Rethought process

ElectricEddie · Aug 4, 2015

With hindsight I can see how my previous thread and questions were utterly unthoughtout with regards to my design process and I thank admin for shutting it down, but the reason I have come on to here is for help as I have previously read through a bunch of threads on here and it seems as if it's full of knowledgeable people. As admin quite rightly pointed out I am inexperienced but I am extremely determined and believe it or not quite intelligent. :

: And I am in the position where this installation is down to me so I need to make sure I do it right.

so I have been doing some thinking and calculations and I have realised that apart from being not properly protected the 16mm te + 16mm Earth combo was the incorrect cable choice and total overkill so I'd like to get that out the way straight away and I'll be pulling it out tomorrow.

now I have calculated the maximum demand for each of my sub main consumer units and because of the way that I have segregated them they will all be around the 35-40a mark apart from the kitchen one which will obvs be a bit higher due to having the cooker, washing machine etc on it I will calculate them 100% tomorrow on site.

according to the ccc chart and my method of installation 6mm/10mm swa will be sufficient dependant on the exact calculations (as it's sort of borderline at that ampage) to power each submain am I right in thinking then in that case that I could run those to a 4 way distribution board with a mains isolator on the incoming side and install each line conductor into a 40a(dependant on exact max load calcs) mcb and then use my split load boards for each of my final circuits which will be rcd protected.

I hope now this is a more sensible/professional properly calculated method and I welcome any constructive input/criticism.

thanks again

Ed

davesparks · Aug 4, 2015

You will not achieve any discrimination using MCBs as the OCPD of the submains
You need to calculate the cable size properly, not just look in the ratings tables. The values in those tables are intended to be used with your required ccc after application of de-rating factors.

And it's not ampage, it's current!

Leesparkykent · Aug 4, 2015

http://www.napitonline.com/downloads/CP 2 07 P 10-11 16th Cable Sizing 1 of 2.pdf

Leesparkykent · Aug 4, 2015

Fuses are frequently used in conjunction with MCBs because they have a higher short-circuit breaking capacity. However, the two devices present quite different time-current characteristics. By comparing the curves for the two devices, it is possible to determine whether full overload discrimination is achieved or, if not, at what levels of overload current it is achieved.
Short circuit discrimination involves comparison of the total let-through energy and pre-arcing energy for the prospective fault level concerned. Discrimination is achieved if the total let-through energy of the MCB is less than the pre-arcing energy of the fuse, at all values of fault level.

ElectricEddie · Aug 4, 2015

I get it ! thanks

GEOFF S · Aug 4, 2015

ElectricEddie said:
With hindsight I can see how my previous thread and questions were utterly unthoughtout with regards to my design process and I thank admin for shutting it down, but the reason I have come on to here is for help as I have previously read through a bunch of threads on here and it seems as if it's full of knowledgeable people. As admin quite rightly pointed out I am inexperienced but I am extremely determined and believe it or not quite intelligent. :: And I am in the position where this installation is down to me so I need to make sure I do it right.

so I have been doing some thinking and calculations and I have realised that apart from being not properly protected the 16mm te + 16mm Earth combo was the incorrect cable choice and total overkill so I'd like to get that out the way straight away and I'll be pulling it out tomorrow.

now I have calculated the maximum demand for each of my sub main consumer units and because of the way that I have segregated them they will all be around the 35-40a mark apart from the kitchen one which will obvs be a bit higher due to having the cooker, washing machine etc on it I will calculate them 100% tomorrow on site.

according to the ccc chart and my method of installation 6mm/10mm swa will be sufficient dependant on the exact calculations (as it's sort of borderline at that ampage) to power each submain am I right in thinking then in that case that I could run those to a 4 way distribution board with a mains isolator on the incoming side and install each line conductor into a 40a(dependant on exact max load calcs) mcb and then use my split load boards for each of my final circuits which will be rcd protected.

I hope now this is a more sensible/professional properly calculated method and I welcome any constructive input/criticism.

thanks again

Ed

Hi Ed

Design order

Assessment of general characteristics, Loads, external influences, earthing, diversity,maximum demand etc.

Installation circuit arrangements, number of DB, number of circuits, ratings, load, etc

Volt Drop, calculate VD from origin to load

Protection against over current, ensure cables and equipment are suitably rated

Protection against electric shock, ensure disconnection times are met

Isolation and switching, ensure appropriate devices are installed for isolation maintainance

Follow that sequence and you will be fine.

Sit down and go throug all the stages, tehn post your design and we can all go over things.

Cheers