201 forum posts
Sorry not flying, but respect the body of knowledge out there.
I have just moved into a new house with a kitchen containing two single ovens, rated at 3.46 KWs in addition to the normal kitchen appliances, eg fridge, gas hob, dishwasher, toaster, kettle, microwave etc. Washing machine and dryer in utility room.
Hopefully a simple question. We were told two ovens would be individually wired back to the distribution board with separate breakers to avoid a potential overload on the ring which has 32 amp breaker.
However, we have discovered both ovens wired to single kitchen ring main. The electrician says it meets the regs, His trade association said its bad practice and only ovens up to 2kw can be put on a ring main, but not interested in offering anything further.
Can anybody offer advice and perhaps be able to share any info from BS 7671. Its expensive to buy and I probably couldn't find the right section.
|Denis Watkins||21/02/2017 19:46:30|
|4310 forum posts|
Is the house new just to you, Neil
Or New Built?
|Mike Etheridge 1||21/02/2017 20:02:22|
|1543 forum posts|
I was an electrical engineer rather than an electrician. My last housing design job was the re-building of a block of flats in Upper Norwood. that burnt down on Christmas day 2007. Certainly I would have included for two dedicated circuits for the cookers. Other dedicated circuits would include a fridge freezer point (s) in kitchens. There are other specific requirements for the current edition of the IEE regulations which may have been supplemented since I gave up work 6 years ago. However a 'Part P' qualified electrician should have all the answers and someone normally responds to these questions.
|Paul Marsh||21/02/2017 20:03:53|
3939 forum posts
Ideally the total load of any one appliance must not exceed 13amps, which is the rating of a single appliance. So a 3Kw kettle will draw 13amps at UK voltages. If using 4Kw, then you will be overloading that socket, unless as you indicated, you have 2x 2kw ovens, which is fine, but normally ovens are wired into their own MCB/RCBO not on the ring main.
Added to this, the supply wires must be a high capacity. (and the MCB a 40A one)
Edited By Paul Marsh on 21/02/2017 20:06:15
|Tim Tolhurst||21/02/2017 20:38:11|
|11 forum posts|
Are they 3.46kW per oven or in total? Either way, you can't technically overload a (compliant) ring on a 32A mcb with 30A, but it doesn't leave much headroom for anything else. If 3.46kW is the total then its probably ok, same as a kettle and toaster going in the morning, but if there are 2 at 3.46kW then thats really bad practice, though not dangerous. An electrician will also allow for 'diversity' when installing appliances - roughly between half and three quarters of the total wattage (I haven't done the calculation, sorry) but it essentially derates the appliance load.
|Chris Walby||21/02/2017 21:15:45|
1188 forum posts
I'll have a look in the regs book tomorrow and see if there is any specific guidance, but the traditional method was to have a separate supply to a cooker point (interestingly diversity on the total load could be applied as you had four rings and an oven all pulling current, but not at the same time!).
With the advent of fan ovens which have lower loadings and hobs that are separate from ovens the dedicated cooker point is a bit obsolete, but should be there should someone in future install a combined unit (good practice, but not absolute requirement)
IMHO as long as each piece of equipment does not exceed 13A it can be connected to individual sockets/fuse spurs, the only limit was the area covered by the circuit.
There are other considerations which you mention e.g. what if the tumble dryer is on, washing machine, ovens, dishwasher, toaster/kettle and then the fridge cuts in? It would be reasonable to say that the total expected load would exceed the protective device (fuse or circuit breaker in the consumer unit) causing a nuisance trip.
The tack I would take is it "fit for purpose" for normal expected use (think of all the things you have on during Christmas day? but not everything that could possibly plugged in and switched on) and if it adds up to more than 32A you have grounds for a complaint.
|Dane Crosby||21/02/2017 21:37:47|
245 forum posts
Before I retired as an electrician, The (new |)IEE 17th edition allows diversity for a domestic cooker that has it's own cable and CB/RCBO. I don't know what has changed since then.
We would work out the total Wattage of the beast. Then the total current assuming 230v. The diversity calculation is:
Allow for the first full 10A
Take 30% of the remaining current
Add 5A if there is a socket attached to the DP switch.
Add all together and that figure will then dictate cable cross section area and CB/RCBO rating.
There are a few more mathematical tweeks to apply of course such as how the cable is routed through walls, insulation and suchlike plus cable length versus voltage drop etc etc.
I always thought that the calculation provided a rather low current requirement so always recommended an increase in cable size if the calculated figure was anywhere near approaching the carrying capacity of the cable present.
With those fitted hobs and ovens I generally gave them their own supply from their own CB/RCBO.
I hope this helps.
|Paul Marsh||21/02/2017 21:47:03|
3939 forum posts
Also, the electric cooker must have a local isolation, if you use the switch on a socket with a full load, it will arc, especially if you overload it.
Overloading causes fires, and can invalidate your house insurance.
