Hob & Oven wiring diagram is this ok?

[TonyWarrington]

I intend to use the existing cooker switch to control a new oven and ceramic hob. Due to the existing install of cable being behind a tiled wall and the fact that the switch will not take 2 x 6mm cable I wondered if the following idea would be acceptable (would prefer not to have to remove the tiles etc just to enlarge the space for additional cable):

1. Existing oven connected to Cooker switch via 6mm cable to outlet plate under counter top.

2. Change the cooker outlet plate to one able to accept 2x 4mm cables plus the incoming 6mm. Then run 1 4mm cable direct to the hob and the second 4mm cable to the oven via a 13A fused spur.

Due to the fact that the main power cable is 6mm and the intention is to then split of to 2 x 4mm I was wondering if this acceptable

Rough diagram of proposed install:

 

[jj4091]

depends on the loading of your oven & hob, what are they?

 

[TonyWarrington]

According to the website the oven is rated at 2.6Kw

Hob has 4 rings rated at 1x 2.1Kw, 1x 1.7Kw, 2 x1.2Kw - therefore assume total rating would be 6.2Kw -if all on at the same time.

I have been mulling this over and getting nowhere so far - due I guess to lack of in depth electrical knowledge. So these are my assumptions - which I am sure will need correcting.

1. 6mm cable can support a load of 45amp. My MCB 30amp (converted plug in fuse old Wylex). With everything switched on load would be 8.8Kw - this would require a higher rating MCB
What about applying diversity? Not really much of a clue here (personal view - my logic - shouldn't you allow for worst case scenario?)

2. Oven load on 4mm cable seems ok and protected by 13amp fuse

3. Hob load on 4mm 6.2Kw (all on!). This a little more than I understand cable rating - again what about diversity.

So as you can see getting myself a little confused on best and safe way forward

 

[jj4091]

Given the situation with your C.U. I would suggest the best & safest way forward would be to call a spark in.

 

[TonyWarrington]

Interesting comment about the CU - for the moment wanted to leave it as is (house built in 1984). I did recently speak to a spark about this situation - his comment not to worry too much you need to consider diversity and all should be ok!!!

Have had a look and found that TLC have Wylex Plug-in MCB's
Rated 40 Amps - said to be a direct replacement for the original rewireable fuse.

In this case I would imagine that it should be ok - the current capacity of the cable exceeds that of the MCB.

What do you think?

 

[Adam_151]

I think that its very unlikely that your CU ws designed to take protective devices in excess of 30A...

 

[TonyWarrington]

I think that its very unlikely that your CU ws designed to take protective devices in excess of 30A...

Not sure I understand your comment. Do you mean that the actual design of the CU would make it unsafe to have a protective device over 30A or that the actual physical size of MCB could be an issue?

This is the MCB I mentioned looks the same physical size as existing MCB's:
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=77992&ts=75007
or
http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/Wylex_Standard_Range/index.html

 

[RF Lighting]

Do you mean that the actual design of the CU would make it unsafe to have a protective device over 30A or that the actual physical size of MCB could be an issue?

Both.


The MCB backing peice has a small tab to prevent it being used in an under-rated CU.

If you just plough on regardless and stick a 45A device in a CU rated at 30A per way then expect to find this shortly.

This does happen.


Alot.

 

[TonyWarrington]

Ah - now I understand!!!

I appreciate your time and efforts the pics certainly drove the point home. I certainly had/have no intention of just going ahead and installing 40A MCB without seeking advice first - hence my questions on this forum.

Will check the CU make, model specs etc. As far as I know it is an original Wylex installed when property built in 1984/5 - seems that it probably will not be suitable for upgrade.

So it seems that I have 2 options:
1. Get the CU replaced at an estimated cost of £350 - £450 (this property is beginning to turn into a money pit )

2. Have been told by a spark that my original proposed install would be ok (still have some doubt in my mind about splitting to 2 4mm and whether diversity should be taken into account)

Can/should the original plan to keep existing CU and 30A MCB be used (or modification suggested)?
Thanks
Tony
Edit There may be a third option - just remembered that next to the cooker cable outlet is another outlet that was wired to the elec ignition of the gas hob. Will need to check what circuit this is on and cable size - with my luck it is probably on a lighting circuit. I know it sounds strange just remembering but I am working on the house just at weekends not occupying it.

 

[kevindgas]

why not install a new standalone RCD and MCB for your cooker circuit next to existing board
then make sure the cables are big enough for the loading you intend? its a lot cheaper than a new main board

 

[Paul Barker]

Agree with Kevin, would involve an electrician but cheaper than a whole replacement cu.

 

[TonyWarrington]

Have now checked my CU Wylex Cat No 604/N. Notice on CU Total Load not to exceed 60Amps. Fitted recently MCB's 3 x 5A, 1 x 15A, 2 x 30A (direct replacement for original wire fuses).

Does this still mean that the cooker 30A MCB should not be upped to a 40A MCB
RF Lighting mentioned CU rating per way and the fact that you should not exceed this eg stick a 45A device in a CU rated at 30A - I understand this but in the case of my CU cannot find the max rating per way - just the total load (Sorry if I am seen to be labouring this point - I just want to learn and be safe)

This is a very small property Lounge, Kitchen, 1 Bedroom, and Bathroom - I cannot see how I could get anywhere near the max load quoted for the CU.

Kevin and Paul Thanks for your suggestions. I am trying to avoid this if at all possible - would require major upheaval to tiled walls and kitchen cupboards - but appreciate that I may have to consider this option.

 

[Steve]

not to exceed 60 amps generally means its an older one that will not take devices above 30 amps.

 

[jj4091]

In your second post, you asked what was the safest way to do this job. Why do you continue to question the advice you are being given? Don't be so mean. Do the job properly or not at all.

 

[TonyWarrington]

Crafty
not to exceed 60 amps generally means its an older one that will not take devices above 30 amps.

Thanks will definitely stick with 30amp - just wanted to check in case CU was capable - believe it or not I did suspect that upgrade not possible!! .

jj4091
Why do you continue to question the advice you are being given? Don't be so mean. Do the job properly or not at all.

I guess you must be exasperated with me and my questions Please do not be offended that I had further questions after your reply - not intended to be an insult or questioning your knowledge - I also said I just want to learn and be safe

As to the advice I am being given as far as this topic goes I was trying to get a better understanding of my CU and what is possible or not. This is very clear to me now. Further on the subject of advice in general there does not always seem to be a definitive answer and some of the more expert people will disagree on what is acceptable etc. My own spark suggested that my original plan acceptable (diversity). On this forum can be found arguments against this etc.

You are right I think I am trying to be a bit mean and yes I agree "do the job properly or not at all".

I have also realised that the intention to use a cooker switch would not be acceptable due to definite possibility that a kettle would be used therefore adding even more load. So looks like I will need to replace with just a cooker switch!