Rcd vs distributors fuse

[DuncanS]

Here is a little cunumdrum that is probably quite simple. Uk network are coming to upgrade their fuse from a 60A to a 100A so that we can put in another mini consumer unit for a new shower and cooker... The existing consumer unit which serves the house is protected by an 80A RCD with 30mA trip, my question is, should we replace the RCD with a 100A 30mA one or is it satisfactory as the load will be split once the mini consumer unit has been installed? My understanding is the existing RCD can only handle a load of 80A and if the distributors cut out is 100A then we run the risk of burning out the RCD.

 

[telectrix]

no. the RCD will still only handle the load of the existing CU. the shower ande cooker circuits will be through a second CU with it's own RCD/s to handle the extra loads.

 

[Rockingit]

Given that both of those circuits and the fitting of a new consumer unit are notifiable under Part P, I'm a little surprised that your electrician deemed the existing 60A supply to be insufficient, given diversity.

 

[DuncanS]

Given that both of those circuits and the fitting of a new consumer unit are notifiable under Part P, I'm a little surprised that your electrician deemed the existing 60A supply to be insufficient, given diversity.

Accounting for for diversity we are already right on the limit for 60A (it's a big house!) and the new shower is 9.5kw so that's a good 40A extra demand, fairly sure that diversity doesn't apply to a water heater of this kind, then if you apply diversity to the new cooker of 10.2kw that's gonna be around 35A load

 

[Risteard]

There are no hard and fast rules for diversity. You need to work out what you might realistically be using at the same time as using the shower and the cooker. The shower load will only presumably be on for a few minutes at a time. A cooker will cycle on and off.

 

[DuncanS]

So the consensus is that the existing consumer unit with 80a RCD is ok? What about the consumer unit tails? Also fine for those to stay as 16mm, obviously if we put in a 100RCD we'd need 25mm consumer unit tails, but 16mm on the existing one is all good?

 

[Rockingit]

Accounting for for diversity we are already right on the limit for 60A (it's a big house!) and the new shower is 9.5kw so that's a good 40A extra demand, fairly sure that diversity doesn't apply to a water heater of this kind, then if you apply diversity to the new cooker of 10.2kw that's gonna be around 35A load

Oh dear oh dear. Who has worked this out for you, or have you tried to do this for yourself?

 

[DuncanS]

Oh dear oh dear. Who has worked this out for you, or have you tried to do this for yourself?

Typing without thinking, don't worry I can handle the maths cooker will be around 28A and the shower around 40 give or take an amp or so.

 

[Risteard]

I only make the cooker out as just above 20A using the standard formula. Not sure where 35A comes from?

 

[Plonker 3]

, then if you apply diversity to the new cooker of 10.2kw that's gonna be around 35A load

How did you come up with this figure?

 

[Plonker 3]

What other appliances are in the house?

eg showers, boilers, water heaters etc

 

[telectrix]

What other appliances are in the house?

eg showers, boilers, water heaters etc

what current does a mother-in-law draw?

 

[DuncanS]

I only make the cooker out as just above 20A using the standard formula. Not sure where 35A comes from?

Well according to this hand on site guide I'm looking in its 10a + 30% of the full load + 5a for the socket attached.... That if we're going down the road of calculus is 10200/230 = 44.35
30% of 44.35 = 13.3
13.3 + 10 + 5 = 28.3.....
Cant fault my maths

 

[Rockingit]

Typing without thinking, don't worry I can handle the maths cooker will be around 28A and the shower around 40 give or take an amp or so.

It's not about the maths, it's about the principle behind it, plus the fact that I'd put a pint on it that the fuse is being changed unnecessarily. I have a property where the mathematical diversity works out at something like 220A, and it's running quite happily on an 80A DNO fuse.

I'm also amazed that the DNO are changing the fuse from 60A to 100A as 90% of modern meters are only rated at 80A as well.

BTW, a 60A DNO fuse will carry 100A for roughly 3500 seconds before it blows.....do the maths on that.

