Period Electrics

[Flyfisher]

Not really a period property topic, although it could help avoid disrupting existing fabric when running a new cable and I know there are some qualified electricians lurking here.

A friend (yes, really!) has a built-in microwave oven that has just failed. Should be a simple enough job to replace it, I thought.

The failed microwave had a standard 3-pin plug and the cupboard above has a standard socket on the kitchen ring-main. Unfortunately, the replacement microwave cable is not fitted with a 3-pin plug as it requires a 16A supply. It seems that microwaves have moved on over the past 20 years or so and while there are plenty of standalone microwave-only models, all the models designed to be built-in are combination microwave ovens and this particular Bosch model is rated at 3350W, hence the 16A requirement, hence the lack of a standard 13A 3-pin socket

So to my question: can a switched, fused spur with flex outlet be connected to the existing ring-main socket and the microwave connected to the fused spur?

My thinking is as follows:
1. The failed microwave has been connected to the ring-main circuit for about 20 years without any problems, albeit via a 13A 3-pin plug/socket
2. The kitchen already has a separate double oven plus an Aga so the chances of the new microwave being used as an oven is very remote and the microwave-only function is only rated at 900W, so well within the 13A fuse in the switched spur
3. Even if the microwave is used as an oven, the 3350W (14A) rating is well within the rating of the switched spur and associated cabling and only slightly over the 13A fuse rating, so the fuse would take a fair while to blow.
4. Even if the fuse does blow, it would only be a nuisance and wouldn't be an electrical safety issue.

Based on this, I'm fairly sure that a switched fused spur off the existing ring main would be electrically safe, so my real question is would it comply with the latest wiring reg and would a qualified electrician install and certify this solution?

I'm guessing that the only alternative would be to run a new dedicated circuit back to the consumer unit and use a 16A RCBO but it won't be an easy job to run a new cable without significant disruption to the kitchen and walls, hence my question.

Any thoughts?

 

[DaveG]

Is there no option to just replace with a non-oven model integrated microwave?
E.g. https://ao.com/product/bfl523mb0b-b...qUkMzSRulMWjuo5hmlEWnSccrVF9hkF0aAqLKEALw_wcB

Pictured with a standard 3 pin plug?

 

[Craig89]

Electrical topics like this are always akin to opening a can of worms.

As you say the circuit can take the current, provided of coarse the circuit is adequately protected. I have seen radial circuits in 1.5mm cable protected with a 32A breaker "because it was for sockets".

I take it the microwave doesn't come with a cable and just has terminals to wire into like a standard oven?

Are fused spurs designed to handle more than 13A? I wouldn't have thought so?

Changing the socket for a fused spur isnt much different to putting a plug on the end of a cable, wiring it up and plugging it in. I have seen plugs and sockets melt with prolonged spells of 3kW going through them though, 13A fuse intact.

A fused spur would be the better option as its 1 less connection though.

If it was my house I would do exactly as you describe.

As far as getting an electrician to do it and sign it off, not sure. I'm not clued up on every detail of domestic regs. Socket change to spur, can't see anyone having an issue. Getting them to wire in an overrated appliance, even though adequately protected, probably depends who you ask

 

[Flyfisher]

Is there no option to just replace with a non-oven model integrated microwave?
E.g. https://ao.com/product/bfl523mb0b-bosch-series-4-microwave-black-55516-50.aspx?ef_id=Cj0KCQjw2N2_BhCAARIsAK4pEkW76C7pa_m8HAMbQwYFr1IqUkMzSRulMWjuo5hmlEWnSccrVF9hkF0aAqLKEALw_wcB:G:s&s_kwcid=AL!8149!3!!!!x!!&gclsrc=aw.ds&&utm_medium=cpc&utm_source=google&utm_campaign=shopping - mda - microwaves - built in - performance max|shopping - mda - microwaves - built in - performance max&utm_term=smartshopping|BFL523MB0B_BK&utm_content=|paidpla&gad_source=1&gbraid=0AAAAADmq13_rbkAoSqwu8PTdtFQhagJ7Z&gclid=Cj0KCQjw2N2_BhCAARIsAK4pEkW76C7pa_m8HAMbQwYFr1IqUkMzSRulMWjuo5hmlEWnSccrVF9hkF0aAqLKEALw_wcB

Pictured with a standard 3 pin plug?

That was my first thought, especially as the failed unit was just that - basically the same size as a standalone microwave-only unit but with a mounting/bezel kit so that it integrated nicely into the kitchen unit.

