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Scenario

There is a three-phase cable popping out of the wall exactly where I need to put a light bulb.

Is it ok if I plug only two wires to the triphasic cable, (i.e. neutral and live 1) to achieve this?

Further questions

What consequences would this have (e.g. consumption?, stability?)

Context

At home I have identified two types of cables:

  • single phase cables (with their expected three wires inside).
  • three-phase cables (five wires inside).

The latter has two outputs, one in the kitchen for the stove/oven, while there is another cable in the toilet (most prob for the washing machine), which I don't use at all.

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    Wiring practices and regulations vary by country. Many will require a protective earth/ground connection as well as the live and neutral. Can you say which country this installation is in? What colours do your 3 wires have? – Graham Nye Apr 18 at 12:26
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    Germany @GrahamNye , the monophasic are yellow/green, blue, brown, while the triphasic's are yellow/green, blue, brown, black, grey – nicosierra Apr 18 at 12:37
  • Thanks. You have the standard IEC colour codes, as expected, but that exhausts my knowledge of German wiring standards so I'll leave this question for those who are familiar with them. – Graham Nye Apr 18 at 23:25
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If there is a provided neutral wire, for most countries it should be fine,

Consequences:

If it was a large load, e.g. an outlet, you would need to make sure the load on each phase was reasonably balanced,

Having a lighting circuit on a different breaker to the rest of the house lighting is frowned at, as if that circuit trips, the light goes out, equally someone later on perhaps wanting to replace that light socket will switch off the lighting breaker, if they fail to test the light socket afterwards, they will have a nasty surprise.

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In germany, the mains that enter a building are typically triphasic. The three phases are then distributed throughout the house. So, any wallsocket is connected to safety earth, neutral and one of the three phases. In principle that's what you describe. But: an electronic mains socket must be fused somewhere (do you know how your triphasic cable is fused?)! The electritian doing your installation has to take care of that. Working on the mains has the potential to kill you and (at least in germany) you are not allowed to do electrical installations if you are no electritian (and I don't want to encourage you to do so).

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Technically it will work. But you must be aware that in some regions (and this can change within the same country) single phase is not achieved by using one phase and neutral, but by using two phases. You must check this otherwise your light bulb may explode. One way to know that is to open the fuse box and check how circuit breakers are connected: With one phase and a neutral or with two phases.

There is also the safety concern. A tri phased cable will be protected by a tri phased circuit breaker and it won't work as well as a single phase breaker. Instead of using the cable right away, I would look where it goes, and if it goes straight to the fuse box and it's not needed for anything else, replace the tri phased breaker for a single phase breaker with preferably a much lower amperage (2A for example). Tri phase breakers have usually very high amperage which is less safe. Even worse, it may be connected to the general circuit breaker of the building.

If the cable is used for something else, add a small box with a single phase circuit breaker at the end of the cable, just for this light bulb. Seems a lot for a light bulb, but later, you don't regret it.

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No, primarily from the point of view of safety. It would be unsafe to have a light bulb in isolation (not being part of the exclusive lighting circuit for the house).

Secondly, should there be an inverter for the lighting circuit, the toilet would be under darkness during a power failure.

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  • There are savety concerns, but whether or not the toilet is under darkness depens on how the installation is fused. – Sim Son Apr 18 at 16:57

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