Cooker becomes Oven and Microwave....any ideas?

[timhoward]

I'm interested what people's gut reaction solution would be.

Kitchen design and customer said freestanding cooker. So I first fix 10mm, and boxes for cooker switch with socket, and cooker connection plate.
No one thought to tell me it became hob (elsewhere, that bit isn't a problem), and a vertical cupboard which will contain an Ikea Microwave combi thing (<13A which came with plug) and an Ikea oven (3.490 Kw so 15A max which came with bare wires). Instructions for oven do not specify an overcurrent device..

One has fan, one has motor, I can't really claim they are fixed resistive loads, chop plugs off and connect them!
I also don't really want to be even thinking about putting a socket on 10 sq mm cable.

Any creative ideas? (It would be really handy to know if the fan was fused internally in the oven....)
I can only think of tiny consumer unit at the moment. Any help welcome!

 

[Electro-tech]

This situation within kitchens I think has never been successfully discussed with a sound solution., leaving the installation of a dual cooker plate & everything connected to it regardless of individual appliance circuit protection.

 

[timhoward]

This is a joke, don't take me seriously anyone, but an MK grid plate with a 13A and 5A fuse in parallel briefly came to mind!

 

[Electro-tech]

Okay, fed from a dual cooker plate, isolated from Cooker Main switch? I take it this will be behind where the free-standing cooker was to be?

 

[timhoward]

Okay, fed from a dual cooker plate, isolated from Cooker Main switch? I take it this will be behind where the free-standing cooker was to be?

Cooker isolator faceplate is in a corner cupboard / walk in pantry.
Cooker plate (currently single but can swap) is behind an adjacent fitted cupboard unit which is where microwave and oven will slide in.
Plenty of access to plate luckily.

(I wasn't serious about paralleling fuses)

 

[UNG]

This is a joke, don't take me seriously anyone, but an MK grid plate with a 13A and 5A fuse in parallel briefly came to mind!

Why not it is a solution to overcome a problem

 

[Electro-tech]

Connect the oven to the dual plate & MK fuse for the microwave socket outlet. Parallel fuses is better, seriously.

 

[UNG]

(I wasn't serious about paralleling fuses)

What is wrong with paralleling fuses and how is it different to paralleling MCB's when we install a CU

 

[Pretty Mouth]

I'd put a single socket next to the cooker outlet, spurred from the terminals of the outlet in 2.5mm, for the microwave. The oven I would wire directly into the outlet.

I see your concern about overcurrent protection for the fan part of the oven, should it stall, but I can't see a 16A MCB providing that protection, bearing in mind most domestic extractors say either max 3A fuse or max 6A MCB.

I would assume this protection is provided by the oven itself.

 

[timhoward]

What is wrong with paralleling fuses and how is it different to paralleling MCB's when we install a CU

Connect the oven to the dual plate & MK fuse for the microwave socket outlet. Parallel fuses is better, seriously.

I don't think either of you realised I fleetingly entertained joining both ends of both fuses together to construct a 20 ish amp fuse...
(We don't connect a load to more than one MCB at once.)

Anyway, the microwave isn't really a problem, I could put a short length of 6mm from a dual plate into a single socket or FCU.
It's more the oven bothering me as an appliance that generally draws 15a max is on a 32 or 40amp breaker.

I'd put a single socket next to the cooker outlet, spurred from the terminals of the outlet in 2.5mm, for the microwave. The oven I would wire directly into the outlet.

I see your concern about overcurrent protection for the fan part of the oven, should it stall, but I can't see a 16A MCB providing that protection, bearing in mind most domestic extractors say either max 3A fuse or max 6A MCB.

I would assume this protection is provided by the oven itself.

Thanks. It's a good point that even fusing it down won't actually help the perceived problem.

 

[LastManOnline]

I see your concern about overcurrent protection for the fan part of the oven,

I think this is the area where we as electricians can make life un-necessary complicated for ourselves. Our role has always been (and still is) to ensure the wiring we install is adequately protected. It is the appliance manufacturers responsibility to ensure his product is designed to be safely connected to the electrical installation. Or he must state in the manufacturers instructions what extra steps need to be taken.

 

[Electro-tech]

I don't think either of you realised I fleetingly entertained joining both ends of both fuses together to construct a 20 ish amp fuse...
(We don't connect a load to more than one MCB at once.)

Anyway, the microwave isn't really a problem, I could put a short length of 6mm from a dual plate into a single socket or FCU.
It's more the oven bothering me as an appliance that generally draws 15a max is on a 32 or 40amp breaker.


Thanks. It's a good point that even fusing it down won't actually help the perceived problem.

Now I have got it, thinking the microwave on a 5A & thinking you were to chance the oven on 13 A. Silly Me !

 

[timhoward]

@LastManOnline - point taken. I do always feel some responsibility when connecting anything hard wired though!

As I said before the oven manual didn't actually say anything other than max power. I've also just checked online and it adds:

I'd therefore have to assume that as they only specify minimum 16A fuse, and no maximum fuse, they have taken care of things inside the box. I'm a lot happier now knowing I haven't failed to comply with the instructions.

Thanks a lot everyone.

 

[Lucien Nunes]

Even a 3A fuse won't protect a fan motor from catching fire, so there is no relationship between the external OCPD for the oven and its current-using components. The manufacturer really ought to state what protection is required, which is quite likely determined by the need to protect the internal wiring against short-circuit.