But the kitchen unit opening is about 450H and that unit is only 382H (including the bezel), which would leave a gap of about 70mm. That could be filled with something of course, but it wouldn't match the rest of the fitted kitchen units, so not ideal.

 

[Flyfisher]

Electrical topics like this are always akin to opening a can of worms.

As you say the circuit can take the current, provided of coarse the circuit is adequately protected. I have seen radial circuits in 1.5mm cable protected with a 32A breaker "because it was for sockets".

I take it the microwave doesn't come with a cable and just has terminals to wire into like a standard oven?

Are fused spurs designed to handle more than 13A? I wouldn't have thought so?

Changing the socket for a fused spur isnt much different to putting a plug on the end of a cable, wiring it up and plugging it in. I have seen plugs and sockets melt with prolonged spells of 3kW going through them though, 13A fuse intact.

A fused spur would be the better option as its 1 less connection though.

If it was my house I would do exactly as you describe.

As far as getting an electrician to do it and sign it off, not sure. I'm not clued up on every detail of domestic regs. Socket change to spur, can't see anyone having an issue. Getting them to wire in an overrated appliance, even though adequately protected, probably depends who you ask

A can of worms indeed.

The microwave has a fixed cable but the cable is just 'tails' with no plug connected, so it's designed to be connected to a flex outlet, presumably a non-switched or fused one if it's on a dedicated circuit. I think these are available as 20A units, so well within the 16A requirement. The shop that supplied the microwave initially suggested spurring-off a non-switched, non-fused outlet from the existing socket until I pointed out that would mean the microwave was connected directly to the ring main and therefore protected only by the 32A MCB in the consumer unit, which didn't seem right to me.

I agree that a fused spur is likely only rated at 13A, given that's the maximum fuse that could be fitted (I'm guessing that 16A fuses are not available!).

I also agree about the risk of connecting a 13A plug and simply plugging into the existing socket, for exactly the reason you mention and would hope that a fused spur would better handle a constant 13A load. Having said that, I know that EV charging cables that plug into 13A sockets are limited to about 10A for precisely the reason that a prolonged 13A load could overheat a standard 13A plug and in an EV charging application the load could easily be on for 12 hours or more.

Hmm

 

[Craig89]

A can of worms indeed.

The microwave has a fixed cable but the cable is just 'tails' with no plug connected, so it's designed to be connected to a flex outlet, presumably a non-switched or fused one if it's on a dedicated circuit. I think these are available as 20A units, so well within the 16A requirement. The shop that supplied the microwave initially suggested spurring-off a non-switched, non-fused outlet from the existing socket until I pointed out that would mean the microwave was connected directly to the ring main and therefore protected only by the 32A MCB in the consumer unit, which didn't seem right to me.

I agree that a fused spur is likely only rated at 13A, given that's the maximum fuse that could be fitted (I'm guessing that 16A fuses are not available!).

I also agree about the risk of connecting a 13A plug and simply plugging into the existing socket, for exactly the reason you mention and would hope that a fused spur would better handle a constant 13A load. Having said that, I know that EV charging cables that plug into 13A sockets are limited to about 10A for precisely the reason that a prolonged 13A load could overheat a standard 13A plug and in an EV charging application the load could easily be on for 12 hours or more.

Hmm

What is the CSA of the cable fitted to the microwave out of curiosity? Ultimately it's the only thing that matters.

I "think" I have seen 15A plug top style fuses before. Sockets and plug tops of old probably could have handled it, unfortunately most things nowadays are made to be just good enough and no more.

 

[Flyfisher]

Not sure and can't easily check as the unit is in Wales and I'm back in Norfolk!

It looked like a 'substantial' rubberised 3-core flex, probably 1.5mm2 at a guess.

 

[a twig]

So if I understand you right there is a single gang socket above that you are now proposing to change to a fused spur?

Bit rusty but I think to do it properly you’d need to work out the max load on the existing ring, apply diversity and then work out if the wiring and fuse on that ring will take it. If it does then probably good to go.

Bit of a faff though, I’d honestly just buy a 450mm high built in microwave only with a plug. AEG do a few that are 450mm high. Miele do one as well

AEG - MBE2658SEM - Built-in Microwave Oven | DIY Kitchens

AEG - MBE2658SEM - Built-in Microwave Oven from the DIY Kitchens microwaves range.

diy-kitchens.com

 

[fernicarry]

can a switched, fused spur with flex outlet be connected to the existing ring-main socket and the microwave connected to the fused spur?

You may run into the same problem as you mention with EV chargers if the oven function was selected and it was left on for a period of time. As you say a 13A fuse won't be fussed about blowing under a 14A load. That said it will be thermostatic so will cycle on and off.

