Cooker wiring dilemma

Mark Waistnidge · Mar 8, 2017

A house renovation, including kitchen extension, all plastered out with a 10mm twin+e feed in place for the 7.3kw induction hob.
The single oven and the combi microwave should have been spurred from the kitchen ring as each being 2.9kw. Neff have decided to up them to 3.45kw for the oven and 3.65kw for the microwave.

Is there anyway i can make the 10mm cable thats in position work for them all or do i need to ruin the new decorating?

Help appreciated.

Electric Magic · Mar 8, 2017

Have a look at your copy of the wiring regs (appendix 15 gives some relevant info regarding fixed loads) and, if you've got a copy, the on-site guide also gives some guidance on cooker circuits. If I had mine to hand I'd give you a page number, but I don't

Hellmooth · Mar 8, 2017

Even at 4800kw combined that wouldn't have been ideal 'spurred' off the ring.......20 amps? Any appliance over 2kw is recommended to be on its own supply.

Octopus · Mar 8, 2017

Who decided the appliances could be spurred off the ring... Bad decision imho

Wilko · Mar 8, 2017

Hi Mark - assuming installation method allows 50A plus and that you are using 50A mcb (?) and the manufacturers have kW rated at 240V, then the max draw is 60A. IMHO this is unlikely to happen in a domestic situation, as there is only one cook etc. And even if it did happen it would only be for a few minutes till ovens and elements reached temp and started to cycle. The OSG diversity ref is Table A2 p117, which seems to say 10A + 30% of 60 = 28A for domestic, and 30.4A + 12.2A + 8.6A = 51.2A for a shop or hotel.

So it seems the OSG agrees with my thoughts for domestic, but computer says no for commercial use. Please don't rely on my calcs, but I hope this helps

Mark Waistnidge · Mar 8, 2017

Thanks for the replies guys.
This job has fell on me half started, through the builder im doing another project for. I dont like finishing other peoples work but I could really do with keeping him onside and not having him get some of his work done elsewhere. The last spark has packed up, he doesnt work for himself anymore.
The appliances being spurred i agree isnt ideal. But do-able, the kitchen does have a dedicated ring. Although yes, life would now be easier if a radial had been run.

I'll check my books tomorrow as they are in the van, but without checking them my thinking was;

Installing a 50a mcb, run the 10mm to a duel cooker connection unit, from that 10mm to the hob one side and 2x 2.5mm to the oven and microwave from the other side. I wanted to check with diversity. And the 2.5mm cables are well above the appliances max draw so being fixed loads i can get away without overload protection.

Am i on the right track?
Appreciate the help.

Midwest · Mar 9, 2017

My calculations after applying diversity are (230v);

7300W = 16.51A
3450W = 11.50A
3650W = 11.75A

So I make that a total of 39.76A? (it is early for me!).

Assuming this is a domestic install, and the supply cable installation method allows, I would suggest the single radial would be suitable, although the termination would easier, if 2 x radials had been installed (dual outlet, 3 into 2 doesn't go). The microwave may well come fitted with a standard plug top, but IMO it shouldn't be plugged into RFC, what does manufacturer advise?

Wilko · Mar 9, 2017

Midwest said:
My calculations after applying diversity are (230v);
7300W = 16.51A
3450W = 11.50A
3650W = 11.75A

Hi MW - can I ask the reference for this method please? I only have the OSG

Phil Thompson · Mar 9, 2017

That's some wattage for a microwave, it's double what the commercial ones in the 2 bakery/cafes I do work for, and they are double the wattage of my own household de longhi microwave.

Is it also a grill & oven as well as a microwave?

Midwest · Mar 9, 2017

Table A1, page 116 OSG (Hope I've got it right now

)

Midwest · Mar 9, 2017

Phil Thompson said:
That's some wattage for a microwave, it's double what the commercial ones in the 2 bakery/cafes I do work for, and they are double the wattage of my own household de longhi microwave.

Is it also a grill & oven as well as a microwave?

It probably doubles up as an expresso maker as well

Wilko · Mar 9, 2017

Midwest said:
Table A1, page 116 OSG (Hope I've got it right now )

Think they may have changed the calc sometime. That table in my OSG shows 10A + 30% x rest + 5A for a socket if there's one. Either way, calc is ok for 10mm

Mark Waistnidge · Mar 9, 2017

The micro is one of these combination things. It grills and stuff too. I was surprised and expected a plug head on this.
The calcs add up, but im not totally happy running 2.5mm cable from 10mm and on a 50a mcb.
It just isnt sitting right...

Wilko · Mar 9, 2017

Mark Waistnidge said:
The micro is one of these combination things. It grills and stuff too. I was surprised and expected a plug head on this.
The calcs add up, but im not totally happy running 2.5mm cable from 10mm and on a 50a mcb.
It just isnt sitting right...

Understood. Subject of some debate here

Maybe 4mm flex so the cpc of the 10mm doesn't change size over the last metre ?

The Ghost · Mar 9, 2017

Ok crazy idea. Submain box three way come off of that for all the appliances.

Phil Thompson · Mar 9, 2017

Vortigern said:
Ok crazy idea. Submain box three way come off of that for all the appliances.

If possible I'd favour this option too. But I guess it depends on the client and their decorating choices if that's doable or not.

The Ghost · Mar 9, 2017

Well I spose if they don't go for that then they will have quite a few more decorating choices to make when you chase in a couple more circuits, is there enough spare ways at DB? @Phil Thompson ?

Phil Thompson · Mar 9, 2017

Dunno, thankfully it's not my job, I was just agreeing with your thoughts lol

The Ghost · Mar 9, 2017

Hellmooth said:
Even at 4800kw

Yeh that is a bit much for a ring

Mark Waistnidge · Mar 9, 2017

The ring would have been fine as no other appliances in there. Only a toaster and dishwasher.
I think my best option is the duel cooker connection unit and run 4mm flex to them.
No chance of a sub CU in there. I did think of this.

Dave OCD · Mar 9, 2017

You could fit a 3 way CU in the adjacent base unit perhaps ?

Mark Waistnidge · Mar 9, 2017

Yeah i have considered options on this. Its a very expensive, fancy kitchen. I cant see them going for it. Client is back on site monday so im going to throw some options there way.

Hellmooth · Mar 9, 2017

Vortigern said:
Yeh that is a bit much for a ring

Lol whoops

Phil Thompson · Mar 9, 2017

If you are going to bring it to them then draft a file explaining the increased wattage, the issues this creates and why, and then list their options, with each option give a price and a list of what will need to be made good.
Don't forget to suggest the option that they switch the 2 ovens for suitable ones. Most likely they should be able to do this without much hassle.