The obvious way to wire it is to use a multi-core as you suggest, with JBs at convenient locations, and 3 or 4 core flex to each light or sensor. You don't want to be taking all those cores into what can be very cramped terminal boxes on the devices. You could consider using surface boxes and putting a 2/3 pole switch on top that can isolate an individual device for troubleshooting.
On the other hand, while 8 lights may be sensible (I don't know what your property is like), 8 lights and 8 sensors does seem overkill and will mean a lot of connections - for 8 sensors on one cable, you'll be looking at 11 core cable (1 permanent line, 1 neutral, one earth, and 8 switched lines); for 8 lights you'll be needing 10 core cable (8 switched lines, 1 neutral, 1 earth). And at each JB you'll need to connect them all between the "in" and "out" cable on the JB as well as to the tee'd off circuit. Depending on how things are located, you could tee off more than one sensor or light from a single JB - less JBs and less connections to make.
As to cable sizes, you don't need 2.5mm cable from the CU - you only need whatever is appropriate to the total prospective load and the overcurrent protection. Typically a lighting circuit would only be 1.0mm² and on a 6A MCB; from memory for most installation methods, 1.0mm would be OK with a 10A MCB. If using 0.5mm² cable, then you'd need to fuse appropriately - but that might be OK on a 6A MCB (CBA to check in the books right now).
And you might want to fit some smaller rating MCBs in your relay cabinet to split the light and sensor supplies - otherwise a single fault anywhere in the system will take out all your lights. So, for example, you might split the sensors across two circuits, and the lights across two more - so a single fault could only take out half of them. You would want two pole switching devices, that way you isolate the neutrals as well which means you can close an RCD in the event of a N-E fault in one part of the circuit - but you do then need to be very careful to avoid circuits with a live on one circuit and the neutral on another as that would be dangerous. But it does increase the amount of cabling needed.
And as already mentioned, make a detailed drawing - give every "circuit"* a number, number all the wires to match, and make sure that a copy of the drawing is where anyone needing to work on the system can find it.
* In this case, circuit means one or more wires electrically connected. So "5" could be a wire that loops from one relay, to another, to another, to a terminal, and then out through the cabling to more than one light.
The one thing that's guaranteed to have someone scratching his (or her) head and ripping stuff to bits till nothing works (and it can't be put back together) is to have a box full of a gazillion brown wires, none of which are labelled, and which have at least 17 (1 permanent live, 8 sensor switched lines, 8 switched lines out to lights) different functions.
Hi and thanks for all of your advice,
!
I think your post is agreeing to most of my idea with the following changes...
I will split the lights and sensors into 2 circuits as you suggest, thanks, this will allow for a circuit fault without losing all lights / sensors and will also allow me to isolate from the relay enclosure one of the circuits.
I will ensure that it is very clear as to which are on each circuit to avoid any confusion.
I will use multi coloured multi core cable and label accordingly.
I have detailed below each JB and the enclosure details which I believe should be OK if I am OK to join into the multicore as described to minimise joins?
So for each circuit....
Sensors..
1 good sized JB per sensor mounted in an accessible position in the loft.
1 T&E cable in a radial circuit supplied from the relay enclosure (appropriate MCB) for the sensors, (permanent live, neutral and earth), hence 3 x 4 port (a port spare) wago type connections per JB.
The multi core (now just 4 core due to splitting into 2 circuits) I was planning to use only for the switched lives and I was going to only join into the appropriately coloured core for each sensors, hence 1 x 3 port wago (port spare again).
So 2 cables into and 3 cables out of each JB and 4 wago's per JB.
Lights...
I was going to use 1 good sized JB per relay mounted in an accessible position in the loft.
The multi core (now just 6 core due to splitting into 2 circuits) from the relays / relay enclosure I was planning to use as per your post and I would join into the earth and Neutral and appropriate switched live only, hence 3 x 4 port (1 port spare) wago's.
So 1 cable in and 2 cables out per JB and 3 Wago's per JB.
All JB's numbered and a folder containing Info about each one.
The relay enclosure would then have..
1 T&E from the CU coming in.
2 T&E going to the 2 sensor circuits.
2 multi core coming in from the sensors carrying the switched lives.
2 multi core going out to the lights carrying carrying the switched lives, neutral and earth.
Thanks again for advice and in advance for hopefully more
Mark.
