Shower & diversity

[HarryHotspur]

Interested to hear people's thought around installation diversity when adding a 7kw (=> 30A) shower onto an installation with a 60A DNO cutout fuse - existing circuits include 30A socket ring, 30A cooker*, 15A immersion (rated 3kW => 13A), 5A lights (6 fittings @ 100W => 2.6A?); and on economy 7, 2 x 15A storage heaters* and 15A economy 7 immersion* radial.
(* = actual current demand not known, rating plates missing/not practicably accessible).


OSG Appx A table A2 seems to suggest 100% of 30A shower circuit plus 40% of current demand on the rest. Difficulty appears to lie in the unknown current demands. I could make some guesses, but grateful for your views. Gut reaction is that it's iffy, however diversity only carries the status of guidance, however unwanted blowing of the DNO cutout is a Very Very Bad Thing (TM); and overload of the DNO supply (while the DNO cutout sits there thinking about blowing while making a brew, having a smoke and scratching any part of its anatomy that might require scratching) is worse.

 

[Ducksy]

Shower & diversity

Forgive me if I'm wrong but I'm sure a electric shower is classed as instantaneous water heater and 100% fl is applied

Oh and 7kw will draw 30 amps

7000/230 = 30

 

[HarryHotspur]

Yes. That's reflected in the "100% of the 30A shower circuit" element. Makes sense particularly because this shower is either on at 7kW or off. My understanding was that, rather than think about diversity per individual circuit, Appx A table A2 allows you to take a whole-installation approach of 100% of biggest + 40% of rest. I will confess to finding the whole Appendix confusing and (in my opinion) not very clearly written.

 

[HarryHotspur]

Re: Shower & diversity

(What's the saying, the test of whether you really understand something or not is whether you can explain it so your granny understands? Assuming your granny isn't a professor of electrical engineering.)

 

[MDJ]

Did you allow diversity for the cooker?

 

[HarryHotspur]

The 30A figure for the cooker is the circuit fuse rating. Actual cooker rating is unknown, so can't do first 10 plus 30% of rest plus 5. Plus I thought you weren't supposed to do "diversity on diversity", so you can't do the 40% on a figure on which diversity has already been calculated.

 

[HarryHotspur]

Re: Shower & diversity

Thanks on the 7000/230=30 by the way. Has the new Pope been seen wearing a big white hat yet? ;-)

 

[MDJ]

The 30A figure for the cooker is the circuit fuse rating. Actual cooker rating is unknown, .

No then?

 

[HarryHotspur]

No cos (first 10 plus 30% of rest) of an unknown is an unknown, and if you're doing 100% of biggest + 40% of rest on installation as a whole does it matter because you can't apply diversity to an already-diversity-adjusted figure?

 

[HarryHotspur]

(Just looked at the curves and the blow time for a 2x overloaded 60A 1361 fuse is about half an hour. Add swift half down the pub to the smoke, brew and selective scratching.)

 

[MDJ]

No cos (first 10 plus 30% of rest) of an unknown is an unknown, and if you're doing 100% of biggest + 40% of rest on installation as a whole does it matter because you can't apply diversity to an already-diversity-adjusted figure?

Fair enough, but you are dealing with a small fixed supply and guessing the cooker but asking if it's okay haha, come on man get a grip, you want a actual answer but are guessing the load.

 

[HarryHotspur]

Fair enough, but you are dealing with a small fixed supply and guessing the cooker but asking if it's okay haha, come on man get a grip, you want a actual answer but are guessing the load.

Um, not really. I'm not after an is it OK answer, I'm asking for people's input and how they'd approach it, given that for some of the loads there is NO CHOICE other than to guesstimate it or assume the circuit protective device rating current as worst-case. And as I say I think that to apply diversity to the cooker (which you can't anyway as the max current draw is unknown) and then further diversity with the 40% factor would be to fuc<BkSp><BkSp>all into error.

 

[Richard Burns]

So on 0.4 rule of thumb you have 100A, so this cannot be correct since it is already working, from the circuits description it sounds like a flat so very unlikely to pull much even though it is all electric. If the two immersions are interlinked then they cannot be on at the same time, storage heaters are on when other things are off in general so you are looking at 0.4 x 50A plus 30 A equals about 50A demand, perhaps.

Generally it is not easy to calculate exactly and would tend to be worst case and with the delay in fusing time any shower would be off before the fuse goes!

 

[Rob Smith 643]

How about a real scenario whereby a domestic property supplied by 60amp cut out/fuse on 16mm tails with existing circuits of 40amp shower, 32 cooker, 32 hob, 2x 32 rfc, 16 pv and 2 x 6 lighting is asking for 10.8 kw shower to be added. Using a 'biggest plus 40% of rest' would yield a max demand of 102 amps and probably more if other diversity calcs were used. But the cut out/fuse holds on the property as it stands (though think the tails should have been upgraded at cu change two tears ago) so the max demand must be less than 102 amps? If the new shower is added this will surely drastically effect max demand calcs into the region of 128amps, applying same calc style which can't be a safe method to adopt in this scenario with two showers on the system. Any advice to aid clarification of this position would be welcome. And I'm happy to have the p taken but not the part p

 

[telectrix]

in the real world. wait and see if the dno fuse pops, then they'll come and fit a 80A.

 

[Engineer54]

In the real world you'll be lucky to see 45-50A being drawn for more than a few minutes in any typical house, and probably less in a flat. Seems that these 5 week training courses don't cover real world ''max demand/diversity'', and experience hasn't yet taught them that BS7671 formulas covering max demand/diversity factors don't tend to work too well, ...ok not at all!! lol!! (apart from the cooker diversity formula that is)

 

[Risteard]

Correct me if I'm wrong, but is it not unlikely that someone will be showering in the middle of the night whilst the storage heaters are energised, whilst also simulataneously cooking and have the immersion heater on?

 

[Rob Smith 643]

Re: Shower &amp; diversity

in the real world. wait and see if the dno fuse pops, then they'll come and fit a 80A.

The acid test...

- - - Updated -

 

[Rob Smith 643]

Correct me if I'm wrong, but is it not unlikely that someone will be showering in the middle of the night whilst the storage heaters are energised, whilst also simulataneously cooking and have the immersion heater on?

sounds like my missus on christmas eve, thankfully only once a year

 

[Engineer54]

You could say the diversity comes from the amount of time a shower is going to be in use for!! For most, that's going to be around 10 minutes per person, and not necessarily one after the other, in quick succession!! lol!!