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I am trying to wireup a UPS (old one bought for my PC once) to power one of my ceiling lamps so it works as an emergency light. (Stupid idea? Yeah? :P)

But since the ceiling lamp wiring is already done and in place, I have only access to its switch where you only have the live wire. The neutral runs as common for all other lights on that circuit.

Since I want to power this specific light from the UPS, how am I going to get its neutral sorted? I read somewhere that you should never connect the output of the UPS to its input (may be on its user's manual).

If I connect the UPS and mains neutrals together, will it end up in chaos? Please advice. Thank you.

See the diagram below: enter image description here

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  • If the neutral is just a straight wire through the UPS, then this would have no effect. If not, then this would be a very bad idea. I wouldn't connect the neutral at all. (Of course, this may be a bad idea for other reasons.)
    – Daniel Griscom
    Jul 30, 2016 at 13:17
  • @DanielGriscom Oh I didn't know it could be a straight through connection. Looks like I need to take the UPS apart and have a look. Or may be just use a multi meter?
    – Ε Г И І И О
    Jul 30, 2016 at 13:23
  • It would probably be obvious upon disassembly, but a multimeter could also be used. But again, this may be a bad idea for other reasons (as in this almost certainly isn't to code).
    – Daniel Griscom
    Jul 30, 2016 at 13:46
  • Can you not get a dedicated emergency light fixture, or emergency ballast with built-in backup supply?
    – ThreePhaseEel
    Jul 30, 2016 at 15:06
  • @ThreePhaseEel Yes I can and I have such lamps too. But I just wanted to use the existing ceiling lamps, hence the UPS.
    – Ε Г И І И О
    Aug 4, 2016 at 12:43

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If the neutral is just a straight wire through the UPS, and it almost certainly is, then this would have no effect. If not, then this would be a very bad idea. I wouldn't connect the neutral at all. (Of course, this may be a bad idea for other reasons, not the least of which is it would certainly violate code.)

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    Actually, if the neutral runs straight through, then re-connecting it would create a parallel neutral, which is bad (and against code).
    – Ben Welborn
    Aug 4, 2016 at 21:45

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