Sub board from sub main

[nottz_spark]

Hi,
I have quoted up a job and would like to know if anyone can see anything wrong with my design.
From the 100A main cut out the tails go to an 80A switch fuse and out of there to consumer unit.
I need to run a sub main to provide another consumer unit upstairs,whiere there is a possibility of a further consumer unit if the client decides to make a 3rd flat. So my plan is...


1. remove tails from incoming side of 80A switch fuse and put in double pole henley block.

2. run tails from henley block back into switch fuse.

3. take additional feed from henley block into 63A switch fuse for sub main

4. run mains from out of switch fuse into new consumer unit upstairs.

all cable calculations have been done and comply for shock and overcurrent protection,so the main question i am wondering if it would be acceptable to install a 2nd 63A switch fuse for an additional ( 3rd ) consumer unit , either by henley blocking from 63A submain or from the henley block in position at the intake..( the 2nd consumer unit is already protected by a 63A switch fuse ).....i know there would then be no discrimination between the 2 63A fuses but something is telling me it will be ok as the DNO's main fuse is protected,hence it is a matter for the client......but i would invite the resident experts to ave a look at my plan and let me know if it is ok?

 

[Octopus]

3 flats off a standard single phase supply - really?

 

[Guest55]

3 accommodation units
3 switchfuses feeding 3 CU's
seperate tails from henly blocks

if youre doing it like this then it will work fine.
not sure where your discrimination issues are as none of the switchfuses are in series ?

 

[nottz_spark]

i assure you the cable calculations have been done and a seperate supply was my first suggestion but as usual the finances mean looking for an alternative to a new supply. original board is using less than 50 Amps and sub board will require approx 35 amps after diversity has been applied, so do you think this is a bad idea?

 

[Octopus]

Where are the 3 seperate meters going to be fitted, or is this a HMO type set up?

 

[Guest55]

if there are gas boilers for heating & hot water then you should be ok
if its all electric heating , HW & power then no chance will 1 x 100A fuse stetch to 3 flats

 

[nottz_spark]

3 accommodation units
3 switchfuses feeding 3 CU's
seperate tails from henly blocks

if youre doing it like this then it will work fine.
not sure where your discrimination issues are as none of the switchfuses are in series ?

i was under the impression dicrimination meant the local fuse operating as opposed to a fuse upstream?

 

[nottz_spark]

Where are the 3 seperate meters going to be fitted, or is this a HMO type set up?

there wont be 3 seperate meters just potentially 3 seperate consumer units...maybe you got the impression of seperate meters by me describing them as flats,that is just how i am visualising the job but in reality it will not be seperate for renting out purposes so 1 meter and 1 bill is sufficient.

 

[Guest55]

the only fuse upstream of the switchfuses is the service fuse and there is no requirement to discriminate with dno devices.

and so long as there are no mcb's bigger than 40A in the CU's , then you will have discrimination with the 63A switchfuses.

 

[nottz_spark]

the only fuse upstream of the switchfuses is the service fuse and there is no requirement to discriminate with dno devices.

and so long as there are no mcb's bigger than 40A in the CU's , then you will have discrimination with the 63A switchfuses.

if as you say there is no requirement to discriminate with dno fuse then i have made a mistake in believing there was a requirement to do so,or maybe its good practice,as opposed to a requirement that you discriminate sub with main fuse?

 

[telectrix]

i'd do as you say for DB2, but if and when you come to fit DB3, feed the tails from the henly blocks at the intake with a 2nd 63A switch fuse. then you won't have 2 63A fuses in series. and as biff says, max. 40A MCBs in DB2 and DB3.

 

[nottz_spark]

i'd do as you say for DB2, but if and when you come to fit DB3, feed the tails from the henly blocks at the intake with a 2nd 63A switch fuse. then you won't have 2 63A fuses in series. and as biff says, max. 40A MCBs in DB2 and DB3.

that is the logical way forward,thank you for your help guys.

 

[Knobhead]

As you’ve applied diversity to the two sub boards loading and added to a real reading of 50A for the main board. You’re in trouble (in theory).

The thing I love about diversity is it’s the biggest load of codswallop out! BS88’s are about as subtle as a brick, that’s why I like them as well.

 

[nottz_spark]

As you’ve applied
diversity to the two sub boards loading and added to a real reading of
50A for the main board. You’re in trouble (in theory).

The thing I love about diversity is it’s the biggest load of codswallop
out! BS88’s are about as subtle as a brick, that’s why I like them as
well.

diversity has been applied correctly and not in the manner you have incorrectly presumed

 

[Engineer54]

diversity has been applied correctly and not in the manner you have incorrectly presumed

What method of diversity have you applied to these separate CU installations??
I think Tony meant, that applying diversity is often a hit or miss affair, and unless you have many years under your belt in the industry, it'll be more often a miss than a hit!! The maximum demand diversity formulas you'll find quoted in BS7671 are the next best thing to being totally useless!!