Cable sizing

[grasshopper]

Hi

Can somebody help me with this please
Looking at the table in OSG. It states if I'm reading it right. That the ratings apply to insulation with thermal conductivity greater than (^)0.04 Wm-1K-1
So if 0.039 W/M2K = thermafloc loose
0.044 W/M2K= superglass
0.035 W/M2K= Rock wool
0.023W/M2K= Kingsland
0.022W/M2K=King thermawooll
Do cables in lofts within these types of insulation require upsizing for ccc.
2.5mm ring circuit wired through loft.

 

[i=p/u]

Yes you add in the factor to the calculation.

 

[i=p/u]

 

[cliffed]

And can you see anyone actually installing it,never mind connecting it

 

[buzzlightyear]

them worked out examples dont work out. .

 

[pc1966]

In reality you are unlikely to know the actual thermal resistance values of a given installation, so it is usually going to be a choice between methods 100-103 (page 74 OSG) for derating factors.

For RFC use Table 7.1(i) on page 65 of the OSG as your ready-reckoner, and you can see the length-limits and allowed installation methods to decide if it is OK for 2.5mm or 4mm needed.

Personally I would always go with the "no RCD" column for length limits as that allows adequate disconnection on the OCPD side alone, which is more reliable than depending on the RCD aspect.

TL;DR Use 4mm if a significant length (say over a meter or so) is routed in 101/103 method.

 

[pc1966]

Forgoet to mention, those methods 100-103 are illustrated on the opposite page (75) of the OSG which helps a bit with understanding the meaning.

 

[pc1966]

Also to add that unless you are above the length-limit for 2.5mm as well, if you only have one section in that method you could use 4mm just for that. Depends to some degree on what you have stock of, as it might be easier to buy a reel of 4mm for a job and simply use it everywhere.