 

[telectrix]

Well according to this hand on site guide I'm looking in its 10a + 30% of the full load + 5a for the socket attached.... That if we're going down the road of calculus is 10200/230 = 44.35
30% of 44.35 = 13.3
13.3 + 10 + 5 = 28.3.....
Cant fault my maths

WRONG. it's 10A + 30% of remainder i.e 30% of 34.35 + 5A = 10 + 10.3 +5 = 25.3A

 

[Risteard]

Well according to this hand on site guide I'm looking in its 10a + 30% of the full load + 5a for the socket attached.... That if we're going down the road of calculus is 10200/230 = 44.35
30% of 44.35 = 13.3
13.3 + 10 + 5 = 28.3.....
Cant fault my maths

Oh but I can. Firstly, you never mentioned a socket outlet incorporated in the cooker switch!

Secondly, you never deducted the 10A before calculating 30% of the remainder. You just added it back on giving an excessively high figure!

 

[trev]

what current does a mother-in-law draw?

It depends on whether she's in full flow or just a slow constant nag.

 

[tony mc]

You need to do your maths again re the cooker as its wrong !

 

[Meggerman]

It's not about the maths, it's about the principle behind it, plus the fact that I'd put a pint on it that the fuse is being changed unnecessarily. I have a property where the mathematical diversity works out at something like 220A, and it's running quite happily on an 80A DNO fuse.

I'm also amazed that the DNO are changing the fuse from 60A to 100A as 90% of modern meters are only rated at 80A as well.

BTW, a 60A DNO fuse will carry 100A for roughly 3500 seconds before it blows.....do the maths on that.

Loading and diversity is part of regulations so you can't knock him for doing it by the book. In reality, some people have 3 electric showers, but don't run them all together.
You are saying your house is 220A with diversity applied, what have you got there??

 

[DuncanS]

Ok we've gone of subject a bit here I appreciate the diversity situation believe me, my question is not about diversity. I simply need to know if my sparky will need to upgrade my existing consumer unit from 80a RCD to a 100A rcd, bearing I mind uk network are upgrading their fuse to 100A because we are having a granny annex fitted with a shower and a cooker, uk network made the discussion to do this as when the annex is finished we will have 2 cookers, 3 showers, 3 ring final circuits which have significant demand due to the 2 living rooms with lots of gadgetry, a garage workshop, a summer House, we also have 3 lighting circuits, a jacuzzi and all manner of ponds and fish tanks all drawing a significant current most of the time.... Please enough about diversity.

 

[telectrix]

praps he's got 2 mothers-in-law. :lol::21:

 

[Risteard]

Loading and diversity is part of regulations so you can't knock him for doing it by the book.

Figures quoted in the OSG are not part of the Regulations. Diversity requires an assessment of the load - there is absolutely and certainly no requirement to use the figures quoted in the OSG!

 

[DuncanS]

You need to do your maths again re the cooker as its wrong !

I challenge you on that, how is it wrong?

 

[Plonker 3]

Ok we've gone of subject a bit here I appreciate the diversity situation believe me, my question is not about diversity. I simply need to know if my sparky will need to upgrade my existing consumer unit from 80a RCD to a 100A rcd, bearing I mind uk network are upgrading their fuse to 100A because we are having a granny annex fitted with a shower and a cooker, uk network made the discussion to do this as when the annex is finished we will have 2 cookers, 3 showers, 3 ring final circuits which have significant demand due to the 2 living rooms with lots of gadgetry, a garage workshop, a summer House, we also have 3 lighting circuits, a jacuzzi and all manner of ponds and fish tanks all drawing a significant current most of the time.... Please enough about diversity.

and what readings did you get when you clamped the exiting installation?

 

[Risteard]

I simply need to know if my sparky will need to

Will he/she not know what he/she will need to do, or am I missing something?

