to consider the need for it before plastering and decorating.

* The current cable is 6mm^2, about ~10m long and deep within a concrete ceiling (and therefore inaccessible). It is a dedicated supply. At the moment there are no appliances connected to it. It previously had a 30A breaker, but we'll be getting a new consumer unit shortly.

* The appliance technical manuals (both NEFF) are very light on electrical requirements.

3 Answers from MyBuilder Electricians

Best Answer

Hi, firstly the answer from Halloways above is spot on, but I would ignore the second answer, firstly 30A is not inadequate (as explained below). Secondly, a kitchen is NOT classed as a special location (in England, but I believe it is in Wales where Hulsea are based) and therefore not notifyable work. There is no requirement for any work to be carried out by a registered contractor (although highly recommended), work can be notifiable direct to building control at a cost. This would only be notifyable if you were to rewire the circuit, which you don't seem to be doing.

Both together equate to approx 55A, however you are allowed to apply some diversity. It would be pretty standard to apply the first 10A plus 30% of the remaining, i.e. 55A-10A=45A, 30% of this is 13.5A. Therefore take 23.5A as your figure, assuming that there is nothing else on the circuit i.e. it is a dedicated supply from your fuseboard? If there is any sockets incorporated into the isolators then add 5A per socket. If they are on their own circuits then work them out separately which will reduce the figures further. If they are on a ring circuit then this will be a problem.

The determaining factors to decide if it is a 30A supply are the size of the cable, the size of the MCB/fuse, and the rating of the isolator.

If the fuse/MCB is a 30A fuse then you have a 30A supply. The cable should then be sized to take a greater current than the breaker rating (as the breaker is there to protect the cable). Determining the cable size isnt as black or white, this depends on the method of installation, type of cable and its length...

E.g. if the cable was clipped dirct to a wall throughout and say 10m then you would get away with a 4mm T&E cable.

If it run through a insulated ceiling void then you could need 6 or 10mm... if its length was excessive then you would need to account for voltage drop and therefore this could increase the size further, worst case, if the cable was installed in an insulated void, in an area with a high ambient temperature, it was grouped with other cables and was a long run then you could require a 25mm cable... however this would be unlikely on a domestic property.

However, there is a bit of an overriding rule with most electrical regs which is 'as per manufacturers instructions...' Check the manual for the cooker, they will most likely state a breaker and isolator rating and a cable size.

2015-02-23T14:40:02+00:00

Answered 23rd Feb 2015

From the initial explanation it says that both appliances will have separate isolation points at separate sides of the room. These appliances would normally have separate cabled supplies to each isolation point.
The rating of the supplies is down to the size of the cable and the length of run from the point of origin ( consumer unit ) and some other factors. The maximum current amount would be the rating of the fuse / mcb protecting the circuits and assuming that the supplies were correctly calculated for the fuse/ mcb rating then all would be fine.

To accurately answer the question I would need to know if...
1. Each existing isolator is feed with its own circuit from the board?
2. What rating of MCB / Fuse protects the circuit/s
3. What size cable is used for the circuit/s

Any good electrician would be able to answer this with a quick site visit to check the variables and a quick test would check the supplies for safety at the same time.

2015-02-22T14:30:02+00:00

Answered 22nd Feb 2015

Hi,

Quick answers; 1 NO, 2 Go to college and spend a few years getting experience, 3 No

Long answer.

1 A 30A supply is totally inadequate for your new appliances.

2 You have to determine cable size and calculate based on the route it has taken from the consumer unit to determine the maximum current carrying capacity.

3 You have given teh required information, the route now needs assessing

The kitchen is a special location and it is a legal requirement to use a registered contractor to carry out the works.

2015-02-22T14:30:02+00:00

Answered 22nd Feb 2015

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