What's going on with this junction box?

January 23, 2022 1:16 PM

I removed a ceiling light in my apartment only to discover a junction box with wiring I did not expect. Photo here. Left to right: two-strand white wire; four-strand blue wire; single-strand red wire.

This is an old (late 1960s) apartment building in NYC, and I'm all but certain the box dates to that time. The white and red wires were connected to the old lamp. It's the blue wire that I'm confused by. (All wires had wire nuts before I removed them.) I was hoping to see a green grounding wire, but alas.

So, what's up with the blue wire? Do I need to be calling in an electrician here? Thanks for your advice.

posted by Conrad Cornelius o'Donald o'Dell to Home & Garden (9 answers total)

I'm not an electrician and it's one of my worst fields, but I believe that blue wire is used when more than one switch turns the light on. The multiple reds and whites lead to other lights in parallel. Is there (or did there used to be) two light switches that controlled that fixture?
posted by bbqturtle at 1:31 PM on January 23, 2022

Response by poster: There is only one switch that controls that fixture. I suppose there is a remote chance that there once was a second switch, but this is a small apartment.

I suppose another version of this question is: Since this is a ceiling light (LED, if that makes a difference) that is firmly out of reach, is it okay to eschew grounding?
posted by Conrad Cornelius o'Donald o'Dell at 1:42 PM on January 23, 2022

The blue wire usually is just a different version of a black always on "hot". It may have even been a black originally that has colour degraded in the last 60 years. Red is usually a switch leg; ie: a hot that is either on or off depending on the position of a switch(es).

Depending on how early in the sixties your building was built you may not have a bond connection (not required most places until '62). Or the box maybe bonded via conduit. It was also common to bond boxes externally such that no bond wire was in the box even though the cable had a bond wire.

Your system won't be any less safe just because you replaced the fixture. An electrician could add GFCI protect to an ungrounded system to increase safety if desired.
posted by Mitheral at 1:48 PM on January 23, 2022 [1 favorite]

As to what is up, the ceiling box is being used as a junction and feeding always on power to different outlets; usually the receptacles in the room. It is one of the standard topologies that can be used to distribute the power of a branch circuit.
posted by Mitheral at 1:52 PM on January 23, 2022 [3 favorites]

To put what others have already explained a little differently, the blue wire is essentially "just passing through" the junction box and likely heading to other outlets or light fixtures in the same or other rooms. You will also note that there are two white (neutral) wires... so, a load "somewhere else " is connected between the blue and white. Just install the new fixture the way the old one was set up. ALSO- it's possible this is a "networked neutral" circuit and the blue wires are on a separate fuse/breaker from the light fixture- I would use caution if you're assuming that "oh, I switched off breaker X or unscrewed fuse X, the light went out so the wires in this box must be safe to handle." Ideally, you would test between the blue and white wires for voltage with an AC voltmeter, test lamp, etc. Next best method would be to shut off all the circuits in the apartment to be sure. Or test & shut off!
posted by Larry David Syndrome at 4:11 PM on January 23, 2022 [2 favorites]

Looks like the preferred term of art is "shared neutrals," not "networked neutrals." My IBEW electrician pal always calls them "networked neutrals" and I assumed he was using industry standard terms. I'm not saying that you do have a shared neutral, but it's possible. It could also just be a single circuit with switched and unswitched legs passing through.
posted by Larry David Syndrome at 5:09 PM on January 23, 2022

Here's the relevant 2017 NFPA Electrical Code (your jurisdiction may have different rules) about Grounding Luminaires... Article 410.42 Luminaire(s) with exposed conductive parts. Exposed metal parts shall be connected to an equipment grounding conductor or insulated from the equipment grounding conductor or be inaccessible to unqualified personnel

Also 410.44 Methods of grounding ex. 1... Luminaires made of insulating material that is directly wired or attached to outlets supplied by a wiring method that does not provide a ready means for grounding attachment to an equipment grounding conductor shall be be made of insulating material and shall have no conductive parts.
posted by drezdn at 7:52 PM on January 23, 2022

It's pretty unlikely the 2017 or even 2020 version of the code applies to this installation as installed and few jurisdictions require updating of the system when making essentially like for like replacements. Which is for example why you can still buy NEMA 1-15 receptacles at home depot even though they haven't been legal for new installs for 50+ years.

It is weird that you have two whites and four blues/blacks. Normally I'd expect either 3&4 or 2&3. I'd be interested to know what wires are in what conduits/cables. The only thing I can think of is the extra blue is feeding a split receptacle that was intended to be switched and that switching was bypassed at this location instead of at the receptacle. It would be weird but I don't think actually illegal as long as either the receptacle was still split or one of the wires was capped at the other end.
posted by Mitheral at 8:46 PM on January 23, 2022

Response by poster: My building superintendent advised me to re-cap the blue wires and proceed as normal. I attached the grounding wire from the lamp to the green screw on the included mounting strap. All appears to be well. Thanks, everyone, for all of your help!
posted by Conrad Cornelius o'Donald o'Dell at 7:15 PM on January 30, 2022 [2 favorites]