Part P

So does a shower qualify for the diversity rule ?? I've never known anyone turn it on to full whack, usually start cool and build up ?????

showers mostly run at a fixed power (often one of several selectable fixed powers) then the main knob selects your comprimise between flow rate and temperature.

I was just throwing my tuppence worth in, but how many people actually start there shower on full power ?? (mines a medium a #7 if you're interested !!!!)

Scoby, unless your shower is thermostatically controlled, then the shower runs at full power most of the time, the temperature of the water is controlled by the flow rate..the faster the flow, the lower the temperature, the lower the flow rate, the higher the temeprature.

Some of the latest showers do have true thermostatic control, but these tend to be the very expensive ones, but even then I would not recommend using diversity on the installation.

Damocles said:

You mean the OSG is inaccurate in the sense that you might come across an installation where the published diversity rules suggest a supply requirement of 150A, yet it has been chugging along nicely on an 80A main fuse for the last 10 years?

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Damocles, now your confusing maximum potential demand with diversity.

A whole electrical installation will have diversity applied to it, and correctly so, however the skill is in judging which circuits within that installation have diversity applied to them and which do not.

No, I was thinking of a house which might have maximum potential demand of 200A, standard diversity calculation suggesting 150A but actual working load running on 80A.

This is an entirely possible scenario. If this lot included , say 3 showers then you might stand a chance of popping the fuse. But if it was a house where someone had been a bit enthusiastic over the number of rings, then it could very easily be over engineered with respect to potential maximum demand.

FWL_Engineer is absolutely right when he says that the diversity for cookers in the OSG has not changed for donkey’s years. It’s based on load research work undertaken by the Electricity Council, which didn’t survive privatisation. The research was done in the 1980’s by analysing the readings of over 50 recording kWh meters metering cookers in households.

FWL_Engineer is right in saying that cooking habits have changed in the last 20 years. Indeed they have; people reheat ready-prepared meals in the microwave to a much greater extent now. Although I can’t see why this should reduce diversity.

And so FWL_Engineer concludes that his experience is superior to the OSG.

I’d just like to ask one little question: How did he gain this experience?

You could connect a cooker a day for the next ten years, but not increase your experience of diversity one iota. Because you install & depart. The only time you return is if something’s wrong. Nor can I see FWL_Engineer undertaking the sort of load research that the Electricity Council did, with no income from it. So how did he gain the experience to opine on diversity?

Perhaps as a cook. Big meal and all four rings are going. But if he tried cooking, he would realise that there is a built-in disincentive to running rings on full all the time. The pan boils over and makes a mess that the cook has to clean up. And if there’s not enough water to boil over, it boils dry and causes even more mess. Except for a short transient when the pans are coming up to the boil, a cook will turn down the rings.

So how did you gain the experience to pronounce on diversity ex cathedra FWL_Engineer?

map, your simply confusing the issue. The thermal rating of any cable is an absolute, however the way it heats is far from simple as there are many factors to be taken into account.

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I have to disagree, perhaps I was not clear, I will try to de-confuse.
Cable ratings are not really ABSOLUTE in the correct sense of the word, they are extremely dependant on how long the cable has already been heating up, and how well, or not, it can disperse any heat, this is why there are derating tables for different mounting methods.
All PVC cables have a zero current rating at the temperature that PVC becomes dangerously soft.
Fuse ratings are not absolute either, they are also thermal which is why your main 100A incoming fuse can carry 200A or so for a good 15 minutes on a winter's day, without popping. It is not slapdash to understand what we are supposed to be guarding against, and to apply approprate logic, particularly in areas where the OSG may well be a little dated - how many people use ring mains to supply 3KW bar fires as their main heating anymore for example?
Circuits do not have to to be given a contimuous rating as if they were to supply peak current under all conditions.
An extreme example, my PC has a 150A switch on surge, but the 0.75mm flex, the 13A fuse and a 32A breaker are all fine with this, as the surge lasts about one half cycle of the mains. By your logic it should be wired in 50mm cross section welding grade wire !
The key is that the circuit protection MUST operate before the wiring gets dangerously warm, and there is no danger beyond that, only inconvenience if it nuiscence trips.
Actually if you are a quick showerer, a 45A shower will run on a 32amp breaker and 4mm cable, in open air at least, and if the trip ever fires, the cable will only be warm to the touch, while I don't reccommend it there are many such installations out there, that have been operating for years without problems. The cooker is more complex because of the pulsing load the thermostats present, but a time integrated load needs to be considered, not just the peak.
This is needed elsewhere too.
There is a very good reason why the substation that feeds ~300 houses is sized assuming about 20a per house on average - to do otherwise is an irresponsible waste of copper, not to say poisenous transformer oil, and leads to inefficient use of power as the maginitising current (which leads to higher no-load losses) is greater in proportion the the size of the core.. Oh, and the underground cable to the house may well be only the imperial equivalent of 10mm or so itself, depending on the era when it was put in.
Is that any clearer?

regards M.

To follow on from mapj1's comment, the average demand for a non Economy 7 domestic customer at the bulk supply point is under 1 kW.

That's from the demand profile used by the settlements system for allocating demand and energy at a bulk supply point to various suppliers.

Is evry 1 buying enuf red/black colour cable b4 new colurs come in?
I'll just claim to be a gypsy. Prescott dont like messin with them.
erm soz this got in wrong subject lol

trying said:

Is evry 1 buying enuf red/black colour cable b4 new colurs come in?
I'll just claim to be a gypsy. Prescott dont like messin with them.
erm soz this got in wrong subject lol

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What language is this?

Is trying trying to be trying?

It's written by someone who still thinks they are on their mobile, with a limit of 450-ish characters.

trying:

With a PC, mate, there is a proper querty keyboard where you can type out all the words in full, and wonders of modern technology mean this can be uploaded and hosted on the internet in full. Good, hey?

securespark said:

Is trying trying to be trying?

It's written by someone who still thinks they are on their mobile, with a limit of 450-ish characters.

Click to expand...

Actually the limit on the length of an SMS message is 160 characters!

Anyway. maybe he's using a WAP-capable phone to read/post this forum?

Cheers,

Howard

HD

My lowly 3410 offers a max of 459 characters...

Are you really "securespark" Or is it "Ban_all_sheds" In Disguise