New wiki article - diversity

[John Rumm]

Interestingly I don't think the statuary bit of the building regs does call up BS7671 - the approved docs may, but they are not actually binding.

In reality it is very hard to lay down hard and fast rules for some of these things. Cookers, heaters, showers etc are quite easy since they are either fixed loads or loads which average out quite a bit lower than the plate rating due to the influence of thermostatic control. Socket circuits however are far harder to call for domestic settings (at least outside of the kitchen anyway).

Not if you can justify your design criteria.

A good example of what I meant above - you split a circuit for reasons not related to total load, so its safe to assume that in the absence of other changes in use etc, then the total load should be no different from before.

Hopefully the notes make it clear that even if the main fuse were 80A it ought not be a problem.

As to the total, my first thought was to add another ring. That just happened to take the total up to something feasible - while having the extra column way over.

[Tim Watts]

Robin ( [e-mail]) wibbled on Sunday 30 January 2011 21:43:

The OSG is a guide, no more no less.

I use the OSG, but I will ignore it in favour of BS7671 - and I will also override that on occasion[1].

[1] eg use of 16A commando sockets in domestic property. Not strictly allowed by 17th due to unshuttered sockets, but mitigated by use of fully interlocked socket, not to mention putting it out of reach of minors.

The point I'm making is one should take note of the rules, and the guides for they are written by people who know what they are doing. But to follow them blindly if one understands the motive behind them and can engineer an equally acceptible solution, is not necessary.

Cheers

Tim

[Robin]

My apologies: you are of course right. (I think I was thinking of the regulations which tell suppliers not to connect to an installation which they think may not comply with the British Standards.)

[John Rumm]

This one probably:

to as the "British Standard Requirements" in the body text)

[Andy Wade]

Just look> You will note that while the final load including diversity is within

But this is *not* an example of 'Robin's problem' (at least for a 100 A supply). The fact the sum of the individual circuit design currents is way over 100 A isn't relevant, other than to demonstrate the need for applying diversity in the first place. I suggest that the Ib sum figure is either removed or used to explain why diversity has to be applied.

To demonstrate Robin's problem, repeat the table with the addition of, say, a 45 A shower circuit. Then you have >100 A in the RH column and the issues can be discussed.

I think the final paragraphs (after the valid stuff about short duration overloads not being too much of a worry) are clutching at straws and should hardly be promoted as a design method. Supply impedance is largely outside the designer's control, and can reduce if the DNO upgrades the street mains.

[Andy Wade]

I agree. I've got four socket circuits here, two rings (kitchen & upstairs) and two 20 A radials covering the remainder of the ground floor. The radials suited the floor plan quite well and the re-wire was done in 1991, when the spectre of the UK adopting Shucko sockets had raised its head again. Anyway the OSG diversity calcs gave an ADMD of over 100 A, so I scaled back some factors to give a 100 A documented design, and applied for a 24 kVA supply when requesting re-connection (we ran the house on a temporary builders supply for a month or two).

A chap came out from Eastern Electricity (as was), inspected the service cable and cut-out and said they'd be happy to fit a 100 A fuse. When I'd finished and they came back to connect the tails the guy put the original 60 A fuse back in, saying "it's OK, you won't have any problems." Nearly 20 years later and after no further visits (other than a meter change), I guess he was right.

[John Rumm]

Erm yes oops - you spotted my deliberate omission of the second example ;-)

Yup, I quite like it being there since it does illustrate the effect of applying diversity over the group of circuits.

It does touch on one part of Robin's comments though in the sense that the deign load is in excess of the main switch capacity in all likelihood.

Yup, sounds like a good plan.

I did not want to promote it as a design method - as you say its not something you can control or even predict changes to. Although it is a contributory factor that will preserve an apparently seriously overloaded main cutout in some cases.

ok, added a new section at the end to cover Robin's points better.

Not sure what (if anything) to say about main switch capacity though. Any thoughts?

[Robin]

I'm drowning again: the OSG has the footnote on page 96 (page 87 of the

2002 edition):

"It is important to ensure that the distribution boards and consumer units are of sufficient rating to take the total load connected to them without the application of diversity."

If that doesn't make the sum of Ibs relevant again to disconnectors etc what does it mean please? Or is it just ignored?

Possibly an idiot boy question: would it be possible to provide instead a pointer to where to find further and better particulars of ways to calculate diversity using factors other than those in the OSG?

I take Tim's point that the OSG is only a guide - but it is also, as I understand it, also accepted in practice as providing "safe harbours". That is to say, it would be rare if an installation within the OSG's parameters were rejected. But even a DIY-er is not locked into the OSG: eg when calculating cable sizes there is behind it BS7671 with all the details. But BS7671 is silent on just how to work out diversity.

FWIW I am expecting the answer "no" as the discussion here has been more extensive and helpful than anything I've found in the books commenting on BS7671.

[ARWadsworth]

That only applies to 6, 7 & 8 of table 1B. A CU supplying storage heaters would be a good example as all the heaters come on at the same time for a considerable time.

[Robin]

Ok, thanks. (I think I'd subconsciously dismissed that explanation as it seems to make the footnote otiose: you get exactly the same result without it given the table says "no diversity allowable" for those 3. But I suppose it adds emphasis.)