Kitchen Appliances Power

[rich42]

My kitchen has a ring main, in 2.5mm2 cable. It was installed in Pre Part P days.

The appliance loads on the ring are:

Dish washer 2.4Kw
Fridge 0.25Kw
Tumble Dryer 2.5Kw
Microwave 1.3Kw
Toaster 1.6Kw
Clothes Washer 2.3Kw
Kettle 3.0Kw total 13.35Kw = 58A @230vac

Now I know these figures are maximum ratings from the appliance rating plate, (fridge figure is a guess) and it would be unlikely that all the appliances would be in use at max power together.

How would you go about designing the installation to the current regs?

Regards, Rich

 

[EFLImpudence]

How would you go about designing the installation to the current regs?

Do it exactly the same.

A (32A) socket circuit, whether ring or radial, with any number of sockets is just that.
What else can you do? Limit the number of sockets to two?

It is up to you what you have working at the same time.
If you should happen to overload the circuit then the fuse/MCB will disconnect the supply before the circuit is damaged.


Part P merely states that the work shall be done to ensure safety.

That's it. There is no more.

 

[SUNRAY]

My kitchen has a ring main, in 2.5mm2 cable. It was installed in Pre Part P days.

The appliance loads on the ring are:

Dish washer 2.4Kw
Fridge 0.25Kw
Tumble Dryer 2.5Kw
Microwave 1.3Kw
Toaster 1.6Kw
Clothes Washer 2.3Kw
Kettle 3.0Kw total 13.35Kw = 58A @230vac

Now I know these figures are maximum ratings from the appliance rating plate, (fridge figure is a guess) and it would be unlikely that all the appliances would be in use at max power together.

How would you go about designing the installation to the current regs?

Regards, Rich

Has your current set up ever been a problem?
We have all of that and never given it a second thought, don't forget the iron, boiler, TV/radio, under cupboard lights, vacuum etc.
In addition ours serves a couple of outside sockets so lawn mower, pressure washer. occasional welder/grinder, vacuum cleaner, 3KW of blue lighting for NHS earlier this year etc
Think about the most likely items to be in use at the same time, such as the dishwasher is most likely not used at the same time as cooking appliances so they cancel and clothes washer and drier also cancel.

Quite often 2 ring finals are used, one for the sockets which takes care of the kettle and toaster and quite often the microwave 4.6 or 5.9 = 20 or 26A [leaving 12 or 6A for other purposes], the second ring takes care of the rest 7.45 or 8.75KW = 33 or 38A
Suddenly the figures don't look so bad.

While fault finding on an intermittant kitchen RCBO tripping, it was always assumed the circuit was being overloaded. The customer had an energy monitor with a clamp on the main tail so I moved it to the kitchen ring only. Over the course of the 2 weeks or so it showed the maximum power as a graph of a 24 hr period and the customer noted down salient points, the peak consumption was about 8.4KW[read from the graph] = 36A and the time over the whole period of exceeding 7.5KW [settable threshold] or 33A was just under 1 hour [read from a i minute counter] bear in mind the power on those monitors is calculated from the measured current assuming 230V.
To put this into perspective this was a big property with 4 adults [2 sisters i Think] and a number of small children in the winter, so I imagine all appliances got a lot of use.

All calculations done in head so not 100%

 

[333rocky333]

Current regs would require Rcd protection

 

[rich42]

Thanks for the replies. This came about after reading some messages on here, about cable current capacity, which got me thinking.
I think my worst case load has been the clothes dryer, washer and kettle all in use at once. I do not get any tripping issues, so i must assume the load is within the parameters of the CBs (RCBOs as well)

Rich

 

[JohnW2]

It might be yet again an appropriate time to re-post this snippet from the 2002 edition of "Wiring Matters" ...

Kind Regards, John

 

[ericmark]

In the days when we had washing day, the washing machine was dragged to centre of floor, and no cooking could be done while doing the washing, you would not want to anyway, dirty cloths and food does not really mix. So the oven and hob would not be used for cooking at same time as washing machine, and the washing machine would be filled with domestic hot water, it would not be heated in the washing machine.

Today the washing machine is often cold fill only, and also put where it can run without really affecting proper kitchen use, it does not have a mangle and tends to have lower spin speeds so as not to damage cloths, so the air drying time is increased. However there seems to be a move from 3 kW to 2 kW so may be a load for longer time, but lower load.

Personally the washing machine is in the utility room and has nothing to do with cooking. Can't really see how the two mix. But it seems house builders do mix the two, so we have to live with it, but when mothers kitchen was re-wired, the whole kitchen ran off a 40 amp MCB supplying a sub board and the 40 amp MCB never tripped. Neither did any of the RCBO's in the sub board.