The plug on my "granny" charger did melt. Its only 10A and only runs for 3 or 4 hours at a go. Its not the plug pins themselves that go but the connection between the fuse and the fuse holder has enough resistance to heat up over time and that heat conducts down the live pin making a bit of a mess of plug and socket. I rewired it with an industrial plug but its on a dedicated 16A breaker.

If you want to limit it to the 900W microwave function then you could try a 5A fuse. No idea what the inrush on a uwave is like.

The shop that supplied the microwave initially suggested spurring-off a non-switched, non-fused outlet from the existing socket until I pointed out that would mean the microwave was connected directly to the ring main and therefore protected only by the 32A MCB in the consumer unit, which didn't seem right to me

Definitely not right. Unless you downrate the ring main with a smaller MCB but that could be a bit limiting for a kitchen.

 

[paulc]

If you want to limit it to the 900W microwave function then you could try a 5A fuse. No idea what the inrush on a uwave is like.

A microwave has a ruddy big transformer inside, so inrush current could be quite high. As for a 13A fuse, these will pass 20A indefinitely, but get very hot in the process. I wouldn't want to be using a standard 13A plug & socket for that.. It would either need an industrial plug or be hard wired in to a suitable outlet plate.

 

[Flyfisher]

So if I understand you right there is a single gang socket above that you are now proposing to change to a fused spur?

Bit rusty but I think to do it properly you’d need to work out the max load on the existing ring, apply diversity and then work out if the wiring and fuse on that ring will take it. If it does then probably good to go.

Bit of a faff though, I’d honestly just buy a 450mm high built in microwave only with a plug. AEG do a few that are 450mm high. Miele do one as well

AEG - MBE2658SEM - Built-in Microwave Oven | DIY Kitchens

AEG - MBE2658SEM - Built-in Microwave Oven from the DIY Kitchens microwaves range.

diy-kitchens.com

Not quite, I was proposing to add a fused spur next to the existing socket for future flexibility but I think it amounts to much the same thing electrically.

I'm also rusty on the diversity rules but as fernicarry pointed out, even in oven mode it wouldn't be a constant 3350W load due to the thermostatic heater cycling, though I've no idea what the effective 'duty cycle' would be or what diversity factor to apply to be compliant with the regs.

As for buying a suitable microwave-only replacement unit, that was my first suggestion as it would have been far simpler by far but we couldn't find one online (I'm now rather embarrassed to admit) and my friend insisted on buying a replacement unit on the day and the local shop only had the combination models. It was only when I opened the box that I discovered it didn't have a 13A plug fitted and thus a quick and easy job turned into a mini-saga! Funny how that seems to happen so often

But thanks for the link - I'll pass it on with a strong recommendation to return the combination microwave for that one.

 

[Flyfisher]

The plug on my "granny" charger did melt. Its only 10A and only runs for 3 or 4 hours at a go. Its not the plug pins themselves that go but the connection between the fuse and the fuse holder has enough resistance to heat up over time and that heat conducts down the live pin making a bit of a mess of plug and socket. I rewired it with an industrial plug but its on a dedicated 16A breaker.

A bit of a digression, but I've used my 'granny charger' cable many times and often for 12 hours or more without any problems. I mentioned my friend lives in Wales and the drive from Norfolk is about 230 miles, which I can comfortably do without charging en route and I arrive with about 25% battery remaining. I immediately plug in the 'granny charger' and leave it charging overnight, so it's often charging for 15+ hours but I've never had a problem with overheating.

 

[ThornhillMike]

As luck would have it my brother-in-law sparky is staying with me at the moment. He is qualified and tells me that this is the solution - a 20A Double Pole switch as a replacement for the existing socket. You may get one with a flex outlet, if not you'll need to break a small entrance hole for the microwave flex. This will handle everything on the ring and then you can rely on the (presumably 32A) RCBO that will be covering the ring.

 

[Flyfisher]

I hesitate to question a qualified sparky, but the unit clearly states it requires a 16A connection, which I interpret as meaning it requires protection via a 16A fuse or MCB or RCBO.

Directly connecting it to a ring will mean it is only protected by the 32A ring circuit MCB. Will that comply with the wiring regs?

Am I missing something here?

 

[Craig89]

I agree with Flyfisher here, if connecting into a 32A circuit then the microwave requires its own protection if designed to be fed off a 16A circuit. The chances are that the flex on the appliance has 1.5mm CSA cores and if the device went faulty to pass 20 - 30A, the MCB will have no dramas but the flex will not be so happy about it. Not to mention anything internally within the device that is designed to be limited at 16A