UK Manufacturers instructions

Hi - fwiw I’d have used the DP switched 13A fused spur too … can you advise the microwave make and model? Nowadays, I think we are required to consider manufacturer’s instructions but not to follow them blindly.

Planctonboy said:

The never ending debate about supplies for cooking appliances. ???
I've just received an email from a customer that I'm in the process of quoting for electrical mods is part of a kitchen re-fit.
When I first talked to them they were talking about having an induction hob rated at about 7.5KW and a single oven rated at 3.5KW. As a rule of thumb I usually say its OK on one circuit when diversity is applied as described in the OSG - and I usually make sure there is no socket outlet on the cooker control. If its a double oven I would recommend an additional circuit be installed. In recent years I've noticed an increasing trend where the manufacturer fits a supply cord to the oven (most induction hobs come with one too). Problem is that the oven supply cord is usually a bit of 1.5mm 3 core flex, and I cant really connect this directly to a 32A supply without first fusing down to protect the cord (not to mention the internal oven wiring). So I usually need to install an additional bit of circuit protection somewhere to protect the oven. I know that every electrician has a slightly different approach to this issue. And I've seen some very creative (dangerous) installations too.

Anyway, I have a question which has just arisen. The customer also wants a microwave-combi oven to be installed. They sent me the details of the load requirements of the appliance - 2.1KW. All sounds fine so far. I'm thinking well that can probably just go on a 13A Switched fused spur. However the manufacturers instruction calls for circuit protection to be 16A? So now I'm looking at a new 16A radial just for the microwave. Does anyone know why they would ask for an appliance that is pulling less than 10A to be on a 16A supply? Maybe it's a European thing where the wire using radial circuits only. But if I deviate from the manufacturers instructions that's a non compliance according to BS7671.

Any ideas ?

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Probably because BS 7671 advises that any load of 2KW and above needs to be on a dedicate circuit

Wilko said:

Hi - fwiw I’d have used the DP switched 13A fused spur too … can you advise the microwave make and model? Nowadays, I think we are required to consider manufacturer’s instructions but not to follow them blindly.

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Hi Wilko
Its a Zanussi ZVENW6X1.

Wilko said:

Hi - fwiw I’d have used the DP switched 13A fused spur too … can you advise the microwave make and model? Nowadays, I think we are required to consider manufacturer’s instructions but not to follow them blindly.

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I

Pete999 said:

Probably because BS 7671 advises that any load of 2KW and above needs to be on a dedicate circuit

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Yep I did consider that too. However there is a bit of a grey area there. I've always wondered why its OK to have a plug in 3KW heater which will likely be used for much longer periods than it is to hard wire a 2KW appliance onto a spur. I guess we're back to assessing load and diversity again.

My understanding is that the appliance cord is protected from overload by the appliance being a fixed load

Mikegh said:

My understanding is that the appliance cord is protected from overload by the appliance being a fixed load

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It's arguably not a fixed load but the manufacturer has assessed the loading of the cable and sized to suit.
So it follows that the same justification can be used to omit overcurrent protection for the flex.

ferg said:

It's arguably not a fixed load but the manufacturer has assessed the loading of the cable and sized to suit.
So it follows that the same justification can be used to omit overcurrent protection for the flex.

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I've heard that said before, but I don't get the logic. Why don't we then just connect appliances to ring circuits by means of an unfused plug ?? Or wire directly onto a cord outlet plate ?
Manufacturers have specified the cord to match the appliance rating and that's fine under normal conditions. But what happens when a fault occurs? will a 32A MCB trip in time to ensure no damage to the appliance cord ? Depends on the size of the fault current. What if the oven started drawing 20A for some protracted period due to a fault ??
I'm not saying you're wrong, but it depends on your take on it.

Planctonboy said:

I've heard that said before, but I don't get the logic. Why don't we then just connect appliances to ring circuits by means of an unfused plug ?? Or wire directly onto a cord outlet plate ?
Manufacturers have specified the cord to match the appliance rating and that's fine under normal conditions. But what happens when a fault occurs? will a 32A MCB trip in time to ensure no damage to the appliance cord ? Depends on the size of the fault current. What if the oven started drawing 20A for some protracted period due to a fault ??
I'm not saying you're wrong, but it depends on your take on it.

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It's exactly the same argument for a fixed load, ultimately it's your call to make and if you are not comfortable with something then you shouldn't do it.

As for the 2.1KW microwave Personally I see no issue with putting it on a fused spur, that would be my preference.

Manufacturers instructions should be taken in to account, so if in your judgement a 13a fuse is appropriate then that is how you should do it. Incidentally 2.1Kw is nowhere near the maximum rating of the accessory and as 13A < than the 16A circuit, it is arguably safer.
So personally I can't see a good argument against it.

There are an awful lot of appliances at or around that rating with 13a plugs attached to them by the manufacturer.

ferg said:

It's exactly the same argument for a fixed load, ultimately it's your call to make and if you are not comfortable with something then you shouldn't do it.

As for the 2.1KW microwave Personally I see no issue with putting it on a fused spur, that would be my preference.

