8mm but you need a 10mm hole with a bit of the internal part of the hole being widened to 12 mm for the grey part that the white cap fits onto.
I had no idea until the engineer arrived that the Openreach FTTP devices were powered and that the leads were very short. Nowhere does BT or Openreach specify this crucial information. I am furious, especially because the engineer left without saying that our BB would be cut off anyway, so we had no BB - with important Zoom meetings imminent. (Fortunately an EE emergency box has arrived) Had he told me, I’d have rigged up a temporary extension cable, pending a concealed spur feed to a new double socket very close to the location of the new kit.
I had to cancel the appointment, and have spent a day so far running a spur cable, hacking out a back box. Another few hours connecting the spur, and fitting a panel to conceal the spur (white conduit in the pic), and, in due course, the new FTTP cable.
The current cable enters via a hole about 10mm. I hope that the FTTP conduit fits.
How flexible is the FTTP cable from the internal entry point to the new kit? Once it’s exited the white circular cover pictured above (thanks for that - very helpful) it will just need to bend quite gently before following the same path as the current cable.
My install was not flush to the wall, so looks like it is hanging off. I've never been happy about it. A bit slap-dash.
I tried to upload a pic of ours but the file size was too large.
Anyway, apart from our cap being white & the ONT being white & smaller, the actual quality of the installation looks like this.
I assume that the engineer who turned up with two devices requiring had the top one on this page - the modem and the battery pack.
https://www.bt.com/help/broadband/whats-an-openreach-modem-ont
I think this socket should be close enough lol.
that link is an old ONT and there is no longer a battery back up the 2 sockets are for ONT and the SH2
ONT looks like this about 4" square
Openreach haven't used battery back-up packs for some time, but they say a double socket's required in case the router is going next to the ONT.
This is the ONT that Openreach use now.