 

[telectrix]

all your sparky will need to know is whether your good for the payment. he/she's been to college for 3 years so you don't need too worry about the technicalities. i managed to blow suppliers fuse with a new cooker and hob, but then again it was a 30A 3036, fitted in 1926.

 

[Risteard]

I challenge you on that, how is it wrong?

See post #16.

 

[DuncanS]

Oh but I can. Firstly, you never mentioned a socket outlet incorporated in the cooker switch!

Secondly, you never deducted the 10A before calculating 30% of the remainder. You just added it back on giving an excessively high figure!

Sorry mate but OSG doesn't say to do that, what it says 10a + 30% fl of connected appliances in excess of 10a + 5a if s/o is incorporated....

 

[DuncanS]

Oh but I can. Firstly, you never mentioned a socket outlet incorporated in the cooker switch!

Secondly, you never deducted the 10A before calculating 30% of the remainder. You just added it back on giving an excessively high figure!

But the on site guide doesn't say anything about 30% of the remainder it just says 10a +30% of full load + 5a if socket incorporated. I can read and here it is right in front of me Appendix A page 111.

 

[Risteard]

Sorry mate but OSG doesn't say to do that, what it says 10a + 30% fl of connected appliances in excess of 10a + 5a if s/o is incorporated....

Are you absolutely positive?

 

[KFH]

On my house I worked out diversity using the OSG at over 120A it has run successfully fo r > 30 years on a 60A fuse. This included a 9kW pool heater that was left on for long periods of time (Weeks unfortunately for my wallet) as well as 9kW shower, pond pumps, computers, the odd heater, oven, microwave, immersion heater and kettles.

If the 80A RCD has been working happily on a 60A supply fuse it will continue to do so quite happily on a 100A fuse as the load going through the RCD has not increased unless you change something.

Should you not be asking your electrician what he recommends and if you trust him/her taking thier advice. If you have worries get a clamp meter on the supply to see what you are actually using.

 

[DuncanS]

Are you absolutely positive?

Unless I've been stuck by selective blindness.... Check it out page 111

 

[Risteard]

Unless I've been stuck by selective blindness.... Check it out page 111

My OSG is nearly 100 miles away so I'm afraid I'll have to check later (or allow someone else to). Unless they have introduced an error in the most recent incarnation of it, however, I can assure you that it doesn't say that.

 

[GMES]

Page 110 osg household cooking appliances
1st 10amp of the rated current plus 30% of remainder plus 5% for socket outlet.

 

[Octopus]

Here is a little cunumdrum that is probably quite simple. Uk network are coming to upgrade their fuse from a 60A to a 100A so that we can put in another mini consumer unit for a new shower and cooker... The existing consumer unit which serves the house is protected by an 80A RCD with 30mA trip, my question is, should we replace the RCD with a 100A 30mA one or is it satisfactory as the load will be split once the mini consumer unit has been installed? My understanding is the existing RCD can only handle a load of 80A and if the distributors cut out is 100A then we run the risk of burning out the RCD.

So who is "we" then? I'm guessing that you are still sparking.

 

[Andy78]

Ok we've gone of subject a bit here I appreciate the diversity situation believe me, my question is not about diversity. I simply need to know if my sparky will need to upgrade my existing consumer unit from 80a RCD to a 100A rcd, bearing I mind uk network are upgrading their fuse to 100A because we are having a granny annex fitted with a shower and a cooker, uk network made the discussion to do this as when the annex is finished we will have 2 cookers, 3 showers, 3 ring final circuits which have significant demand due to the 2 living rooms with lots of gadgetry, a garage workshop, a summer House, we also have 3 lighting circuits, a jacuzzi and all manner of ponds and fish tanks all drawing a significant current most of the time.... Please enough about diversity.

Your thread 7 weeks ago quoting this exact situation made out you are using this job for your NAPIT assessment. Now you say you have a sparky in doing it for you. Which one is it ?

