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 Post subject: Diversity Calculation
PostPosted: Mon Apr 16, 2018 7:18 pm 
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Hi All,

Having bought a single circuit 4bed flat here I've been rewiring it circuit by circuit as I go room by room. Although it never tripped before with a 15A MCB for the whole flat (all wiring was 2,5mm and we don't do ring mains here), I've obviously wanted to design it better so got the elctrician added 6mm tails to my 25A main switch, after which is a 25A RCD (I know RCD isn't measured in amp carrying capacity but it does say 25A max on it) and then I have in the CU currently:

one new 16A MCB with new circuit for all sockets in 3beds/ lounge (8 sockets)
one new 16A MCB with new circuit for sockets in one bed (700W heater) which will extend to kitchen
one new 16A MCB connected to old wiring which feeds lighting (100W), bathroom (a waterheater 2kW), kitchen (fridge 400W, Oven 2.4kW, Washing machine 800W, socket for kettle 2kW). But don't seem to have any problem with it tripping even when most stuff is on at the same time.

So just the kitchen to do and plan is to get rid of the old wiring in the kitchen (except lighting) and extend my 2nd 16A socket circuit to kitchen so in total it will supply:

1 bedroom socket (700 W heater max)
1 kitchen socket (2kW kettle max)
Washing machine socket (800W max)
Fridge socket (400W max)
Oven (2.4kW max) (directly wired)

That's a theoretical max of 6kW (as per supplier's instructions) on a 3.8kW 16A breaker. BUT...I used all this stuff along with other PCs/televisions in the other rooms and the water heater when I had just one 15A circuit and it never tripped before and I suppose only the oven/heater will be using max power for any length of time.

So is there enough diversity there? Or do I need a seperate 16A 2,5mm oven circuit? I'd like to avoid an extra circuit as decorated the hall where CU is with only the 3 circuits in mind not the fourth, and also would need to buy more 2,5mm (which comes in 100m rolls!). I'll never get an electric hob (bottled gas costs 16€ for 9 months!) and not put anythign else liek a dishwasher (no space) and I don't liek kitchen gadgets so only present set-up is important!

Appreciate the advice.


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PostPosted: Tue Apr 17, 2018 12:07 am 
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Diversity is not easy, for example it is likely you could switch on all lights, however the oven at 2.4 kW will start cycling on/off once warm.

There are tables in the on site guide as to what one should allow, however it has never been updated, so we have a new induction hob which puts all energy into the food not into the room so clearly must use less, however old 4 ring hob rated 8 KW and new one also nearly 8 KW so using on site guide no allowance is made for new one using less energy.

So looking at the RCD there are two ways to limit current, one is incoming so a 60A DNO fuse means no more than 60A or 3 x 16A MCB's means no more than 48A however both are larger than the 25A marked on RCD so that does seem wrong.

There are a number of ratings on an RCD, 30 mA refers to it tripping between 15 and 30 mA, 63A or in your case 25A is maximum switching current under normal conditions, and the 4.7kA is the current which it can switch under fault conditions.

Personally I like the ring final with 32A as it allows better for items switching on/off, the 20A radial or 16A radial are far easier to overload, the big book in the back suggests that every fixed item over 2 kW should have it's own circuit, I think with a ring final that is over kill, but with radials of 16A then maybe. However it is the person you signs the paperwork to say they have designed, there is no hard and fast rule.



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PostPosted: Tue Apr 17, 2018 9:52 am 
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Hi Eric,

I'm in Spain so I am the designer and the one who "signs it off". The 25A is the max power I can get from the company without them doubling the price of every unit of electricity! Due to this pricing strategy (and maybe as it's warmer so less need for power) it's quite normal to have such a "low" total and that fits with the 6mm tails I had fitted (2,5mm tails before!). The RCD is actually RCBO in English or whatever the one is that is a MCB and diferential in one at 25A but I have a seperate 25A main MCB where the tails come in before it just to be sure. So that all can't be changed and just whether I need a seperate oven circuit is all that's left to decide. No ring mains allowed either and for that I'm happier as a circuit is simply loop in loop out so, as long as chased and tubed correctly wiring a circuit is no more involved than changing new plug sockets.

