Diversity Factor

[reds42]

I have to say I was suprised at just how far off the mark the maximum demand calculation described in the onsite guide was when I applied it to my house installation compared to real life measurements - even in worst case scenario of christmas dinner.

Real life loads on most circuits are far below those assumed in the recommended calculation.

 

[JohnW2]

I have to say I was suprised at just how far off the mark the maximum demand calculation described in the onsite guide was when I applied it to my house installation compared to real life measurements - even in worst case scenario of christmas dinner. Real life loads on most circuits are far below those assumed in the recommended calculation.

Indeed, the OSG guidelines are generally over-cautious and, in a good few cases, totally outdated.

Sockets circuits are probably the worst. When the guidelines were written, the common domestic situation was probably just one 30A ring. However, recent new-builds and re-wires will commonly have at least three 32A rings ('upstairs', 'downstairs' and 'kitchen') and to have to count those as 57.6A (32A + 12.8A + 12.8A) is really pretty unrealistic (ridiculous?) in most cases.

Albeit fairly trivial in terms of the big picture, the OSG figures for lighting circuits are obviously ridiculous with LED lighting.

Even the 'standard diversity calculation' for cooking appliances is perhaps unnecessarily cautious, particularly for 'small appliances' such as ovens - a 2kW-3kW oven will cycle 'off' a lot once it has got up to temperature, so it's probably unnecessary to consider all of the first 10A. I don't have any electric cookers but a while back undertook (and posted here the results of) some experiments with a 1700W deep fat fryer. As you can see, it starts cycling on/off in under 10 mins and within about 15 mins settles down to an average of well under half (nearer tio a third) of its 'on' consumption....

Kind Regards, John

 

[reds42]

I have a 7.2Kw induction hob and 5.2Kw double oven. If you get the timming perfect in a non real life situation I can get them to draw their maximum load for a relativley short time (a few minutes). Once Ovens are up to temp and pots on hob have boiled and reduced to a simmer the draw was a lot less than half maximum (Can't remember exact figure of top of my head but it was suprising).

The draw on the kitchen ring with a washer dryer and dishwasher running was also much lower than expected. The on/off duty cycle of the heating elements as shown in your deep fat fryer is applicable to a lot of appliances.

 

[JohnW2]

I have a 7.2Kw induction hob and 5.2Kw double oven. If you get the timming perfect in a non real life situation I can get them to draw their maximum load for a relativley short time (a few minutes). Once Ovens are up to temp and pots on hob have boiled and reduced to a simmer the draw was a lot less than half maximum (Can't remember exact figure of top of my head but it was suprising).

Quite so. Averaged over any reasonable period of time, the current draw is always going to be well under half of the theoretical 'peak' figure.

The draw on the kitchen ring with a washer dryer and dishwasher running was also much lower than expected. The on/off duty cycle of the heating elements as shown in your deep fat fryer is applicable to a lot of appliances.

Indeed. It's probably time to wheel-out the oft-posted extract from 2002 'Wiring Matters', showing the current draw following the simultaneous start of a washing machine, tumble dryer, dishwasher, kettle and toaster (the later two staggered) - the total hardly ever got above 30A, and was half, or less, of that for much of the time...

Kind Regards, John

 

[JohnW2]

I have a 7.2Kw induction hob and 5.2Kw double oven. If you get the timming perfect in a non real life situation I can get them to draw their maximum load for a relativley short time (a few minutes). Once Ovens are up to temp and pots on hob have boiled and reduced to a simmer the draw was a lot less than half maximum (Can't remember exact figure of top of my head but it was suprising). ... The draw on the kitchen ring with a washer dryer and dishwasher running was also much lower than expected. The on/off duty cycle of the heating elements as shown in your deep fat fryer is applicable to a lot of appliances.

In a new thread that I have just started https://www.diynot.com/diy/threads/real-world-maximum-demand-and-diversity.532583/] (click here) [/url] I have presented some real-world 'maximum demand' data from my installation.

Kind Regards, John

 

[hard-work]

That seems a rather odd thing to say - since, were it not for that "63A" sticker, you would presumably have had no reason to start this discussion?I would be one of those 'few' - and, as I have said, and for what it's worth, my view of the common sense is that you really should not have any concerns. However, as you do seem to have concerns, it seems that you probably need someone with sufficient knowledge and understanding to advise, hopefully reassure, you.

Again... I have sufficient electrical knowledge. That is abundantly clear. I do not need an arm waving spark on site. It is a calculation that could fail this installation. I asked for any others who have encountered something similar and how they got around the 63A fuse limit, if they did.

 

[John D v2.0]

It's not the calculation that would make the installation non compliant it's the excessive demand that would.
The way around it i can see has been suggested in the thread, either increase the capacity of the supply, or reduce the maximum demand by some suitable demand management eg priority switches or reducing the amount of electrical equipment.
Regarding the using an MCB to limit things, remember the regs require circuits to be designed to not have a small overload of long duration. So the circuit design must match the predicted demand, not the other way round.
And the reason the mcbs trip slowly is that they are matching the curve of the cable warming up, so they should perform their function of dealing with overload appropriately.

 

[JohnW2]

Again... I have sufficient electrical knowledge. That is abundantly clear. I do not need an arm waving spark on site.

If you say so, but not all of us would agree.

It is a calculation that could fail this installation.

I agree with everything that John wrote. It is not 'the calculation' that might cause the installation to 'fail' but, rather, the facts underlying the calculation.

In particular, as John wrote, an installation can/should 'fail' if MCBs are used in an attempt to 'limit' an anticipated possible load.

Kind Regards, John

 

[reds42]

One different approach could be to super insulate the flat to reduce the heat loss, add heat recovery ventilation etc., then you would need lower power underfloor heating.

 

[JohnW2]

One different approach could be to super insulate the flat to reduce the heat loss, add heat recovery ventilation etc., then you would need lower power underfloor heating.

Indeed - although I'm far from convinced that, in the case we are discussing, there is actually any issue which needs to be addressed.

IF 'load restriction' really IS needed, there are legitimate and acceptable ways to do it (albeit not necessarily 'simple'), with 'prioritisation' if required - e.g. things could be arranged so that the heating could not be switched on when the cooker was on, and that the heating (if already on) would switch off if the cooker were switched on (and similar with cooker vs. dryer or whatever) - but, again, I'm far from convinced that the OP actually needs anything like that.

Kind Regards, John

 

[hard-work]

If you say so,

I do say so.

 

[hard-work]

One different approach could be to super insulate the flat to reduce the heat loss, add heat recovery ventilation etc., then you would need lower power underfloor heating.

Sensible suggestion, which is being considered.

 

[Simon35]

I do say so.

So why did you post incorrect statements about MCB operation?

 

[JohnW2]

So why did you post incorrect statements about MCB operation?

... and then thanked me when I explained to him about the characteristics/behaviour of MCBs.

Kind Regards, John