Also to get this element out of the way, table A1 on page 110 of the OSG says to use 10A + 30% of the remainder. Table A2 on page 111 of the OSG says 10A + 30% of full load.

 

[DuncanS]

Page 110 osg household cooking appliances
1st 10amp of the rated current plus 30% of remainder plus 5% for socket outlet.

I'm sorry but it just doesn't say that.... I tried to upload a picture of the page but Im not sure it worked. Here is the exact quote from the osg printed in 2013... "10a + 30% f.l. Of connected cooking appliances in excess of 10a + 5 a if socket-outlet is incorporated in control unit" I'm not sure how you guys interpret that as all the variations I've so far seen....

 

[Risteard]

in excess of 10a

And that, my friend, is where it says it. In excess of 10A means not including the first 10A.

 

[DuncanS]

And that, my friend, is where it says it. In excess of 10A means not including the first 10A.

No according to English in excess means, when more than, not not including.

 

[Risteard]

No according to English in excess means, when more than, not not including.

So what does 30% of the load exceeding 10A mean then? It means not including the 10A. The first 10A does not exceed 10A.

 

[telectrix]

i'll grant to duncan the fact that the osg is poorly worded. proof of the pudding will be to fit the bloody cooker. turn it on full whack, then clamp it. after initial 1 minute warm-up, bet no more than 21A ( excluding socket outlet.)

 

[Risteard]

i'll grant to duncan the fact that the osg is poorly worded.

I would agree that it is unnecessarily confusing also.

 

[Rockingit]

But all this is smoke and mirrors, tbh. The OP is clearly trying to do the work himself and doesn't have a clue. The only upside is that he's probably had a bill from the DNO for something that he didn't need to do.

 

[Rockingit]

You are saying your house is 220A with diversity applied, what have you got there??

Not mine, a customers place. Has a 12kW ground source system, four outbuildings, a private water treatment plant, American white goods, illuminated grounds.... and so on. I think the sum total of all the breaker values across 4 DB's is something like 600A. And it's been running very happily on a standard 80A fuse coming from a 22kva pole TX. I've never seen the whole lot pull more than 40ish amps when clamped.

 

[GMES]

I'm sorry but it just doesn't say that.... I tried to upload a picture of the page but Im not sure it worked. Here is the exact quote from the osg printed in 2013... "10a + 30% f.l. Of connected cooking appliances in excess of 10a + 5 a if socket-outlet is incorporated in control unit" I'm not sure how you guys interpret that as all the variations I've so far seen....

I'm sorry but it does say what i said on page 110 you are reading page 111. i am sat looking at it now , 2 diferent tables as Andy as already pointed out a few post's back. Now if your's doesnt say that on page 110 then you have a fake copy there are plenty out there.

Can someone also look this up and confirm that on page 110 it states 30% of remainder
No excuses you should all have your books with you at all times. :toetap05::toetap05::toetap05:

 

[telectrix]

that's not a problem . all he needs to do is buy a fake cooker.

 

[Octopus]

The rules and guidance for calculating diversity are well out of date and need a major rethink. In years gone by the average house had 4-6 fuses.

New builds now can have upward of 10 and my own refurbishment will have 12 when complete. Our house has gas heating and I estimate the maximum demand to be approx. 35A!

 

[telectrix]

when 'er indoors is not cooking , showering, or doing her hair, our house happily draws a massive 3-4A.

 

[GMES]

It's ok for you Tel but our lass as taken out 3 100amp main fuses in a row with a toy of some discription that is to powerful for batteries, I am still none the wiser to what it is.

 

[Engineer54]

Please enough about diversity.

Think you need another 5 weeks at that training centre, because you clearly have no understanding of diversity, or accessing the max demand of an installation. You sure aren't listening to any of the good advice from the more experienced and reputable members here either....

Why you are considering a mini CU for a cooker and a shower is a bit suspect too, especially if your existing CU has only One RCD covering the whole house... Get it changed out for a dual 17th CU that can accommodate these two additional circuits!!