DIY Doctor UK DIY Forums

I have attached two overview schematics for lighting wiring for a single room. The diagrams show the rough physical locations of the lights and switches, but the wiring and junction boxes are schematically placed (sorry it's a bit of a mess).

What would be the most efficient and sensible way to wire up these lights; would it be the first diagram where each light and the switch are routed straight from the junction box, or the 2nd whereby I connect to the junction box and then onto each light in that group?

Why the junction boxes?

Junction boxes should be accessible as and when required / testing purposes.

junction boxes are only often used to add / alter something after the initial installation. You "have a clean slate" so as I first asked, why all the junction boxes?
You might want to look up "loop in lights" or "3 plate lighting"
Much neater than what you propose.

Thanks Mr White. I have looked up "loop in lights" or "3 plate lighting" but can't find a diagram showing a circuit where you have 1 switch that turns on multiple lights, another switch that turns on another group of multiple lights and so on.

Are you suggesting that my circuit should be wired up as shown in this attachment? Going from the Consumer Unit into the ceiling rose of the first group of lights and from here to the switch and the other light(s) in the group and then onto the next ceiling rose of the next group of lights, and so on?

Here is the original diagram updated without Junction Boxes.

You can get special lighting junction boxes, in real terms two types, those with screw connections which must be accessible, and those push in maintenance free connections.

The proper lighting junction box emulates the ceiling rose, it has switched line, permanent line, neutral and earth. The idea is where a ceiling rose is not used, then the special junction box allows a single cable to feed a lamp as many European lamps don't have a permanent line connection.

Often the junction box is located very close to the lamp so it can be accessed through the hole the lamp normally fits into. However thought must be given to fire rating of the lamp if a whole is made in the ceiling.

Lights are normally wired as a radial, that is a single wire from the consumer unit splits and loops to many lamps. The limit is the length of cable before either the earth loop impedance or the volt drop is exceeded.

for 1mm cable with a 6 amp supply it works out at 30 meters, however if we consider the load even distributed then you could double that to 60 meters. That is for volt drop.

For an electrician who has installed many circuits he has a good idea if sailing close to wind, or if loads of leeway, with the latter likely he will only test after fitting, only if he things close to limit will he actually work it out.

But the whole idea is following safe routes as laid out in regulations to keep cables as short as possible. Short cables means better ELI and volt drop.

Although only permitted 3% volt drop on lighting, in real terms modern LED lighting is not that critical, it was the old fluorescent which caused problems. So 95% of the tripping current within 0.4 seconds is normally good enough, so a B6 MCB will trip on the magnetic side between 3 and 5 times rating so 5 x 6 = 30 amp, so 230/30 = 7.66 x 95% = 7.28 ohms for the ELI, using a C6 then 5 to 10 times so half that so 3.64 ohms and D6 is 10 to 20 times so 1.82 ohms for the earth loop impedance.

All just simple maths, and ohms law.

For DIY the problem is a new circuit often needs notifying, and would also need RCD protection, there is no requirement to upgrade old installations, but all new must comply with new regulations which in the main needs RCD protection. Since all my house is already protected I tend to forget other peoples houses may need it adding.