Electric Oven advice ?

It's a built-in double oven. It will be hard-wired to an isolating switch.

Which was fitted when we moved in. However since we were gas, I blanked it. So that's the feed from the CU sorted.

However what spec cable is needed to go from there, through the conduit below and thence to the oven ? Which is going to need a junction box fitted to the wall behind the cooker and the cooker connected to that ?

not sure 6mm will go into the conduit, but 4mm will.

if you use 6mm singles, not T&E, it probably will.

Currys say it's 5.09kW:

For diversity, the rule is 10A plus 30% of the remaining load, plus 5A for an additional socket on the cooker point (irrelevant here).

If it's 6kW that's 26A. So rated current needed 10+(16*0.3) = 14.8A. If it's 5.1kW that's 22A, need rated current = 13.6A

According to TLC's calculator:

Given it's a cooker I'd plump for the high temperature and go with 4mm2.

You might be able to run it off a plug at 13.6A and get away with it (especially if both ovens aren't run at once), but I wouldn't - it's better to be hardwired (and I think regs would expect that).

It might work via a non BS1363 plug - 16A caravan connector or something - but I can't see why to do that unless there's a good reason (disconnecting for maintenance?)

Theo

I would expect it to need a dedicated radial circuit in (probably) 6mm cable, connected to a 32A MCB and with an accessible isolator switch near the oven.

It sounds as if you've already got most of this. Is there a flush or surface mounted cooker control unit with a big red switch (and possibly a 13A outlet as well) with wiring going down to a cooker outlet connection? If so, just connect the oven to this outlet.

I disagree. That diversity is for a cooker that comprises a hob with 4 hot plates and one or two ovens - in this case you just have two ovens and you cannot apply that sort of diversity.

So rate the cable for 26A.

If using H07RNF flex then that is 2.5mm.

Why does the switch need to be red.

From the Curry's site

"ELECTRICAL INSTALLATION: This product requires professional installation to a dedicated cooker circuit (identified by a big red cooker switch) by a qualified installer, such as one of our Currys experts."

A Scolmore CMA504 is not red

... If the cable is in a cold situation. If it gets hot where it runs inside the oven it'll have less ampacity & need derating & thus 4mm cable. So it depends on the oven design.

B&Qs web site helpfully suggests the power output is 13W :-)

(might be true if you only have the light on!)

Yup - it does depend on the actual capacity. A double oven is very unlikely to be over 7kW, so a 32A protected radial will likely be more than adequate.

Normal T&E as far as the isolator, and ideally a length of high temp flex for the final connection. Something like 2.5mm^2 H07RNF should be ok for the final connection. The T&E size being dictated by the length of the circuit and the any other de-rating factors encountered along the way.

Chapter and verse here:

In which case the instructions should say; if the connections are subject to heat from the oven itself. Otherwise I don't why it should be treated differently to any other appliance?

the difference is only that heat reduces ampacity, hence some ovens/hobs that would appear to be fine on 2.5 require 4mm - and some don't.

The main oven of our double oven has 3 elements - top, bottom and rear for the fan. None of the settings use more than 2 elements and some only one. The top oven has 2 elements and only one of the 5 settings uses both. Some settings only run the top elements at half power. So it would seem reasonable to apply diversity, especially with the thermostat creating an intermittent load as opposed to continuous running.

That is my point. In which case the instruction would advise accordingly.

Generally the terminal connections are away from an area of heat and so no need to uprate the cable.

Wouldn't the rated power in the installation instructions take that into account?

I meant from a max power the oven would take.

While I agree the thermostat should also reduce average current I'm not sure how that should be applied to diversity. Currently I don't think it is in any calculation.

There do seem to be a lot more combination ovens around these days that have Microwave as part of their suite of facilities. I guess tis is a reaction to more costly electricity, but many of those seem to plug in. Brian

Thanks for all the answers.

When we moved in, there was above the (gas) hob a dedicated cooker switch with (new) wiring feeding into it from the CU. However it had never been taken below the hob to connect up. The fitted single over was simply plugged into the ring main.

Sounds to me like 4mm T+E is more than adequate. The cable will run in a wall, and then in free air to the actual oven. The real challenge is feeding it through the conduit that's been left.

What are the implications of removing the outer sheath and (as a pp suggested) running the wires individually to the junction box, but as a single cable to the oven ?

I see from reality shows that the "big red key" used by the police is also often not red any more.

Our double oven (two different ratings) has LEDs that go on when heating and off when at temperature. It would be very easy to check the duty cycle of each and work out the average load over a period of time.

Diversity is based on the idea of average load from the power and duty cycles of all connected loads and the relatively slow rate of heating of the cable when (temporarily) overloaded. It is just a way of estimating an average load, without knowing all the details accurately.