Manufacturers instructions should be taken in to account, so if in your judgement a 13a fuse is appropriate then that is how you should do it. Incidentally 2.1Kw is nowhere near the maximum rating of the accessory and as 13A < than the 16A circuit, it is arguably safer.
So personally I can't see a good argument against it.

There are an awful lot of appliances at or around that rating with 13a plugs attached to them by the manufacturer.

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My thoughts exactly. I know what you mean about doing what you are comfortable with. That's why there are a variety of approaches by different electricians. I guess I'm very safety conscious when it comes to designing circuit protection so I sometimes do stuff that others might consider overkill.

In my view a 13A spur would be OK for this appliance. The only thing that makes me hesitate is when something goes wrong with the appliance (completely unrelated to the method of connection) and the engineer comes out and says it's not been installed properly therefor the warranty is void. You know how they like to wriggle out of paying anything!! Then the customer points the finger at the installer - me.

Many worthwhile points raised here.

. I've always wondered why its OK to have a plug in 3KW heater which will likely be used for much longer periods than it is to hard wire a 2KW appliance onto a spur

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A general-purpose socket-outlet circuit exists to supply portable appliances as and where the occupants require. If it's a 32A circuit, there's 7.4kW available in that area anyone to do what they like with, including running 3kW heaters but those are the exception, most appliances being small loads. The 2kW installed appliance gets its own circuit so that it and its stablemates don't use up a large chunk of that 32A general socket allocation before even the first portable device gets connected.

IMO in this situation 'dedicated circuit' doesn't mean its own circuit with nothing else on, just one that is installed with CCC reserved for that appliance rather than a general purpose circuit with no control over how the available CCC is going to be used.

What if the oven started drawing 20A for some protracted period due to a fault ??

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An oven rated to use 10A that starts drawing 20A will soon trip or blow its internal thermal protection, or if that fails to act it will self destruct, possibly by catching fire. In this situation protecting the flex against overload seems to be the least of the issues. Most appliances won't endure a significant overload for long without something destructive happening because all the overload normally turns to heat somewhere, often at the location of the defect.

Does anyone know why they would ask for an appliance that is pulling less than 10A to be on a 16A supply? Maybe it's a European thing where the wire using radial circuits only.

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We had a long discussion about a similar thing recently and my take was that it was a failure to alter the instructions, when translating into English, to account for the UK wiring practice. They had simply translated the German instructions which assumed all socket outlets to be 16A. Here, it would likely be meaning: "People who have 10A sockets e.g. Switzerland and Italy, don't connect it to one of those even if it looks tempting, use a 16A circuit instead.' without so much as acknowledging the existence of a 13A plug or fuse.

This is the main reasoning behind BS7671's instruction to consider, not blindly follow, the MI's If they obviously conflict or don't apply, but some other clearly correct solution does, then use that. E.g. if your USA appliance manual refers to a 'green #14 ground wire' you are not required to go out and purchase green 14AWG cable to connect it up, but instead use green/yellow 2.5mm² as a more appropriate solution in the UK.

Why don't we then just connect appliances to ring circuits by means of an unfused plug ??

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Fused plugs are usually needed to provide fault protection, even if not overload protection. This is a result of our use of highly-rated (32A) socket-outlet circuits even for small appliances, which was conceived with the fused plug in mind. Technically, it would probably be safe to connect the oven via an 'unfused 13A plug' but only because it has a 1.5mm² flex that will clear the 32A circuit OCPD. But if 'unfused 13A plugs' were made, people would fit them indiscriminately, so all other appliances - hairdryers, alarm clocks, table lamps, DVD players included - would equally need 1.5mm² flex in order to be protected by the 32A device. Instead, 0.75mm² will clear the 16A protection commonly used in Europe and that is the smallest flex now normally fitted.

Secondly not all loads are fixed, in particular extension leads with 2 or more sockets can be used to connect two full-size fixed loads to one socket. So again an inappropriately fitted unfused plug could be loaded right up to 32A and the 1.5mm² cable with it, by someone plugging three tea urns into a 4-way strip.

All these potential pitfalls are overcome by enforcing the use of a 13A (max). fuse in every accessory to BS1363. Historically there were unfused 2-way adaptors but AFAIK the P&S(S)R proscribed them a while back.

Hi - looking at their spec, it seems the unit may draw about 9A and so a 13A fuse would seem a safe alternative to their recommended 16A.

Thanks Lucien. You've given me loads of useful information to consider when connecting these types of appliances. Sounds like you've been down all the same thought processes as me but you've got a few more answers that I've never been able to get.

Lucien Nunes said:

Technically, it would probably be safe to connect the oven via an 'unfused 13A plug' but only because it has a 1.5mm² flex that will clear the 32A circuit OCPD.

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In most cases it would, but not all, and the thought of that makes me very uneasy.

Strictly speaking a generic 32A B/C MCB only meets the adiabatic for 1.5mm for PFC of 3kA and below (OSG Table B7), and it is vitally important that the fault Zs is low enough to hit the 'instant' magnetic trip as well (so 1.1 ohm or so) otherwise cable roasting follows (e.g. could be around a minute at over 96A for 32A B-curve).

But as you say the real danger would be adding more items to such a cable.

Sure, I wasn't saying 1.5 on a 32 is a good thing, more setting up a comparison for small appliances