The reason two different fuses can't realistically be connected in parallel is that the current won't share in proportion to the rated In of the fuses, or put another way, they don't blow with the same voltage drop across them. The higher-rated fuse has a larger cross-section per unit In so will hog the current and blow first.

A separate deal is whether two identical fuses can be paralleled. Two completely identical fuselinks (not just the same rating but the same internal construction) will track well and can provide effective protection. Indeed there are commercial fuses that consist of multiple elements within one body or even multiple bodies spot-welded together. Historically, ready-made paralelled wire elements were available, I have some in use in an installation.

The problem with two separate fuseholders with identical fuses is that the contact resistance could upset the current-sharing and any significant discrepancy will result in a significantly lowered In because the fusing is related to the square of the current which will increase disproportionately in the fuse with the lower contact resistance.

Going back to the oven, it's not 100% clear to me whether the hob is still running from the same circuit. If not, it might well work on a B20 or certainly a B25, in which case the oven-maker's requirements might be met if one can find out what they are?

 

[APE37]

The way I see it, is there is a oven, hob and combi oven/ microwave. I may be wrong here, but if you change the outlet plate to a dual plate that will deal with the oven and hob, I am sure I have read in the regs somewhere you can reduce the conductor size, as long as less than 3 meters? And the earth wire meets size for the duration of fault/ disconnection time 0.4s and zs, and cables meet current carrying capacity. So as you have 10mm that on 32a that can carry the loads, the oven and hob may have cables pre wired, so as long as they are sized for the load(s) and earth is correctly sized, they will be ok. Can you spur off with for example 2.5 for a socket/ switch fused spur for a combi oven/ microwave? As long as cables meets less than 3 meters and cabke calcs. May need to up the size if not? Just a thought and may completely have the wrong end of the stick.

 

[timhoward]

Going back to the oven, it's not 100% clear to me whether the hob is still running from the same circuit. If not, it might well work on a B20 or certainly a B25, in which case the oven-maker's requirements might be met if one can find out what they are?

Yes, the hob is still upstream on the same circuit so as you say I don't want to reduce the OCPD.
Thanks for the other info - I was never serious about the fuses thing - but good to know some more theory.

Even a 3A fuse won't protect a fan motor from catching fire, so there is no relationship between the external OCPD for the oven and its current-using components. The manufacturer really ought to state what protection is required, which is quite likely determined by the need to protect the internal wiring against short-circuit.

The instructions are a mixture of a couple of helpful sentences on the website and a downloaded manual that appears to contain a copy/paste set of rules.

The website says:

"No plug is included. Installation to be carried out by a qualified installer."
"Minimum fuse = 16 amps"

Fine so far. The complete downloaded guide says:

-Make sure not to cause damage to the mains plug and to the mains cable
• The appliance must be earthed.
• Make sure that the parameters on the rating plate are compatible with the electrical ratings of the mains power supply.
• Always use a correctly installed shockproof socket.
-Connect the mains plug to the mains socket only at the end of the installation. Make sure that there is access to the mains plug after the installation.
• If the mains socket is loose, do not connect the mains plug.
• Do not pull the mains cable to disconnect the appliance. Always pull the mains plug.
• Use only correct isolation devices: line protecting cut-outs, fuses (screw type fuses removed from the holder), earth leakage trips and contactors.
• The electrical installation must have an isolation device which lets you disconnect the appliance from the mains at all poles. The isolation device must have a contact opening width of minimum 3 mm.

It looks like to fully follow the letter of the instructions a 16A socket would have to be fitted. I also believe a lot of these points are copied and pasted from other products. But no mention of a required maximum fuse or overcurrent device.

The circuit is RCD protected (the house happens to be TT). I know that thousands of these must be fitted without this degree of thought but my main concern is (still) a L-N short circuit as I don't trust the internal wiring to withstand the fault current allowed by a 32A breaker.

Ultimately the choice seems to be
a) dual plate, at least 2.5 sq mm T+E for single socket for microwave (433.2.2 allows this as @APE37 mentioned), connect oven in as no manufacturer maximum fuse is specified
b) Stick BG garage board in cupboard and run a 20A circuit for the oven and 16A for the microwave.

 

[timhoward]

The way I see it, is there is a oven, hob and combi oven/ microwave. I may be wrong here, but if you change the outlet plate to a dual plate that will deal with the oven and hob, I am sure I have read in the regs somewhere you can reduce the conductor size, as long as less than 3 meters?

Hob is half way along circuit.
Oven and Microwave at end.
Yes regarding that reg, the OCPD can be downstream in these circumstances, see reply above.
Thanks.

 

[Pretty Mouth]

my main concern is (still) a L-N short circuit as I don't trust the internal wiring to withstand the fault current allowed by a 32A breaker.

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

 

[timhoward]

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

Rightly or wrongly I'm more thinking about the 160A that flows before the instantaneous trip and whether that would start a fire.
As you know, I like to over-think and worry!

 

[Lucien Nunes]

Would this be a problem though? If you've got a L-N short somewhere in the oven, it's dead anyway.

You wouldn't want the short-circuit current to blow an internal CPC or its connector open-circuit and leave the faulty part live.