connection and cable size type for BU oven

[DJM]

We are having the kitchen done, and were going to fit 2 single ovens side by side and a seperate induction hob.

Hob will use current radial fro CU, but will need new runs for ovens.

I was planning on running 2 new radials from CU to DP switches and then onto the ovens (unless anyone has any better ideas which means less CU connections)

Oven 1 has has a total connected load of 3.8KW
Oven 2 has has a total connected load of 3.8KW

My questions are
1) Will 2.5mm T+E cable be sufficient for each radial or should I go to 4mm?
2) Will 16A fuse be sufficient?
3) Ovens suggest connection via H05RR-F 300 V / 500 V Rubber Flexible Cable or H05VV-F 300V / 500V PVC Flexible Cable. Can I substitute T+E instead?

I know this is notifiable, and i will probably get someone to do it, but would like to plan what flooring, walls etc will be affected and have some idea of work involved.
Thanks

 

[JohnD]

2 separate radials is good. 2.5mm can go up to a 20A MCB (subject to installation method) which is usually enough for a double oven (which will be handy if you decide to get one in future). give them 16A MCBs. Do not put them on the RCD side of a split-load as they will probably get slight earth leakage as they get older.

An alternative would be to run a radial in 6mm at 32A and take two FCUs at 13A to feed outlets for the ovens. Or you could use 20A DP switches above the worktop and SINGLE unswitched 13A socket outlets below, with fused plugs for the ovens. The sockets should be accessible so you you can unplug the ovens easily when they have to be pulled out for repair or replacement. You must not use a double socket for two heavy loads as it will overheat and be damaged. edited: i see your ovens use more than 13A so this would not be suitable.

If you have a large kitchen and like cooking, then maybe one day you will want an electric range cooker (although gas hobs are better) so make sure you have at least one of your cooker circuits able to take a 32A load, and run in 6mm.

 

[davy_owen_88]

2.5mm² on a 16A MCB is fine.

If you can ensure the t&e will not come into contact with any hot surfaces then yes you can use it, but for easy of removal it might be worth using flex.

 

[DJM]

Thanks for the replies.
The original idea was a range cooker, but want an induction hob as more efficient than gas and safer. But can't find one at 900 or 1000 to fit so have gone for teh 900 induction hob and 2 single ovens.

We already have a 10mm radial which runs our current cooker, so I was going to use that for the induction hob. The ovens would then be fed by the 2 new radials.

CU is not the most modern and has cartridge fuses, so no MCB and certainly no RCD.

Why I was concerned about 2.5 T+E is that the run will be 10 - 5 metres and there will be some bunching near the CU for at least a couple of metres, and it will have to travel uner a concrete floor for another couple of metres so thought 4mm might be more appropriate.

As for the flex issue, I just wondered why flex and not T+E? I presume running T+E from CU to DP and flex from DP to oven is fine?

 

[davy_owen_88]

Is there enough space in the new CU for the 2 new circuits? You might want to start saving up for a new CU with RCD protection, and also a PIR might be a good idea in the future if the wiring is particularly old.

Each oven will draw about 16A and 2.5mm² t&e is good for 27A when clipped direct/buried in plaster etc. When you say the cables will be bunched together what do you mean? Inside conduit/trunking? If this is the case 2.5mm² may not be suitable unless you can split the cables into smaller groups.

Run t&e from CU to the CP isolator and then flex to the oven - the flex just makes it easier to install/remove the oven, and there is less chance of the cable getting caught between the oven and the wall and being subjected to high temperatures.

 

[DJM]

Thanks for teh reply
I would love to have the system re-wired and sorted out, but the previous owners have had some re-wiring done and upgraded by adding additional CUs. One of these has 2 free spaces so could use these for the ovens. So not ideal, but not dangerous either.

Due to the mix of concrete and suspended flooring, the cabling mostly goes up and through ceiling. For the Kitchen, breakfast room (inc current 10mm cooker cable) runs parralell out of CU cupboard then into a boxed in section down the wall then under suspended floor before separating to specific rooms. The 10mm cooker then runs under concrete floor for a couple of metres and through wall to cooker point. The two new oven radials would have to do something similar. Thats what I meant by bunching, and cabling can't be split into smaller groups.

 

[FatGit]

Why not run the radials in 6mm2 in case someone in the future decided they want a more powerfull over/cooker etc?

 

[davy_owen_88]

Who's going to need 1x 10mm² radial and 2x 6mm² radials in a domestic kitchen? That's a combined capacity of 158A

 

[bernardgreen]

Yes.......a lot of amps

but the voltage drop and therefore the power wasted along the cables is a lot less with 10mm² than with 2.5mm²

 

[davy_owen_88]

Indeed, but it's still overkill. A single 6mm² cable will accommodate almost any cooker on the market after diversity. Since he already has a radial for a hob he only needs to accommodate single ovens, for which the VD on 2.5mm² is well within limits.

I suppose a better solution is one that John gave earlier, which was run a second 10mm² cable and fuse that down for the ovens. That way he gets the best of both worlds plus it's cheaper in labour and materials that 2x 6mm² radials.

 

[DJM]

I'd prefer one run, but as each oven has a total connected load of 3.8kW, that would put me over the limit for FCU at 13A.

I suppose I could go up a cable size 4mm or 6mm and use cooker points with sockets and get some use out of the cabling.

Or would it be acceptable to spur off via FCU for a built in microwave thereby removing that load from teh kitchen ring?

 

[JohnD]

Can you check that loading? Domestic Single ovens sold in tbe UK usually take 10 to 12 Amps. They have an oven element and a grill element, but the switching prevents you turning on both at the same time.

 

[DJM]

One of the ovens will be a Neff B1644 which from the website simply says

Total connected load 3.8kW.
Connection have to be made using a mains connection cable not lighter than H05RR-F 300 V / 500 V Rubber Flexible Cable or H05VV-F 300V / 500V PVC Flexible Cable.

 

[DJM]

Looking at it again, the oven is multifunction and allows various combinations of above and below heat, so it may be the total load of 3.8kW can be achieved in one mode or another, but not in most. So I presume that in that case the use of 13A FCU is not an option.

Is the cable size increas to use a cooker point with attached socket or FCU spur for a microwave in my previous post a runner?