Connecting 10mm^2 to the oven - connectors seem inadequate

diyAlex · 15 Nov 2010

I am just in the process of installing a new Bosch HBN13B251B Avantixx Built Under Double Electric Oven in our kitchen.

The electricians added in a cooker switch fed by 10mm^2 from the consumer unit when the house was rewired a few months ago. We have just got around to buying an oven, so I'm now fitting this.

I've got some 10mm^2 cable - connecting it to the switch was easy enough. But in the oven's connector block, the screw terminals just seem very inadequate for such a thick cable. Should I be soldering on terminals to the end of the cable, which are then themselves attached to the screw blocks or something??

I have actually got it connected by separating the 7 cores of the blue wire and then wrapping these around the screw - 4 on one side, 3 on the other. Then squishing them with pliers and then tightening the screw into the terminal. Then repeated all this for the brown wire. The earth was much easier.

This then looks like all the cores are forming a connection at the screw. As opposed to, say, only a couple of the cores making contact and the others floating about.

JohnD · 15 Nov 2010

what is the power rating of the oven? do the installation instructions mention the cable size?

when you say "oven" I take it you mean "oven" not "cooker?"

10mm was probably chosen as it will be adequate for a huge electric cooker with five rings and three ovens, which I think is not what you have got, so the conection from the wall to the oven need not be in this size. Hobs take much more power than ovens.

diyAlex · 15 Nov 2010

I can't find a rating in the Bosch manuals/tech specs online - I will look at the actual applicance tonight.

Yes, you're right - it is a "built under" oven - i.e. just the oven and it goes under the worktop. It's actually a "double oven" - standard 600mm width but with a small top oven/grill and then a larger main oven.

When I talked about our plans with the electricians, we had in mind a Bosch oven that has the "self cleaning" facility (which I think uses a very high wattage element to burn stuff off). This, combined with the fact that it's a double oven, led the guys to install 10mm^2. Maybe they were just playing safe. In the end, we've downgraded our oven a bit so actually it doesn't have this self-cleaning facility.

And we're looking at a 7.2kw hob, so the guys ran a second 10mm^2 cable + switch for that. This one gets delivered on Wednesday

riveralt · 15 Nov 2010

diyAlex said:
I am just in the process of installing a new Bosch HBN13B251B Avantixx Built Under Double Electric Oven in our kitchen.

The electricians added in a cooker switch fed by 10mm^2 from the consumer unit when the house was rewired a few months ago. We have just got around to buying an oven, so I'm now fitting this.

I've got some 10mm^2 cable - connecting it to the switch was easy enough. But in the oven's connector block, the screw terminals just seem very inadequate for such a thick cable. Should I be soldering on terminals to the end of the cable, which are then themselves attached to the screw blocks or something??

Something like this you mean
http://www.tlc-direct.co.uk/Main_Index/Cable_Accessories_Index/Crimp_Lug_1/index.html

JohnD · 15 Nov 2010

IMO a better solution will be to use a cable size better suited to the oven.

diyAlex · 15 Nov 2010

Yes, a copper crimp lug! That looks like it will fit perfectly!

riveralt · 15 Nov 2010

diyAlex said:
And we're looking at a 7.2kw hob, so the guys ran a second 10mm^2 cable + switch for that. This one gets delivered on Wednesday

10mm is fine up to the cooker switch or even the cooker connection unit.
The key thing here is that your MCB should be rated lower than your cable. Since the MCB is there to protect your cable. E.g. 32Amp MCB - 10mm cable max current carrying capacity 64Amps - 6mm cable max current carrying capacity 47Amps.

Unless your oven and hob are greater than 32 Amps after you have allowed for diversity I would have used 6mm cable from the cooker connection unit to the oven and the hob.

I would suggest that you may struggle to fit a 10mm in the hob terminals since the ones I have worked on recently are the terminal block type and are an exceptionally tight fit.

Work out your total Kw of oven and hob in watts and divide by 230.
This will give you the total Amps.
Then apply this formula for diversity 10Amps +((Total Amps -10)*.3) + 5Amps if you have a socket on your cooker switch.

So say 15Kw total watts = 15000/230 = 65.2 Amps.
10Amps +(55.2*.3) = 16.56 Amps + 5 for your socket = 31.56Amps - which means you are covered for a 32Amp MCB and 6mm cable (unless the cable is derated for some reason like insulation).

diyAlex · 15 Nov 2010

That's very interesting to read about diversity ... not entirely sure I understand what "diversity" is, but it's interesting to see how you relate an applicance's wattage to the cable size that is supplying it.

I have only worked on the (naive) assumption that the cable running from the appliance to the switch should be the same rating as the cable running from the switch to the consumer unit.

JohnD · 15 Nov 2010

you'rer not wrong, but in your case the cable from CU to switch has (very sensibly) been sized so that you can, should the fancy take you, put in one of those huge range cookers with 5 big rings and three ovens, without needing to upgrade it.

riveralt · 15 Nov 2010

JohnD said:
IMO a better solution will be to use a cable size better suited to the oven.

Quite right, so..

JohnD said:
you'rer not wrong, but in your case the cable from CU to switch has (very sensibly) been sized so that you can, should the fancy take you, put in one of those huge range cookers with 5 big rings and three ovens, without needing to upgrade it.

for the oven you have and the hob you are getting - do the maths and then pick the cable best suited to your needs.

Diversity is used as a means calculating the average use of ovens and hob. It works on the basis that, except perhaps on Christmas Day, your oven and hob will not be working a full blast all of the time. Even on Christmas Day, most modern ovens are fitted with thermostats that are constantly checking the temperature and turning the oven on/off as appropriate. Diversity is also applied to lighting circuits, ring final and radial circuits. But not to hot water systems like electric showers.

ban-all-sheds · 15 Nov 2010

diyAlex said:
Yes, a copper crimp lug! That looks like it will fit perfectly!

You'll need a tool like this:

to attach the lug.

JohnD · 15 Nov 2010

quite right

note that it is not the same tool as is used for red, yellow and blue insulated crimps, which are more common.

PrinceofDarkness · 15 Nov 2010

What a waste of 10mm cable for the final connection to this slight load of less than 5kW. 4mm cable or even 2.5 butyl flex would have served just as well.

There's no imperative for the final connection to a cooking appliance to be the same size as the fixed wiring to the consumer unit. The reason for this is quite simple: Such a fixed load isn't capable of being overloaded - you cannot switch on more than the maximum load, which in this case is less than 5000W. Therefore, the main consideration is that of short-circuit protection coordinanation between the final cable connection and the circuit fuse/MCB.

Modern ovens (such as this one) have very small terminals and terminal housings - so it's madness to try to cram an unnecessarly oversized cable in.

Lucia.

stardanny · 16 Nov 2010

For your info:

Bosch HBN13B251B Avantixx: total connected loading 4.9kw.

diyAlex · 16 Nov 2010

Thanks for all this feedback guys!

Looks like I have grossly over-engineered the cable from the switch to the oven

In my defence, this is the first kitchen I've built and I didn't want to get the oven out of the packaging until needed in case I scratched it. So I was working on assumptions (therein lies the weakness!).

diynot

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Connecting 10mm^2 to the oven - connectors seem inadequate

PrinceofDarkness

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