Cooker cable

[Graham.]

My kitchen currently has a dedicated cooker circuit, 6mm cable from a

32A MCB.

New kitchen planned, I now want a new induction hob and a double oven fitted, the max ratings are 7.2 & 4.2 kW respectively, lets call it a total of 12kW or 52.17A

AIUI you apply diversity by subtracting 10A then reducing the remaining figure by 30% that's 22.65A, about a third of the carrying capacity of 6mm.

The thing is that the electrician is telling me that my 6 mil cable is not big enough for both appliances, and he wants to run a new cct for one of them.

Total length of run is about 14m, run in the void under the floor except the last 4m in a duct through a chase in a concrete floor. I'm not sure what derating to apply, but surely I've got ample headroom.

The electrician saw the oven I am to use, but to be fair, he had no idea which hob I had chosen, but I can't imagine any ordinary 4 ring induction that would break this power budget.

So, sanity check time. Have I got anything wrong? Any advice? (Apart from the obvious).

[Michael Chare]

My new 4 ring Bosch induction hob actually has a variable current limiting feature. The most current it will draw is 20 amps, and you can have a lower setting. It and a double oven share a cable to a 30 amp fuse.

[Graham.]

Your hob is 17A max for cable sizing purposes.

[Michael Chare]

What makes you think that? Apart from anything else the current limiting feature is a variable. I could set 13 or 16 amps instead.

[Graham.]

That's why I said "max", if you set it to 16A or 13A, the diversity factor has less impact, 14.2 and 12.1A respectivly.

What size cable is it?

[Bill Wright]

I ran separate cables to separate consumer units (we have three) so that if there's a trip on one means of cooking she's still got the other.

Bill

[Tim Watts]

Have you asked him for his diversity calculation?

[Michael Chare]

I am not certain of the size of the cable but I do know the fuse is 30 amps. But how do you arrive at the 17A?

[Graham.]

AFAIK it's 100% of the first 10A of the total max load plus 30% of tbe remaining load. You can have a single socket too if you allow anorher 5A..

[spuorgelgoog]

That's so considerate of you, Bill, thinking of her like that.

Owain

[Dave Plowman (News)]

True. But wouldn't it be better to arrange things so there never is a 'trip'? A trip is an indication of a fault being present. If it's something that happened so regularly that you need three independant feeds, something is very wrong indeed.

[Michael Chare]

So how did you get to the number 17?

[John Rumm]

For hobs with current limiting built in, I would be inclined to take the limit as the load with diversity already applied and not apply any (or at least much) further reduction.

[John Rumm]

So far, so normal ;-)

Indeed. Allow an additional 5A if there is a socket fitted to the cooker point...

For domestic applications, its likely to be fine. Its certainly big enough in the sense that it won't suffer damage while protected by a 32A MCB.

The only question that remains is could you get nuisance tripping of the MCB under full load if you turn on all the hot plates and the ovens at full chat. The reality is the MCB is unlikely to trip at full load for going on for a quarter of an hour, and that seems like ample time for the heating elements to start cycling on their stats.

You could probably count that as installation reference method B, which gives you a 38A max continuous load capacity.

I can't fault your logic. So it really comes down to discussion with the electrician...

Personally I have seen plenty of double oven + 6 - 7kw ish hobs installed on the single circuit, and don't recall tripping being an issue.

[dennis]

I have two single ovens on one feed, they have a maximum of 7.2kW each stated (I can't see how though as the elements aren't that powerful).

However the 32A breaker has never tripped and I don't imagine running the three elements in each cooker at the same time for any length of time unless I am silly enough to put them both into pyrolysis cleaning at the same time.

The hob is on its own circuit as I think that might be pushing it a bit too far. 8-)

[tabbypurr]

reminds me of the entire flat on a 5A rubber feed.

NT

[Dave Plowman (News)]

When I moved in here, there was a 30 amp wire fuse before the CU - which was much newer than most of the wiring. Lighting was all in lead. A few 13 amp sockets all on a couple of radials. Water heating was an immersion. Manage like that for a few weeks - including using fan heaters - until a basic re-wire was finished. The

[Michael Chare]

It is quite interesting. If you just have the hob I would be inclined to agree. However I also have an oven which can draw at least 10 amps. So in the case of my hob which has rings totalling 30 amps the diversity rule would count that as 9 amps (30% of 30). But the current limiter is set to 20 amps. To actually draw 20 amps, 2 rings need to be on max and a 3rd on partially on.

[Bill Wright]

Very wise words Dave. I won't bother with my car safety belt in future. I'll simply arrange things so there never is an accident.

Bill

[Bill Wright]

It's just one facet of my general loveliness.

Bill