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.