FAQ: rewiring light circuits
The questions that come up most often when changing a light circuit. They refer to installations in Germany.
What is a junction box?
A junction box is a box in the wall, usually above the door near the ceiling, where the cables branch: the feed from the fuse box, the cables to the switches and the cable to the lamp.
Newer installations often have no junction box. The cables then run from switch box to switch box, and the connections sit behind the switches.
Do I need a neutral (N) in the switch box?
A normal switch doesn’t. Many smart devices do: a Shelly 1 Mini or the wired Hue wall switch module need live and neutral in the box they sit in.
The battery Hue wall switch module, the Senic smart switch and the Shelly 1L work without neutral. Whether neutral reaches a box depends on which cables lead into it. klemm.app works this out for your boxes.
May I do this myself?
In Germany, work on fixed electrical installations may only be done by a qualified electrician. For installations behind the house connection, § 13 NAV requires a registered installation company.
klemm.app is a planning tool: you see beforehand what needs doing, which device fits and what material you need. You can print the plan and give it to your electrician.
Which wire has which colour?
Today, under DIN VDE 0293-308: green-yellow is the protective earth (PE), blue is neutral (N), brown, black and grey are live conductors, switched wires or travellers.
If a blue wire is used as live or switched wire, it must be marked brown or black at both ends. The protective earth is never used for anything else.
What is different in old installations?
Before 1965, other colours were common in Germany: red for protective earth, grey for neutral, black for live. Some old buildings have only two cores without a separate protective earth.
klemm.app works with today’s colours. In old installations, a qualified electrician should first check what is where.
Why does a Hue bulb need permanent power?
A smart bulb receives its commands by radio. If the wall switch cuts the power, it goes offline and reacts neither to the app nor to motion sensors or schedules.
So the circuit is rewired to give the lamp power at all times. Switching is then done by radio, for example with a Hue wall switch module behind the old switch.
What is the difference between a switch and a push button?
A switch stays in its position; a push button springs back after you press it. Push buttons don’t switch the lamp directly but send a pulse to an impulse relay or a smart relay.
How accurate is the plan?
The plan assumes a textbook installation: standard colours and the usual cables for the chosen circuit. If yours differs, enter it in the calculator, for example swapped colours or cables with more cores.
Before every step, check with a two-pole voltage tester that the box looks like the plan.