Cooker and Hob Query?

[whazza22]

Hi, doing a rewire and the customer is having a cooker with a separate hob in the kitchen.

Do I need to put 6mm cable in for the cooker and continue as a radial for the hob, or would the hob go on the kitchen ring main?

Thanks

 

[Robo]

I would run a 6mm to to the oven and a seperate 6 mm to the hob. Then if you loose one circuit you still have the other. However providing the loading isnt to high you could run 2 x 6mm from the 45a isolator to each item, use a double box isolator tho. easier to work with.

If the hob is gas and just has a 13a plugtop for igniter id conect thing to ring. Make sure switchs are relevant disances away from hob etc.

Robo

 

[chiplard]

Personally i would use one of these;

Click 45a Easyfit Dual Appliance Cooker Outlet Plate

 

[whazza22]

That would be ideal!

Is there a requirement that there must be an isolator for the cooker or can you wire straight from the consumer unit into an outlet plate?

 

[telectrix]

must have isolator for cooker. go with chiplard's sggestion unless oven comes with a plug on it ( some do ). then oven can go on ring

 

[Johno]

You need to know how much power each takes before you can make a decision really.

 

[whazza22]

Just found out from customer that they will be getting a 7900w Hob and a 4900w cooker.

What would be the best way to go about this?

 

[Ponty Massive]

That' 35a for the hob and 22a for the cooker. When you say cooker do you mean oven? That's a lot of juice to be pulling down one circuit!!! If this is the correct power for both units then your gonna need 2 individual circuits from the CU with 2 seperate isolators. Haven't got the BRB to hand but depending on the installation method, may even need 10 T+E for the hob with a 40a mcb.

 

[whazza22]

What about diversity?

Diversity for cooker 10A + 30% of remainder

7.9kw+4.9kw = 12.8kw

12.8kw/230V = 56A

10+(30% of 46A) = 23.8A

So would a 32A with 6mm be sufficient?

 

[ringer]

I've never been entirely comfortable about applying diversity to these new high-powered cookers, but then do not have enough experience to be sure if circuits designed using the diversity factor really are safe in this situation.
I look at it this way, if all current using devices on a circuit were switched on, we would need to design using larger cable sizes. But in reality (typical usage) no-one ever switches everything on in their house at the same time. It is on this basis that diversity is applied. But with a cooker circuit, particularly when preparing large meals, all ovens and hobs ARE likely to be powered up at the same time, drawing full current.

 

[Ponty Massive]

I wouldn't be too comfortable with that. Using diversity to take it down from 56A to 24A, just doesn't sit well. If that's the case just stick it on a 2.5 ring!!

Do it right, do it once!

 

[whazza22]

Ok well what about two separate circuits?

7900w/230v = 34.34A (6mm cable on 40A MCB?)

4900w/230v = 21.30A (6mm cable on 32A MCB?)

Any thoughts on that instead?

 

[malcolmsanford]

Use the diversity as out lined in the OSG guide it's perfectly adequate. Remember your protecting the cable not the appliance

So the

7900 would be 17.5amps + another 5amps if the plate as a socket.

4900 would be 14amps + another 5amps if the plate as a socket

Personally for design I would run a 6mm^ for the oven with a 32amp and a 4mm^ 25amp for the hob. That would give you diversity of circuits as if one appliance trips the other is still on.

Your the designer only you can choose after you discussed it with your client. A point to remember about the diversity and cookers. Yes there could be times ie Christmas where all elements are on, but an MCB or RCBO must be capable of runing at 1.45 times it's Ib for an hour before tripping. So that 25 amp breaker on the hob in scenario one would need to be capable of running at 36.25amps for that hour.

The other train of thought is run a 10mm^ with a 40amp protection device and split the supply at a single cooker point to each appliance. That way you will future proof if someone decides in the future to have a combined unit.

 

[telectrix]

remember that the loads are thermostatically controlled, so even if all are on at the same time, they are not necessarily all drawing current. my rule if thumb is to apply diversity as BRB, and then add 20% for an extra safety factor, so my calc. for this situation would be 24A + 5A = 29A ( to nearest whole no. ) so would use 6mm and 32 or 40A MCB. However, it would be ideal to use 2 6mm on 32A circuits if only to be doubly safe and have the convenience of independent circuits. one point could then incorporate a socket outlet.

 

[fivestar]

Just a thought but 4900w seems a lot for an oven - is this the combined total of oven and grill perhaps? In which case it is not normally possible to have both on at the same time (I do realise it could of course be a double oven - but even so still seems quite high)