We have an external garage and a 4 core (brown, blue, green/yellow, black,) cable is supplied from the main property. The garage is supplied with an electric door opener, wall sockets and an internal light. The cable in the garage is a 3 core (brown, blue, green/yellow) cable that needs connecting.

I assume (need to measure) the black wire is connected to a switch in the main property. As I will not use this, I'm wondering the best way to handle the black. Should I just connect this to a connector block and leave it at that?

Thanks for any pointers

  • You can't assume anything. You need to check how the cable is connected at the supply end.
    – Owain
    Apr 7, 2020 at 16:55
  • Many thanks for the reply. Yes I plan to measure before. My question is rather what to do if it is attached to a switch that i don't plan to use.
    – Nuphonic
    Apr 7, 2020 at 17:35

4 Answers 4


It's hard to say. Euro power is 3 phases of 230V each arranged in 3-phase "wye". (whereas America said "Wye not!") Phase-center is 230V. Phase-phase is 400V. Draw an equal triangle with 230mm from the center to each corner. Now measure corner-corner. You got it.

Germany is fond of delivering all 3 phases to houses. So one possibility is that someone brought 2 of the 3 phases to the garage for 400V tools. In that case simply terminate it off.

Another possibility (common American trick) is that the second phase wire is switched. Say there are lights in the garage or outdoor lights, the second phase is meant to be used to feed them. Then they can be turned off from the house. If all your garage outlets/lights are currently under the control of the switch, then check the black wire - it may be always-hot. Move the receptacles to that wire instead of the switch. Yes, it's OK to share neutral in that case.

  • thanks for the quick response. I have a hunch it is the latter but will confirm
    – Nuphonic
    Apr 7, 2020 at 17:36
  • It is uncommon to supply only 2 phases. In old installations, 4 wires meant 3 phases + Ground without Neutral, but in this case here it is most likely the second possibility: 1 phase, black unswitched and brown switched or vice-versa.
    – xeeka
    Apr 7, 2020 at 17:50
  • 2
    There aren't any 400V 2-phase tools. The European equivalent of US 240V tools is 3-phase (at least nominally, so you need a proper 3-phase receptacle, typically the big IEC kind).
    – TooTea
    Apr 7, 2020 at 20:20
  • Hi just to close this. The black wire was indeed connected to a switch in the main property and there was an extra grey wire that was cut back which I missed the first time. Thanks everyone for the help
    – Nuphonic
    May 3, 2020 at 7:30

Just to elaborate Harper's answer:

4 voltage measurements should be done.

  1. brown to blue
  2. black to blue
  3. brown to black
  4. blue to green/yellow

And a possible switch should be searched, easy done if an extension cable (drum) is pulled back from the garage outlet into the main house and a radio/TV or audio system is connected to the drum plugs in order to hear the interruption when testing all switches.

From which year is the garage installation/wiring?

Normally it should have a Fehlerstromschutzschalter (new names are RCD or GFCI) upstream, even if it is an older installation.

  • I'd also check the assumed live wires to the assumed earth wire directly, though this is redundant by a strict application of logic
    – Chris H
    Apr 8, 2020 at 13:00

Wiring colours in modern installations are supposed to be harmonised across Europe, in which case you've got

  • Earth (Gn/Yw, or bare wire sleeved at the ends). This is always earth unless you've had some very dodgy work done in the past. A voltmeter between this and some independently earthed metal should show near-zero volts; if you see 0V, and you've got a resistance meter, it should show only a few Ohms (I get 1 Ω from my earth line to my incoming water and gas pipes, but my conveniently-placed garage door frame isn't earthed).
  • Neutral (blue). Blue has also been used for one of 3 phases, in some parts of Europe at least, but not combined with brown and black like that.
  • Live (brown)
  • Another live (black) - probably. This fits with your assumption, but black has also been used for neutral in fixed wiring.

Another possibility is that there are 3 phases supplied, and 2 have been taken to the garage for different circuits at some point in the past - perhaps lighting on one and sockets on another to avoid too much load on one.

I'd check carefully that there isn't a grey wire cut off short, because brown/black/blue/earth isn't a common combination, but brown/black/grey/blue/earth would be 3-phase and neutral. If there is a grey, don't assume it's not live until you've checked.

It's possible that the cable used wasn't quite the right on for the job but had the right number of cores, hence why I've suggested a few additional possibilities, some of which may not be proper in Germany (or here in the UK) but creative shortcuts can't be ruled out except by testing.

  • 1
    There was a grey wire Chris which was cut short. The black was simply connected to a switch in the house.
    – Nuphonic
    May 3, 2020 at 7:31

The cable from the property needs to be connected to a subpanel at the garage. The subpanel will then have a main circuit breaker for each of the phases in the cable and if one of the phases is not going be used it just gets connected to a main circuit breaker and the breaker is left turned off.

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