Submain

[Spaced]

Hi there, just after a little advice. I have been asked to replace just the kitchen ring in a flat in a converted hotel. The problem is that each of the 12 flats is supplied from a distribution room 2 floors below. My initial thought was to run a submain in25mm SWA to a 17th Ed CCU via a DP isolator. However upon closer inpection, one flat appears to have recently been re-wired in 16mm flat t&e directly from a Henley. The run length is approximately 12M and will be clipped direct. Would this be an acceptable alternative if a dp isolator is used and would you consider it advisable to use an RCD (100mA)

Kind Regards

Chris

AC Electrics

 

[Spudnik]

Hi Chris,

Depends on the route of the submain.

If it has been concealed in a wall less that 50mm and isnt mechanically protected then the RCD needs to 30mA.

Clipped direct does not need RCD protection but you will need to install a switch fuse at the front end and not a DP isolator.

 

[Spaced]

Thanks Jason, excellent. The complete run will be clipped direct via a riser. Just to clarify, 16mm T & E clipped direct with an 80A switched fuse on the front end?

I'm having a nightmare with this tonight, does the system log you out after a set period?

Regards
Chris.

 

[old dog]

swa is a better job but if you do use t&e run a 16mm earth with it

 

[BillR3]

Hi Chris,
There are many governing factors to be considered when carrying out this sub main installation. Obviously characteristics of the supply origin and assessing the maximum demand for the flat accommodation load. From these figures your design current will have to be calculated an appropriate protection device selected and a cable that will offer you a sufficient capacity for the existing installation and an allowance for expansion in the future. The main obstacles in selecting the cable are the conditions in which it will be run and have been mentioned beforehand in this post but the most problematic C factor is thermal insulation as this can half the current carrying capacity of your selected cable so bear this in mind when selecting. If you are going to be installing a twin RCD board within the development containing 30ma and 100ma main controlling RCD's then this could be problematic with discrimination if the sub main is buried at less than 50mm in the walls on the run to the flat as this would also require 30ma protection. This means that the outgoing circuits being controlled by the 100ma main RCD would not trip and cause the trip at the supply source which in turn would kill the power to whole unit and opens up a series of new problems. If you were to select 16 T&E for your sub main supply you would have to check through adiabatic equation that your s min was capable of taking the fault current generated for the proposed load.
I am only trying to highlight the problems that you could encounter in carrying out this job without first investigating every aspect of the installation and it's design criteria. Everything that I have stated above can be interpreted in different ways and there are many approaches to completing this work.
Hope this helps.
BillR3

 

[Spaced]

Hi Bill, thanks for your reply. The complete run will be clipped direct and as Old Dog suggested the earth will be run in separate 16mm. The board will be a 17th ed but I wasn't planning to have an RCD at the source. I can't make any final calculations yet as the customer is still designing the spec of the kitchen.

Regards
Chris.

 

[daver]

Just nit picking, but your seperate earth does not have to be 16mm. As long as the CPC is = to or greater than the CSA of Line Conducter, then you are fine. You already have 10mm in the T&E (Table 54G)

But do your calcs first

Just trying to save some pennies

 

[chocolate]

My question is what else was connected to the Henley block ? My other question is whether the rewired flat has an electrical installation certificate ?

 

[old dog]

Just nit picking, but your seperate earth does not have to be 16mm. As long as the CPC is = to or greater than the CSA of Line Conducter, then you are fine. You already have 10mm in the T&E (Table 54G)

But do your calcs first

Just trying to save some pennies

hi you are correct however i dont use t & e 16mm as a rule but if i do i run 16mm
with it belt and braces and it stops the dno sucking his teeth and having to show him
it complies i like to keep it simple

 

[Spaced]

My question is what else was connected to the Henley block ? My other question is whether the rewired flat has an electrical installation certificate ?

Hi Chocolate, the Henley is part of the installation for another flat and all of the flats are privately owned. Is that what you meant?

 

[daver]

hi you are correct however i dont use t & e 16mm as a rule but if i do i run 16mm
with it belt and braces and it stops the dno sucking his teeth and having to show him
it complies i like to keep it simple

Fair Comment Old DOg