... No, for a *household* cooking appliance the "first 10 A plus 30% of remainder" rule appears in both Tables 1A and 1B of the OSG. Therefore it applies to *both* the design current of the cooker circuit itself and to the loading presented to the installation.
Well 5 kW is nearer to to 22 A. Applying the diversity rule gives you
13.6 A, so a 16 A circuit would be OK, but a 13 A 'spur' wouldn't. In practice though I guess most designers would opt for a 20 A radial circuit in 2.5 mm^2 giving a generous diversity factor of ~ 90%.
By your argument a typical free-standing cooker (12 kW, flat-out) would need a > 52 A circuit, meaning a 63 A fuse or breaker in practice. As we all know though, a 32 A circuit (~60 % diversity) is fine [OSG p. 154].
Hey, don't laugh. I did that once. Temporarily, of course. I had designed and built (with a colleague) an interactive exhibit which used four or six hot air guns to heat the air for a hot air balloon. The final install location was to be near a distribution board where we could quite easily sit a 32A breaker and proper take-off, but for "proof of concept" purposes we had to make do with a couple of standard 13A sockets in the workshop...
Worked a treat in the workshop, but they made me redundant before I could install it properly, and the people who did the installation didn't really have time to get it right. I gather it still (two and a half years later) isn't working. Getting a 10ft high balloon to fly on demand and within the sort of attention span a typical 11 year-old has is no easy task.
Guaranteed method: teach them a maths class while the job's being done outside on the lawn. It could take half the day, you'll have their attenmtion alright.
I see an Athlon CPU heatsink plus cooler fan on there, held in place with plastic ties round the plug, which is 2mm out to enable this. Fan is run off a wallwart in the next socket. The cable ties creep, ensuring poor thermal contact, the fan blows the hot air off the heatsink onto the wart, and the plastic round the plug pins slowly turns a shade darker each year.
I can't put a date on it. I was about to say Christmas dinner, student hall of residence, 1983, but I must admit that I'm not sure it was party poppers that were propelling the sprouts back then. I'm pretty sure I'd done it by 1990 though, which is a date I can be certain I'd played with the things.
Two 13A plugs, and a whole cabinet full of DIN-rail mounted connectors, contactors and a transformer. Transformer was for LV remote switching of the contactors, one contactor per hot air gun, four installed, but contactors for two more. Even I was impressed.
There is a large house in Ravelston, Edinburgh, with remote switching for curtain closing, light switching, TV switching Etc. Etc. Etc. And it's all done from a bank of LV relays and alpha-numeric keypad panels sited around the house. It took us four weeks of very long days to create it, but the finished product is pretty impressive, though I say it myself. :-)
Done that with a PC to program sequences and to allow for control using a web browser. Didn't bother with keypads though.. much more impressive using a tablet PC running a flash program.
You could dim lights, switch appliances, change channels on the TV, etc.
It even had face recognition and took videos when people went into certain places. It sent SMS and email alerts if it decided you were an intruder.
I was working on routing the TV to the tablet PC when I had to leave it all.
Ah, um, yes. I see. Delicious stuff, this pie. What did you say it was called again? Mumble? Dumble? Oh, *H*umble!
Fairy neurf. For an entire cooking installation - hob + oven(s) - it indeed seems entirely reasonable to me to consider the design load as diversified, as you simply can't draw the flat-out load for any serious length of time. For a 4-ring hob - well, it depends on your style of usage! Chez nous, it's common enough to be doing large pans of pasta, sauce, soup, and fried-things to feed medium-sized hordes of transient teenagers; again the simmerstats mean that the full-on load won't last long, but I'd still be chary of connecting through a 13A plug-n-socket and a 1.25mmsq flex (OK, I sneaked that headroom-lowering bit of spec in without any evidence just to bolster my point ;-) than a 20A DPsw and
2.5mmsq hob feed.
S'like that with diversity, innit - as the Good Book says, its values are 'only for guidance because it is impossible to specify the appropriate allowances for diversity for every type of installation... The figures given in Table 1B therefore may be increased or decreased as decided by the engineer responsible for the design of the installation concerned.' Applying which, I'd be happy connecting a 5kW hob to a 13A socket (well, FCU) for Granny, where Granny's been given such a hob as a Christmas pressie, there's no heftier cooker circuit to use, and she's living on her own; I'd be most *un*happy putting a 5kW hob on a 13A accessory in a communal-use kitchen (student kitchenette, yoof hostel, etc) - and to be fair the Good Book would be unhappy too, as it's not 'household' - and like the OP I'd be chary of the arrangement in a bigger-than-single-person household.