For a type B MCB to trip on the magnetic trip looking at 3 to 5 times the thermal rating, and the thermal rating is an average, so a 32 amp supply can have a 40 amp load as long as not sustained for too long, cable also takes time to warm up, so the time the MCB/RCBO takes is not a problem.

The problem is as the designer we don't know what load and for how long, all we can really do is say it didn't trip in last house we rewired in that way so should not trip in this one. Even the guides don't really help.

If we compare an induction hob to a halogen hob, the maximum draw on the induction is often higher than halogen, however we are cooking the same food on both, and there is less waste with an induction, so the average use with an induction hob is lower than a halogen.

The same with an oven, a range cooker has a load of iron to heat up, and really warms the room, so although the same element size to a oven built into a wood carcase the average must be far higher. The lighter in weight the oven is, the less power it is likely to use.

I would say ideal is dedicated circuit to immersion heater, hob, oven, and cloths drier is the ideal, but in real terms only the immersion heater as it runs for maybe 5 hours and the hob as over 3 kW really needs a dedicated circuit.

 

[SUNRAY]

It might be yet again an appropriate time to re-post this snippet from the 2002 edition of "Wiring Matters" ...

Kind Regards, John

Good heavens exceeding 30A by 12 minutes and peak current of 35A.
assuming that pattern is repeated every day for 2 weeks, which it obviously won't be, gives under 3 hours.

Thefollowing is for a house holding 2 families with small children

Over the course of the 2 weeks or so... the peak consumption was about 8.4KW[read from the graph] = 36A and the time over the whole period of exceeding 7.5KW [settable threshold] or 33A was just under 1 hour .

That's close enough for Government work

 

[JohnW2]

For a type B MCB to trip on the magnetic trip looking at 3 to 5 times the thermal rating, and the thermal rating is an average, so a 32 amp supply can have a 40 amp load as long as not sustained for too long ...

I'm not quite sure what you mean by 'an average', but a B32 is required never to trip with a continuous current of 36.16 A (32 x 1.13) and to trip within 1 hour with a continuous current of 46.4 A (32 x 1.45).

I would say ideal is dedicated circuit to immersion heater, hob, oven, and cloths drier is the ideal, but in real terms only the immersion heater as it runs for maybe 5 hours ....

I'm not sure how immersion heaters got into this discussion about kitchen electrics, but mine is very rarely 'on' continuously for much more than 2 hours, even if the cylinder is totally 'cold' at the start. By calculation (and ignoring losses), a 3 kW immersion should increase the temp of 140 litres of water by 44°C in about 2.4 hours.

Kind Regards, John

 

[JohnW2]

Good heavens exceeding 30A by 12 minutes and peak current of 35A.
assuming that pattern is repeated every day for 2 weeks, which it obviously won't be, gives under 3 hours. ... Thefollowing is for a house holding 2 families with small children ....

Over the course of the 2 weeks or so it showed the maximum power as a graph of a 24 hr period and the customer noted down salient points, the peak consumption was about 8.4KW[read from the graph] = 36A and the time over the whole period of exceeding 7.5KW [settable threshold] or 33A was just under 1 hour

Indeed. Since I'm digging up old stuff, it could be salient to re-post here something I originally posted in November 2019, as follows ...

----- copy of old post starts here -------
In another thread (click here) , there has been extensive discussion about the estimated ‘maximum demand’ of a domestic electrical installation and the associated concepts of diversity. It has been pointed out in that discussion that real-world ‘maximum demands’ are usually surprisingly low.

To illustrate this, I present here some data relating to the last ~2.5 years (about 1.35 million minutes) of my electrical installation. Although data is captured roughly 5 times per minutes, I have it recorded as averages over each 1-minute period, and it is those 1-minute averages which are presented here.

It has to be said that it is a very atypical domestic property. Although now usually only occupied by two people, it is very large, with a 3 x 80A electricity supply and about 30 final circuits. There are no large electrical cooking appliances (only microwave, fat fryer, bread maker, toaster etc.) and the two electric showers are virtually never used. There is, however, a silly number of refrigeration devices! Something like one third of the total electrical consumption results from nocturnal hot water heating.

The first graph below shows the distribution of the instantaneous (1 minute) total demand, indicating that for the vast majority of periods the total demand is under about 5A. The small peak around 15-16A relates to periods when the 3kW immersion is the main load.

The second graph shows the same data, but in terms of cumulative figures, indicating that some 80% of all minutes have a demand below about 4A, about 92% of all minutes have a demand below about 14A and about 98% of all minutes have a demand below about 20A.

