2

US 120v system.

I’m replacing a light fixture in my living room. When i took the fixture off, the two wires broke. Therefore I can’t identify how the fixture was previously wired.

There are five wires in the ceiling box. ALL OF THEM ARE BLACK. Two of them are nutted together and are far up inside the box, and I’ve determined these are unrelated to my work, probably just a hot using the box as a junction. The other three are the confusing part.

I used a tone tracer to identify the hot wire coming from the switch. I double checked this with power on - contactless sensor senses power in the wire when the switch is on and not when it’s off. Ok, so now I know which is the hot.

Now this is the part I’m confused about. First I need to add that I have a switched floor outlet in the living room. Since I pulled the fixture out and all the wires came loose, this outlet is dead even if the switch is on. Contactless sensor DOES detect voltage at the switch AND at the outlet even if the switch is off. (Sensing at the outlet is irrelevant though since it’s a split outlet and thus I’m just sensing the always on socket.)

There are two more wires in the ceiling box. One has no voltage sensed at all in any configuration, so I’m reasonably sure this is the neutral. But the other wire always has voltage on it, EVEN if the wall switch for the floor outlet is off and the switch for the ceiling light is off. Nothing in the ceiling box changes when I flip the floor outlet switch, but devices on the floor outlet do not work.

I obviously know that the hot of my new fixture goes to the hot I found coming from the ceiling light switch. What I’m not sure of is what to do with the other two wires + the fixture neutral. My hypotheses for what’s going on are:

  1. The second live wire is actually the neutral for the extra floor outlet and it should be tied to the wire with nothing sensed by the contactless tester. However this is concerning because this wire reads live EVEM when the wall switch is off.
  2. The second live is a hot that is meant to lead to the switch for the floor outlets. Also doesn’t make sense since the switch always has voltage even with that wire disconnected in the box I’m working in.
  3. The wire with no volts is actually the hot for the floor outlet and thus it reads no volts. Also unlikely because of #2.
  4. Voltage testers can sense voltage on isolated neutral wires and perhaps #3 is indeed the case. But in this scenario, shouldn’t the switch be turning another wire on and off? This leads me to believe #1 is the most likely.

What would be the next things to test to verify my hypotheses?

I fully intend to mark the wires! The previous homeowner was a DIYer who clearly cut corners and did shoddy work.

  • If you had a spur circuit whose neutral is disconnected... And you had appliances plugged into it that were switched on, e.g. Because they are a TV or USB charger that acts as a vampire load 24x7... What voltage would you expect to see on the dead side of the neutral? (Hint: if it was somehow 0 volts despite no neutral, why would you even need a neutral?) – Harper - Reinstate Monica Sep 15 at 22:50
  • So could it be that the second live is the neutral going back to the breaker box and I’m just sensing other non switched neutrals? – fdmillion Sep 15 at 23:02
  • Yes, my thought is both the remaining two wires are neutrals; one to the panel and the other from that half of the circuit that is now disconnected. Loads on that circuit are floating the neutral to hot voltage. That would be a very reasonable/typical/common arrangement. – Harper - Reinstate Monica Sep 15 at 23:49
  • 1
    You were correct. I joined the two extra wires and the fixture neutral, and everything works again. Also marked the neutrals with white electric tape! If you want to write as an answer I can give you the green check mark. :-) – fdmillion Sep 16 at 0:38
3

You've already identified switched-hot. The old lamp would have needed it, of course.

The old lamp would have also needed neutral, and you've identified one of those.

Lamps generally do not want/need always-hot, but if there are 2 or more cables in a lamp box, fair chance always-hot is in there. Since the lamp doesn't tap it, it's most likely that undisturbed 2-wire bundle in the back. Typically you have two always-hot's in a lamp box when always-hot is also being carried onward to other points of service - outlets and the like.

Generally always-hots and always-neutrals show up in pairs. E.G. AH+N from supply and AH+N onward. You have two always-hots, so you might expect two neutrals. (All neutrals are always). You don't have 2 neutrals identified (yet).


So what's left? A wire that reads "hot". And this wire was disconnected as part of removing the old light, which makes it very likely a switched-hot or neutral.

When you run through the possibilities -- a multi-wire branch circuit's second "hot" is out; that wouldn't be connected to the lamp.

Switched-hot onward to another lamp would leave this box awfully starved for neutrals - you need a neutral supply, a neutral for onward power, and supposedly a neutral for this other lamp! And you don't have another lamp.

Now, when an onward circuit is partially disconnected -- obviously if you disconnect the hot, the hot reads 0V. But if you disconnect the neutral, current moves through the loads and tries to return through the neutral, and it can't. So it lifts the neutral up to line voltage. Or phantom voltage lifts it somewhat.

That is exactly what we are seeing. And it covers the neutral we'd expect as the two neutrals for the two always-hots. So, join the mystery wire to the known neutral and the lamp; you should be all set.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.