947 forum posts
I'm not a professional electrician but I have always been led to believe that running a cooker off a ring main is bad practice. Every installation I've been involved with has the cooker fed by its own cable and MCB in the consumer unit.
|Geoff Jackson||21/02/2017 22:21:40|
|160 forum posts|
As a plant electrician in thepastand having a17th Ed leccie working for me till I retired last year, cookers should be on a seperate circuit and MCB/fuse, not on a ring main and in heavy duty twin and earth cable. Putting em on a ring is a lazy way and could invalidate home insurance IMO.
721 forum posts
Are the to ovens 3.46Kw each or is that together ? if 3.46Kw each then they must have there own dedicated circuit as you cannot have a any appliance more than 3Kw on a ring(any cooker oven,,hobs with a rated power of 2kw or more must have own detecated circuit ,see 17th editon appendix 15). if less than 2kw each then you can plug them in or have a 13a fuse spure for each oven but the total usage of the ring circuit needs to be checked along with the layout so the load /s are balanced equally distance on the ring(so one side of the circuit dosent have all the load).
as for the overcurrent reg if you whant a good read see chapter 43 of bs7671
|Phil Green||21/02/2017 22:39:29|
1574 forum posts
Maybe I misunderstood but I thought what Neil meant was that both cookers were wired to one common but dedicated ring, ie not the sockets ring. Sockets on one ring, two cookers on another. But maybe I misread it.
|Mike Etheridge 1||21/02/2017 22:54:57|
|1543 forum posts|
This was extracted from the contract schematic drawing I developed for the replacement block of flats at Upper Norwood. It was meant to comply with the 17th Edition of the IEE Regulations at the time in 2008 but I think there have been recent changes ?. The extract shows a typical wiring schematic for a flat including dedicated circuits we required.but only one cooker circuit. Also shown is a typical conduit and outlet box arrangement. I hope the drawings are big enough?
Edited By Mike Etheridge 1 on 21/02/2017 23:04:39
Edited By Mike Etheridge 1 on 21/02/2017 23:06:40
721 forum posts
If this the case, a dedicated ring for the two ovens would work and not essentially be electrically unsafe but it does not meet the wording of a ring ciruit in the regs in appex 15. To meat the regs in this case itshould be easy to split the ring circuit and make two dedicated radials feeding each oven with 16Amp MCB's feeding each circuit at the consumer unit. (it is how i would have done it in the first place)
|Rich too||22/02/2017 06:28:50|
3059 forum posts
We replaced our oven and hob (hob previously gas), and. I would say very few are rated low enough to use the ring main. I ended up finding a piece of kit that allowed the oven and hob to share the same dedicated supply wire - effectively splitting the supply.
201 forum posts
Many thanks for all of your advice which will certainly help me in my ongoing discussions with the builder. In particular I hadn't thought about the potential insurance angle so will again check.
The slightly bizarre aspect of this situation, is that the Building Standards Department of the council have accepted the electrician's 'sign off' certificate, albeit they are not electrically qualified, as part of the documentation for issuing our Completion Certificate. So I suppose if anything did go wrong and there was an insurance issue I can clearly prove the council signed it off. A simplistic view I know but it does highlight the catch 22 situation you can end up in!
For clarity, the load of the ovens is rated at 3.46 kw each and individually connected to the same ring main via a 13 amp spur, which I also believe is unacceptable for the reasons some of you have highlighted, so that at least will have to be changed. There is only one ring which everything I mentioned is or could be connected to, no separate ring for ovens.
|Keith Simmons||22/02/2017 11:05:30|
|451 forum posts|
That's bad. I had moved to my present home over 10 years ago and the previous owners had the house rewired a few years before I moved in.
I have had problems for years with circuit breakers tripping and had then found out (according to the electrician below) that the tradesman had done the rewiring in the house and got the electrician to connect it all to the fuse box.
The rewinding was substandard and I had to pay the different electrician to check and replace some of the wiring.
I hope you will be able to sort it out with the cooker circuit.
Edited By Keith Simmons on 22/02/2017 11:06:32
|Mike Etheridge 1||22/02/2017 12:48:40|
|1543 forum posts|
In future ensure all electrical installation work is carried out by an NIC ECI or ECA approved contractor with part P qualified electricians. There is / was an arrangement with the ECA to guarantee any electrical work carried out by their members. If any works proved to be unsatisfactory then the ECA would arrange to have the installation rewired as necessary at no cost to the client.
I am amazed to think that a substandard installation has been signed off by a local authority by someone without the necessary qualifications. That said I worked directly for two local authorities for about 35 years but eventually both authorities dispensed with all engineers,architects,surveyors and I got made redundant twice. Other than by out sourcing work to consultants,I wonder now who is employed in Building Standards Offices country wide to sign off or approve electrical works ?
|117 forum posts|
Building Control accept the electricians certificate at face value, as the electrician is supposedly qualified and as a competent person is able to self certify.
The 2 loads of 3.46kW should not be on a ring main.
I would refer the matter to the NICEIC as they are the governing body. Invite them to do an inspection.
|Nigel R||22/02/2017 14:51:40|
3710 forum posts
Not exactly on topic, but...
For a short time (and not that long ago) I owned a house which had precisely one fuse for the entire electrical supply. In a bakelite box.
I guess that was considered to be OK in the 60s when it was built, long before sparkies had to be properly trained or anything.
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