When the UK regs say each appliance over 2kW needs its own circuit, is that a safety thing or just a convenience factor. Here each socket is rated 16A as is 2,5mm2 so is there anything dangerous about just having one 16A circuit for the 6kW potential and if it trips remember to not use the kettle and oven at the same time (though I have tried switching on fridge, kettle and oven from cold on full at the same time on the same circuit protected by a 16A now and it won't trip it so maybe the maximum's are never actually reached or just reached for the first second or so which isn't enough to trip the MCB?).

I'm thinking along the lines of as long as it's not dangerous not adding an extra circuit; I survived on one 15A without a trip for 6 months so 2x16A and 1x10A can only be better!


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PostPosted: Tue Apr 17, 2018 2:06 pm 
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Spain changes everything, and so does RCBO rather than RCD. I don't have a clue as to Spanish rules, I have worked abroad, but not Spain. It seems a bit like caravan wiring where each electrician has their own ideas. But basic rules.
Quote:
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

Note number (iii) if the MCB is too big then it will take out the 25A RCBO rather than trip just that circuit, when our RCD rules changed so we have 30 mA not 100 mA in the UK except for caravans and boats both which normally have 12 volt lighting, we have to have at least 2 RCD's as otherwise too easy to loose lights and sockets together, and if you have something faulty plugged in the last thing you want is to also then be in the dark.

I would use 16A RCBO's and a 25 amp MCB feeding them.


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PostPosted: Tue Apr 17, 2018 8:10 pm 
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Hmm I'm not sure if it's a RCBO or RCD - they sell the same RCD in 16A 25A and 40A so I assumed it was an RCBO...regardless as it protects all 3 circuits if it trips it will trip everythgin but the pre wired CUs they sell that which meet the latest regulations all only have 1 RCD protecting everything so maybe that's not a requirement here. I'm struggling to imagine a situation where if the power is cut to everything then no lighting would be dangerous? If the power is cut there shouldnt be any danger.

The regs are only for new build so essentially anyone can do whatever they like (and the PO has done so). I need to keep the 25A MCB as the companies do random inspections when you state your max power as that determines the price/kW so I can't make it 40A (and as the tails are 6mm then 25A is limit anyway).

So I'm just left to decide if I use the one circuit for the 6kW of appliances or a seperate for oven. It seems a bit pointless if there's no safety reason as with o nly 25A in total it may well trip occasionally when all combined anyway


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PostPosted: Wed Apr 18, 2018 6:58 pm 
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In my house the stairs are in the centre of the house, so with a power cut it would be easy to trip on stairs and so be injured, also the loft, no outside light so losing light would present a danger.

So I assessed the risks, and for loft decided best option was always take a torch with me, for landing and stairs I have an emergency light. I also have an emergency light in garage so I can see the consumer unit to reset it. The number of times I have gone into the garage to reset, then had to switch it all off again and switch on garage light then reset a second time I have lost count.

Mother house I also have an emergency light, however a more simple setup, it is a plug in light from Lidi which is triggered by movement or power loss, so it both lights the way when we open the door, and lights when power cut right above where the meter and consumer unit is under the stairs.

I personally think splitting the RCD because of safety also does not make sense, emergency lights are far better. However I have had an unknown fault which took more than an hour to find, lucky I have two RCD's so while looking I used an extension lead for fridge/freezer so nothing defrosted. We are told if you don't open the door a freezer is OK for around 8 hours, however my freezers when there is a power cut show the temperature inside when power came back until the door is opened. And with a near empty freezer the temperature rose to -4 degs C within an hour, two hours and the food needs eating or dumping.

OK with a cheat freezer full then maybe it will keep cool for 8 hours or more, but that is not true for an upright model. As said with caravans and boats there are battery lights anyway, and the caravan fridge will run from gas, however that does not apply to a boat, but even in a boat often 12 or 24 volt fridges and freezers are used.

Unless you fridge/freezer like mine actually shows there has been a power cut, and the temperature it reached, it is very easy for the power to be returned and for the home user not to realise the food has defrosted and frozen again, ice lollies may show what has happened, but fish fingers and beef burgers show little or no outward signs. So you may get ill 6 months after the power cut, and you would not have a clue what had happened, so yes there are dangers with one RCD however only you can assess the risks, and decide if anything needs doing.

I have woke up in the night and seen the landing light on, so got dressed and went outside to reset the RCD. Had the landing light not been on, I would have not realised there was a power cut until morning.



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