OK so I'm obviously being majorly dumb here, but I am still struggling with this diversity issue when applied to kitchen white goods!
Looking at the "connected power" figures (in watts) from a Bosch technical specs publication I get the following figures: (note that where alternatives are stated, these are values for different models)
Dishwasher 1900 or 2500 Microwave 1220 or 1900 Washing machine 2300 Condensing Dryer 1.9kWhr (is this the same as 1900w?) Kettle 3000
It would surprise me greatly if the average family doesn't regularly have all this stuff running at the same time at some time during a normal week. But that lot adds up to 10320watts - 43A on a 32A ring. So where does the diversity bit come in?
I am considering a single oven, designed to run off a 13A supply, but if I add that to the above load it looks like meltdown city!
So, can I take a fused spur off my cooker feed to reduce the ring load or is this not necessary
Peak load when heating, which is only a small part of the cycle.
That's a heavy one - could be running for an hour continuously.
Only on for a few minutes at a time.
But once your single oven is up to temp it will cycle on and off on its thermostat.
It's probably not necessary and if you have an electric hob *that* can take quite a lot of current - although again the rings will cycle on/ off.
Cable heating is a function of current and time. Most wiring and circuit breakers can handle short-term moderate overloads; long term overloads are more worrying.
Iys highly unusual to be boiling a kettle, running a microwave on full power and a washing machine on hot wash heating cycle and a condensing dryer on full dry cycle all at teh same time. But yes, if you do, you will trip the MCB.
Remember many houses are fused at the supply at 60A total. Whole house.
we get rather close to that these days.
Again ovens are cyclical - on-off.
If you had 50A running in the kitchen for any length of time, that's about enough heat to heat a fairly large house., Ive got 4000 sq ft and that's what it takes to heat the whole shebang in winter with -5C and a gale..
So basically, those loads are peak loads., Not continuous loads, yes, you might by dint of turning it all on at once get an overload, but the fact that by and large most people don't, is proof that the diversity thing works.
On Mon, 7 Jun 2010 12:29:35 +0100 someone who may be "TheScullster" wrote this:-
They may at times have this all stuff running at the same time, though probably not that regularly.
The question is then, how long for? The heating elements in the dishwasher, washing machine and drier are not on all the time. They will cycle on and off as determined by the programmer and conditions. The element in the drier may be on for long periods, the elements in the other two are unlikely to be on for more than five minutes at a time.
The kettle is on for not long while it boils, a few minutes even if full.
So even if all these heating elements happen to be on at the same time, how long does it last? Five minutes or so. During this time the wires will become hotter than they would be otherwise, but they can then cool down when the current reduces ready for the next surge.
From the curves a 32A MCB might operate in something like 4,000 seconds if a current of 43A passes through it continuously. 66 minutes. As a rule of thumb a "normal" wiring system can tolerate twice the maximum designed current for an hour before it becomes dangerously overheated (don't try this at home children). Some highly engineered systems are not capable of this and will suffer runaway temperature rises, but house wiring systems are not highly engineered.
Assuming the cooker switch is within 2m of the oven I would run a suitable cable to a fused connection unit and from there run a suitable cable to the oven. Where the first cable starts depends on the precise arrangement, but that cable will be at least 4 square millimetres in size.
That assumes that the existing cooker circuit is adequately sized for the cooker and the oven. There are simple formulae to calculate the diversity for a cookers in a domestic setting.
If neither of these conditions is true then I would look at wiring it to the ring circuit, or if that was not possible due to diversity calculations I would consider a new radial circuit to the oven.
Dishwasher, washing machine and possibly tumble drier all have thermostats on their heating elements (which use the majority of the power) so would run at an *average* well below the peak stated. Rather like an oven does.
Not sure about a microwave. Of all those only the kettle definitely takes a (near) constant load while in use.
I currently have a 6mm feed to the existing separate double oven and hob. The hob in the proposed new layout will be within 2m of the existing switch, so I intended to use the same feed for this. The new oven will be located on the opposite wall, so I was considering putting a joint box in the supply cable (located in the loft above) and dropping down to a second supply point near the new oven. Should this be a second "cooker point", or a fused spur rated for the 32A upstream condition?
On Mon, 7 Jun 2010 13:39:06 +0100 someone who may be "TheScullster" wrote this:-
That is fine.
Not having seen the layout, for future flexibility I would put in a cooker switch near the new oven .
If the protective device for the cooker circuit is rated at 30/32A then I might consider a smaller version but getting cables into this might be interesting.
Depending on the ability of the oven to deal with fault currents, the manual may help with this, I might then add an unswitched fused connection unit somewhere .
If I wasn't too worried about future flexibility I might not bother with the above and go for a switched fused connection unit , having studied its characteristics carefully. One of those is unlikely to be able to break fault currents, but the protective device should break the fault current before the switched fused connection unit bursts into flames and it is unlikely someone will be able to operate the switch before the protective device clears the fault.
The 30 or 32A rating for ring circuits isn't their peak ampacity, but rather the max continuous rating. Its quite normal to pull 45A or more on a kitchen ring circuit at times, and neither the cable, fuse nor MCB objects. A fair few kitchens contain enough appliances on one ring to be able to consume much more if they were all running at full power at one time. Eg: 2.4kW oven, 3kW kettle, 3kW washing machine, 2kW dishwasher, 1.5kW microwave, 3kW toaster and other small loads, 15kW in all.
If possible it would be better to run a new 6mm from the cooker switch connected to the input side of the switch to a new switch for the oven. From the new oven switch run a 6mm to a 35mm deep back box and fit a fused spur to connect to the oven. If at a later date you wanted to upgrade your oven (single ovens are very poor) you can change the fused spur to a cooker outlet plate.
Due to being a lazy bastard I have my tumble drier, dishwasher and washine machine on a 20A MCB (it is a ring but I need to change the MCB). I'll change the MCB when it trips and not before:-)
Vented dryers are worse, 3200W for 1.5-2hrs, although do cycle the heater.
A 32A ring has some "overcurrent tolerance" designed in. #1 Each ring leg must have a CCC of 20A. #2 A ring should have load balanced across the legs. #3 32A MCB/RCBO takes circa 20min to trip at 40A.
Now, whilst #2 may not always be true, #1 should be true by design. Combined with #3 you have a reasonable margin of safety.
I would only be concerned with a Whole House Ring. In that situation you may want to install a dedicated 20A radial to the dryer, or break the existing ring so as to create a 32A kitchen ring and 32A house ring. If the wiring is 4mm you could break it to create two 32A radials - but this is very unlikely, 4mm is not often found due to pre 17th EFLI limiting length and basic economics making it pricey.
If wiring afresh I prefer a few 32A 4mm radial & 20A 2.5mm radials, gives better redundancy & "survivability" with RCD/RCBO.
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