Cooker wiring query

cubix

When you have a single 6mm feed at the back of the cooker from the wall switch and previous install has simply pushed the 2.5mm cable for the oven into the connector with the 6mm cable feeding out to the hob.

As new oven comes with a 13amp fused plug what's the best practice?.

Remove existing connector block on 6mm feed from wall switch and replace with 45amp / 13amp socket similar to below or cut off plug and go like for like as previous install. Has been mentioned if below setup is installed then oven will not be on an RCD via the plug socket as oven feed is not on an RCD. But going the route of cutting off the plug top and pushing into the connector hob feed will not be RCD either.

https://cpcireland.farnell.com/volex-accessories/gb6-vx9700/45a-cooker-switch-13a-switch-socket/dp/PL12814

If small household appliances plugged into the sockets have RCD protection why does a standalone feed for your oven not need one.

kirk.

Basically if you leave the plug on and fit a socket behind the oven the socket is inaccessible for other uses so is basically the same as a joint, oven is still a fixed appliance in practical terms so wouldn't need rcd protection

I assume that's allowed one of the recs would be able to confirm

Personally, I wouldn't be a huge fan of connecting an oven via a 13 amp plug. As over-engineered as those plugs are, they have a tendency to run quite warm at maximum sustained load.

Ovens are likely to draw close to 3kW for a prolonged period, so are better off hardwired.

I don't like the practice here of putting ovens and hobs together onto a single 32 or 45amp circuit. It would make a lot more sense to have appropriately rated circuits, but I'm not really in a position where I can debate Irish regs.

A lot of modern oven manufacturers seem to suggest the oven be connected to a 20amp MCB + RCD or RCBO, yet in a lot of Irish installations they're sharing a 32 or even 45 amp circuit with a powerful hob.

The issue I would have is the lack of protection for the 2.5mm cable feeding the hob. If that overloads, nothing is going to trip.

Modern ovens generally do not have issues tripping RCDs either. That's more of an old fashioned issue with ovens that had issues with insulation of elements or old fashioned solid hobs / ring hobs.

kirk.

https://www.boards.ie/discussion/comment/120741844#Comment_120741844

Ya I think UK boards have been all 30 ma without issues on the cooker ?

Other points are all up for debate🙂I used to run out 2 circuits for oven and hob as you say

My own oven and hob in Ireland are on two separate Hager RCBOs without any issue whatever.

Never tripped. I think there's a lot of old nonsense going on about ovens causing RCD trips tbh. It seems to be a non issue anywhere else.

I asked the electrician to use two separate circuits as they're not very close together and also asked him to use RCBOs which just exceed the minimum requirement for protection in the regs. My thinking was it's one of the few appliances where you've water boiling over onto potentially live areas and people get involved with cleaning surfaces and all sorts of things, so I just reckoned why not...

The induction hob maxes out a 32 amp circuit completely on its own and the oven is on its own 20amp circuit.

Each one has its own isolation switch near by and they're on opposite walls of the kitchen, so it's quite practical.

One small comment:

If I were connecting a high powered 3kW sustained load like that, I would probably have a preference to use a fused connection unit, rather than a plug top.

The 13amp fuse in the FCU will protect the flex of the oven, but the connection is also more robust and not reliant on mechanical springs and prongs. In a new socket that shouldn't be much of an issue, but I just have a preference in general for solid connections when there's a load like that that could be drawing at close to the maximum for hours.

You can get very slick and decorative looking FCUs that come as part of the usual designer socket selections from the likes of Legrand, Hager, MK etc and they will blend very well with your kitchen.

You can then make the oven isolatable with the FCU separately.

The bit I don't like about the installations I've seen in Ireland is where you've a junction box installed (often hidden away somewhere) with the oven just spurred off a 32 or 45 amp circuit, without any protection of the flex.

Does that even comply with wiring regulations? It seems a bit daft to me.

I don't think that's a reasonable way of wiring anything, and it seems to me to go against the logic behind overcurrent protection. The whole purpose of the local fuse is to protect the flex of the appliance when connected to a high rated circuit.

13 amp plugs or FCUs are deemed to be adequate overcurrent protection 32 amp rings, so they should be safe enough in that context too.