But thanks for the clue-by-four about the significance of Table 1A vs
It 'protects' against normal, foreseeable usages; it doesn't protect fully against abuses. So, the terminals and flexes are fine for a 13A sustained load; and nothing sold with a 13A plug 'should' pull more than that. But if you put a 13A plug onto a 16A or 20A peak-draw appliance, the terminals and flex will pass that higher draw, a 13A plugtop fuse won't blow, but things will get warmer than is conducive to best practice. Continental-style hobs are one example, higher-current welders are another. In both cases, the peak currents (as we've discussed to death) don't last long; but the repeated excursions into/beyond the safety margin are a departure from good sense. For example, all's well until the flex ends up better thermally insulated (meaning someone drops a pile of old newspapers on it) - now instead of being in free air it's got a harder time getting rid of its heat, and pressure helping the softened insulation to move gently aside from its wonted place. Hopefully the endgame is an L-E or L-N little-sparks short and the fuse or MCB popping!
However, this case isn't one where the current is /limited/ to the 'design load' by the nature of the appliance.
So although, as Andy Wade showed, the "diversity" of a 5kW hob may only be 13.6A, if the cable is protected by a 16A or 20A MCB, the design current of the circuit then becomes the rating of the protective device.
Oh I agree. I wasn't for a moment trying to suggest that it would be OK to connect it via a 13 A plug or FCU. In any case the "came with flex and fitted plug" bit of the story now seems to have been discredited. A dedicated 16 A circuit is the minimum acceptable, and 20 A preferable. (I doubt that the product standard would allow it to be fitted with a
1.25 or 1.5mm^2 flexible cord in any case.)
As you hinted, the more independently switched loads there are, the safer one's diversity assumptions become - it's the central limit theorem in action, I guess. The OSG's cooker rule undoubtedly evolved in the era when when the four-rings-grill'n'oven cooker was the only sort and it's certainly stood the test of time for that type of appliance. For a plain 4-ring hob alone I guess we'd agree that it's getting a bit marginal.
Even that's dodgy. Granny will die; the house will be sold...
I'd be most *un*happy putting a 5kW hob on a 13A
Absolutely.
'Tis a common source of confusion, especially with lighting circuits where 1A allows no diversity on an individual circuit but 1B allows 66% diversity (household) on the lighting load's contribution to the installation maximum demand.
Diversity is all well and good, but...................
Who here prefers to make the installation as safe as they can? In my view, diversity is good on loads that you know will only demand a surge lasting for a few seconds, but it is not the safest for loads that can exceed the safe limits for minutes at a time.
If you know that a total load runs safely at 32 Amps, then the installation should reflect that fact, and be designed to allow for that safe total loading. The matter of diversity rules could show that the total load of 32 Amps will only be demanded for five minutes at a time though, so why can't I allow for the drop in the running load in my design? I know the cable and switch gear can withstand a higher current flow for that length of time, so this would make my installation much cheaper with the lower graded equipment.
My honest view on diversity ruling is it is not totally safe. OK if a load is known to only demand a start surge that lasts for 2, 3, 4 or 5 seconds at a time, then drops immediately to its constant running level. For loads which you know can demand that higher current for lengths of time verging on minutes, then diversity rulings are tosh. If the load is known to demand higher currents for sustained periods, then the design of the installation should reflect this fact.
Hmm, it seems to me that there is a failing of either standards or design, all you say I accept and is great for fixed wiring etc. but were the safety device is being used by people who barely know how to wire a plug let alone calculate cable / load ratings....
As I may've mentioned before, I knew an 'official' University electrician (he'll have retired by now ;-) who did this for real to run
5-6kW of stage lighting for smaller gigs in the individual colleges (York Uni). He was ever so careful to plug the two plugs into separate sockets with the individual switches *off*, and then turn the switches
*on*, reversing the proceedcake when tearing down.
He had some 32A 'commando' style sockets fitted in at least a couple of the colleges (Goodricke and Vanbrugh, IIRC) shortly after spreading the load across a couple of sockets in separate rooms.
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