The final three graphs are ‘inverted’ versions of the second, the final two being ‘magnified versions’ which show what happens at the extreme tail (moments of highest demand). These show that only about 1% of minutes have a total demand >24A, and only about 0.01% of minutes (about 1 minute every week) have a demand >32A.

Over all the ~1.35 million minutes observed, the mean total demand was 4.35A and the median 2.97A. The lowest demand was 0.35A and the highest 40.36A. Summarising the number/percentage of minutes with total demands above a certain figure. As can be seen, under 2% of minutes had total demands >20A, and almost none had total demands above 30A. For about half of all minutes, total demand was under 3A.

... and here the graphs ...

As above, I don’t pretend that my house is typical, but these figures give some idea, and will undoubtedly surprise some people.

Kind Regards, John

 

[ericmark]

Being lazy I used this calculator and got this

result so around 5 hours, as to if the immersion heats the whole tank, or if it starts cycling before complete will vary, however 5 hours is a far amount of time at maximum 13 amp output. So is about the only item in the modern house rated 13 amp or less that really needs a dedicated supply.

On my sons narrow boat he had a 6 amp MCB feeding the whole boat, with washing machine set to 40°C wash it could do the cycle without tripping. That is in spite of the 2.4 kw rating.

Modern washers use very little water, so it does not take much heating, we hunted for a hot fill, but could not get hot fill and inverter drive, so the start current for motor was a problem with hot fill types.

Seem to remember the RCD (Recreational Craft Directive) stated that an RCD should be at least type A for boats. I will assume the diodes used in the earth is the problem? But it could be because of other devices used.

But as shown in other posts, when you actually measure the electrical energy into a house is very low, unless charging an electric vehicle or heating the house with electric.

 

[JohnW2]

Being lazy I used this calculator and got this ...

Yes, I agree with those figures - but that's for a (pretty unrealistic) 70 °C temperature rise, whereas I was talking about a 44 °C rise (from 16 to 60 degrees).

Kind Regards, John

 

[ericmark]

I know my central heating at 24 kW will not heat the cylinder in 20 minutes, after 20 minutes it starts to cycle. But it does not help the guy wire his kitchen.

Other than the cooker, all the rest in my kitchen was powered from one ring which served rest of house, in all 4 fuses, 30 for cooker, 30 for ring final, 15 for immersion and 5 for lights, and the fuses did not blow, swapped for 32, 16 and 6 amp MCB's and the only one which would trip was the 6 amp for lights when a bulb ionised as filament ruptured.

Latter a second fuse box was fitted, but with tumble drier, washing machine, dish washer, waste disposal, kettles, toaster etc all from same MCB it did not trip. 32 amp is ample for whole house sockets never mind the kitchen.

What was a problem was when I added RCD protection, one for each fuse box, they would have bouts of tripping, then do years without tripping, before I moved I lost two freezers full of food, so this house all RCBO as I feel the less on each RCD the better as less likely to trip.

As to how I would re-wire a house today not sure, I liked the idea of freezer not being RCD protected, but now it seems not an option, I remember my grand fathers house with a fuse for every socket, pre war so pre the 13 amp socket, and I would not want to return to that, and when my veranda leaked and water got into a socket down stairs in flat, I was unaware it had tripped for weeks, it was no harm, but the question is if the supply to a freezer tripped how long before you notice? clearly if lights don't work as well then this time of year will realise at 4 pm, but I do not walk into kitchen and look to see if freezers still working, if a light when on yes, but little green light 2 inches from floor likely I would not notice if not lit.

I intend to fit a rechargeable torch to freezer supply as then it lights with power failure, so more likely to notice it.

 

[denso13]

I know my central heating at 24 kW will not heat the cylinder in 20 minutes

@JohnW2 mentioned 2.4hrs, I dont think your 20 minutes is comparable.

 

[Harry Bloomfield]

I intend to fit a rechargeable torch to freezer supply as then it lights with power failure, so more likely to notice it.

A torch can quickly go flat, so then you have no indication...

Even better, I bought and fitted a battery operated temperature display, with alarm. It is very versatile, settable to alarm on temperature rise or fall, at a programmable temperature. I have the sensor stuck to the inner wall of the freezer, with the display stuck to the outside and set to alarm at >13C. It updates every minute and if above 13C it bleeps loudly several times, once per minute.

I once accidentality left the door ajar, so bought this gadget. It even triggers if SWMBO spends a little too long putting things in, or out. Cost was around £8 delivered from China, from Ebay.