What I'd expect to see with ovens and hobs is two appropriately sized circuits, each with an appropriately rated MCB/RCBO. That's certainly the way it's done in most of Europe these days.

kirk.

The flexes hooked up to a B32 mcb comply with rules afaik

The thinking is that the appliance is a fixed load and won't overload the flex afaik

Where is the potential for overload on appliance flexes

2011

https://www.boards.ie/discussion/comment/120753464#Comment_120753464

The current carrying capacity of the cable must exceed the rating of the protective device. An MCB rated at 32A can not protect the 2.5 mm sq. cable.

An overload could occur on the flex in the same way that it could occur at anywhere else on the circuit.

2011

@[Deleted User] "If I were connecting a high powered 3kW sustained load like that, I would probably have a preference to use a fused connection unit, rather than a plug top."

Agree, a spur outlet would be preferable to a plug top for a load such as this. I see a number of 3 kW ovens on the market that are supplied with BS 1363 plug tops fitted. I have also seen this cause a few sockets to overheat, and get damaged as result.

kirk.

https://www.boards.ie/discussion/comment/120753820#Comment_120753820

Don't think that's correct in relation to oven and hob leads , could be wrong

Can you explain the risk in relation to overload on the appliance flex?

Maybe I'm missing the obvious there

2011

https://www.boards.ie/discussion/comment/120753954#Comment_120753954

Can you accept that an overload could potentially occur on the cable that the flex is connected to?

^^^ Assuming that the answer is yes, then can you explain why you feel this same fault could not occur on the flex?

kirk.

I can see leakage and short circuit , they're a regular occurrence

Not sure where an overload would come from

kirk.

https://www.boards.ie/discussion/comment/120754165#Comment_120754165

You stated the appliance flex must be overload protected , it's up to you to demonstrate why

Cooking loads on the fixed wiring can change when a cooker is replaced and overload might occur

Where does the overload occur on the appliance flex as the loading is matched to the flex ?

I could be completely wrong

Maybe a rec could confirm the sizing on oven and hob leads

kirk.

I believe I'm correct in saying that 13amp appliance leads are not necessarily overload protected

Plug fusing now is <=700w 3 amp , over 700w 13amp

Afaik a lot of 13anp appliance leads are not rated at 13amp so are not overload protected

I remember this being discussed previously

2011

https://www.boards.ie/discussion/comment/120754166#Comment_120754166

An overload could occur under the following cirumstances:

1) A fault within the oven that causes excess current to flow.

2) Damage to the cable. This could be due to mechanical damage, melting of the insualtion due high ambient temperature (possibly from another applicance).

I'm not saying that the 2.5 mm sq. lead on the oven is undersized, I am saying that a 32A MCB won't protect it.

kirk.

https://www.boards.ie/discussion/comment/120754618#Comment_120754618

Unlikely scenarios, short circuit and leakage much more likely

See above post about 13amp leads .This came up before in discussion

I could well be wrong on the cooking leads maybe it's a minimum 4sq

2011

https://www.boards.ie/discussion/comment/120754657#Comment_120754657

A lightning strike, an electrical fire and electrocution are also unlikely yet we take measures to mitigate the risks of these events.

Yes, some cooker leads are 4 mm sq. but the in the OP's case this cable is 2.5 mm sq. Have a look at post #1.

2011

A few extracts from the rules:

I would argue that Iz for a 2.5 mm sq. cable is less than In (in this case is 32A)

Furthermore IS10101 states that the manufacturer's instructions should be followed. In my experience 3 kW ovens such as the one described specify a protective device rating of less than 32A.

kirk.

The rules on cookers specifically ?

We don't allow the connection of a 2.5mm2 flex directly to a 32 amp ring circuit. There's always a plug top fuse or a fuse in an appliance connection unit (spur switch).

Non fused plug type sockets are normally on 16 amp circuits, so the MCB is adequate on its own. This applies to 'CEEform' sockets used in light industrial situations, and it applied to BS546 and old installations that used Schuko back in the day had 16 amp fuses.

