2.5mm ring main and Celotex Insulation

[Roger Perkin]

I am building a garden office and currently have 25mm Celotex in the walls. I am running the ring main around and then want to add some more Celotex.

My question is how much of an air gap do I need to leave for the ring main?

 

[The Ghost]

It is not so much how much air gap as what is the drop in current carrying capacity. Which in the case of 2.5mm² drops down to 17a (ish) in insulation thus reducing from 54a to 34a. How much load do you think is going on to the ring final circuit. I would say ring main but we have a pet rottweiler who will get cross if we do, he is right though!

 

[westward10]

The OP has two threads running for this project.

 

[Ian1981]

The ccc of the live conductors ( iz) should not be less than 20amps in each leg of the ring final circuit.
See regulation 433.1.1 and 433.1.204

 

[davesparks]

You shouldn't leave gaps in the insulation layer due to it allowing condensation to form in the cold voids

 

[Octopus]

Best you read his other thread before contributing....

 

[Ian1981]

Best you read his other thread before contributing....

Noted

 

[ruston]

Must of got SOME of the advice but not all lol.

 

[The Ghost]

should not be less than 20amps

Interesting will check that (again) but umm well if the 2.5 is doubled up how do you read that? as in yes its drops to 17a ish but as it is doubled up it is 34a yes? No? Maybe, don't know...

 

[westward10]

Interesting will check that (again) but umm well if the 2.5 is doubled up how do you read that? as in yes its drops to 17a ish but as it is doubled up it is 34a yes? No? Maybe, don't know...

Any conductor in a 32/30A ring final circuit should not be rated at less than 20A.

 

[davesparks]

Interesting will check that (again) but umm well if the 2.5 is doubled up how do you read that? as in yes its drops to 17a ish but as it is doubled up it is 34a yes? No? Maybe, don't know...

No, the ccc of the conductor used to form the ring should not be less than 20A.
The cable is not doubled up, it is in a ring, the difference is subtle but it is different

 

[Roger Perkin]

OK so I need to ideally not leave any air gaps in the insulation but need to run the ring circuit in there - total load on the cable will be a laptop, 2 monitors, and a 1kw heater and iphone charger - that's it

 

[davesparks]

OK so I need to ideally not leave any air gaps in the insulation but need to run the ring circuit in there - total load on the cable will be a laptop, 2 monitors, and a 1kw heater and iphone charger - that's it

What made you choose a ring circuit for a single office? It seems a bit over the top to me.

 

[Roger Perkin]

I thought it had to be a ring. It was only another 4m of cable to run it back no big deal - but just need to find out if I can bury the cable in the celotex or if I need to leave air around it?

 

[Madocks]

Im assuming your talking about running cable in the cavity? where you have inserted celotex?

There is a requirement in the Building Regulations 2000, Approved Document C, 2004 Edition, Section 5: Walls, paragraph 5.13 b state that no cables other than electricity meter tails are to be located in the cavity of an external wall.

 

[davesparks]

I thought it had to be a ring. It was only another 4m of cable to run it back no big deal - but just need to find out if I can bury the cable in the celotex or if I need to leave air around it?

Which regulation did you get that from?
That is up to you as the designer of the installation to decide, the insulation around the cable will affect the size of cable you use but you will need to do the calculation to work out how much bigger the cable will need to be.

 

[telectrix]

personally i'd run 2 radials. 1 for the heater ona a FCU, and 1 for the sockets. each off a 16A MCB.

 

[Madocks]

personally i'd run 2 radials. 1 for the heater ona a FCU, and 1 for the sockets. each off a 16A MCB.

what about lights?

 

[telectrix]

what about lights?

OP has not mentioned a lighting circuit. i assume that is a separate issue.

 

[Ian1981]

what about lights?

Depending on what the total load is then a 6amp mcb using 1.0mm twin should be fine.
The lowest ccc for 1.0mm is 8amps using table 4D5 depending on other correction factors