Cutting Hole For Back Box In Celotex/Equivalent

[Bobbya82]

Hi,

I have a number of socket holes to cut in Celotex equivalent insulation. I was just wondering is it ok to have the sockets sat tightly into the insulation around the back box and at the back? Or should a gap be left around it and at the back?

Also what size pattress back box Is most commonly used?

I was thinking 35mm, so I was going to cut a hole to the depth of 35mm and then drill a hole through the insulation to pass the cables. Once the 12.5mm plasterboard is fitted there will be a small gap at the back.

Just wasn’t sure if regulations specify a gap around switch sockets cables and insulation?

Many thanks

 

[oldbutnotdead]

Backboxes are fine, if you allow for thickness of skim & pb you only need to cut a 20mm divot in the insulation.
Cables through insulation is more of a sticking point- chemically you're ok with pir (unlike polystyrene) but you may need to derate them (or go one size larger) depending on how much insulation you're going through

EDIT 35s are fine, easier to keep a bit of slack for installation. 50s can be handy for lots of data in 1 box)

 

[Bobbya82]

Excellent many thanks, appreciate it.

 

[flameport]

I was thinking 35mm, so I was going to cut a hole to the depth of 35mm

Cut out 20mm, the plasterboard and skim will take up the other 15mm.
Backbox edge should be level with the face of the wall, not recessed into it.