The risk is if you get a fault in the oven that causes an overcurrent that's less than the rating of the MCB, it will simply burn the flex out

BS1363 plugs and sockets in my view are not safe for sustained loads at their maximum rating. They seem very prone to overheating and melting. Despite being quite bulky, the contact surface area is small on the pins and there are small contact areas on the fuse carrier too. If you get any loose springs on the socket they can get pretty warm. Same if the fuse is not exactly in position in the carrier.

They're constantly talked about by British commentators mostly as if they're some kind of ultra superior design. They really aren't. They have a lot of engineering around shuttering and they've universal earthing, but they're not great at high sustained loads and it's been around since the 1940s. If you want to use those you should really be using CEEform (common in commercial kitchens for ovens, toasters etc) or hardwiring it with a spur.

2011

^^^ There you go. A fuse / MCB with a rating of 32 A is permitted when the flexiable cable is 4 mm sq.

As can be seen in post #1 the cable in question is 2.5 mm sq.

So in conclusion the rules would not permit an MCB of this rating in this case.

kirk.

I stand corrected on tha, thought maybe I'd seen something about 4sq, that's a new rule afaik

My point above stands on some appliance cords not having overload protection on 13amp fuses

kirk.

I was doing them that way back in the day 6sq and 2.5

That rule means u have to run 2 circuits if leads have to match mcb

Risteard81

"433.3.1 General

"Devices for protection against overload need not be provided:

[...]

"b) for a conductor that is not likely to carry overload current, provided that this conductor is protected against short-circuit current in accordance with the requirements of Clause 434 and that it has neither branch circuits, nor socket-outlets"

kirk.

https://www.boards.ie/discussion/comment/120756837#Comment_120756837

That's a rule I had somewhere at the back of my mind

Is that rule on 4sq/32 amp new ? I suppose they just wanted to tighten up on leads

They have a habit of introducing unnessary rules in this country

Whereas the UK tends to apply logic and common sense to rules

kirk.

A lot of talk about overloaded appliance cords but no satisfactory explanations on thread

Risteard81

https://www.boards.ie/discussion/comment/120756883#Comment_120756883

It was also listed in the 4th Edition. However Annex 55A in which this appears is Informative, and not Normative, and therefore does not form part of the Rules. It's more akin to guidance.

kirk.

The 4sq rule will take precedence anyhow

I suppose the logic could be that you might have numptys changing appliances and overloading existing leads

With everything domestic being idiotproofed due to handymen/homeowners undertaking work at least the overcurrent protection is assured at install stage with 4sq

Risteard81

https://www.boards.ie/discussion/comment/120756975#Comment_120756975

It's not a Rule. It's only in an Informative Annex to the Rules.

kirk.

https://www.boards.ie/discussion/comment/120757144#Comment_120757144

Sorry missed that

Presumably smaller cooking leads can be connected to 32amp then according to the general rule 433.3.1

Would need to be guided by manufacturers instructions as 2011 said so may not be doable anyhow

kirk.

Don't think I've ever heard of appliance overload faults unless I'm missing something

Leakage and short circuits on cookers unless there's something I'm missing

Bruthal.

https://www.boards.ie/discussion/comment/120753954#Comment_120753954

Yea it's unlikely an overload occurs on fixed appliances. If the oven has 13 amps of elements, it's unlikely it can draw more than 13 amps except in fault conditions, the magnitude of which are not related to the ovens max load. I'd say other appliances with 13a fuses in their plugs possibly have flexes smaller than the 13a rating.

Bruthal.

https://www.boards.ie/discussion/comment/120757203#Comment_120757203

It's just not likely. Cable damage involves a short, in which case the mcb trips, or the point of damage clears itself as strands of the cable burn away. A sustained overload is not probable. Plenty of fixed appliances have internal wiring smaller than supply cables. And while we will hear it said the protective device is for the supply cable, we still don't see internal wiring in cookers etc getting overloaded, even though it's often only sized for a single hob element.

kirk.

Afaik that rule is applicable to cooking

I don't think there's anywhere on domestic fixed wiring that overload protection is omitted

There was unprotected tails and unfused spurs on rings

Still though the advisory is 4sq and then there's manufacturers instructions

A possible issue with connecting appliances direct is someone uses the appliance lead as an extension to a higher wattage cooking appliance later so 4 maybe safer I reckon