On Sun, 21 Mar 2021 19:32:55 -0700 (PDT), DerbyDad03
wrote:

On Sunday, March 21, 2021 at 8:22:14 PM UTC-4, wrote:
On Sun, 21 Mar 2021 16:22:03 -0700 (PDT), DerbyDad03
wrote:

On Sunday, March 21, 2021 at 4:33:18 PM UTC-4, J. Clarke wrote:
Please forgive me--this is a bit of a rant.

I went to put a new fixture at the top of the stairs--the old one is
physically too small to hold anything bigger than a 60w incandescent
(by physically I mean you can't put the globe on if anything bigger is
in it, and that includes CF and LED that are larger than 60W
equivalent and in that location a 60 just isn't enough light.

Well, went to kill the power to the circuit and discovered chaos.

First:
Turned the switch off
Checked socket with a voltage sensor
Still voltage on the circuit
Par for the course in this house
Switch is in the neutral leg
Add to list of stuff to fix.

Next:
Screwed adapter into socket and plugged in tracer
Traced signal in breaker panel
Not one, but _two_ breakers showed signal
Turned both off
Went back upstairs
Checked for voltage again
No voltage--good
Flipped switch--checked again
Voltage--not good
Back to the panel
Identify third breaker, turn off
Now no voltage

Replaced fixture, turned breakers back on, everything works, I didn't
die.

But now I have the real mystery--how is this effing light managing to
be connected to not one, but _three_ separate branch circuits?


It's possible that you are dealing with an Edison circuit, also known as
a "shared neutral" or "multi-wire branch circuit". Granted, Edison circuits
usually only involve 2 breakers, but I think I could imagine one with 3.
I don't see three.
In a typical Edison circuit a cable with 2 hots and a single neutral has the
2 hots tied to separate breakers. That 3 wire cable runs to a junction box
from which a pair of 2-wire (hot and neutral) cables emerge. Those 2 circuits
share the single source neutral from the 3 wire cable.
And his switch is in the neutral. That's enough to make your head
hurt.

Maybe. We're not there to check. The white wire can be used as a switch
leg but it's supposed to marked as hot. I'm not doubting that JC knows
what he's looking at cuz he's there, just saying that things are confusing
enough here that something else might be being missed.

Sure, I do that all the time with 240 circuits - paint the white wire
with a red sharpie. I have sharpies in my electrical tool caddy for
that reason.

If you only turn off one breaker, you will often get some voltage that bleeds
back through the neutral at the supposed-to-be-dead fixture. If you used a
meter as opposed to a voltage sensor, I'll bet that it's not the full 120 VAC.

Now - although this is not something that I have ever seen - expand that to
a 4 wire source cable (3 hots and a single neutral) to the first junction box
and you've brought that 3rd breaker into the picture.
4-wire cable? It's used in dryer cables but it's a lot larger than
what you'd find in a lighting circuit.

Southwire makes a 12/4 non-metallic cable and 14/4 MC. Both are standard sizes
for lighting circuits.

It can't be something the average electrician would have on his truck.

I don't know if a "super-Edison" (I made that up) would be allowed by code,
but I can certainly imagine someone grabbing some 12/4 Romex and running
one.

I don't see how to do it with only one cable. Using two is _really_
scary.

Again, 12/4 or 14/4 could be used. Yes, 2 cables would be scary, run-away scary.

However, I don't see how it doing it with 4 wire cable could be code considering
that 2 of the 3 hots would have to be on the same phase in a normal residential
situation. Bad! See my other post where a 4-wire multi-branch circuit is mentioned
in the NEC 300.13(B) commentary. 4-wire cable is apparently used in multi-branch
circuits, but it's certainly not something I've ever run across. I'm guessing it's only
used (properly) in a 480V 3 phase system.

And of course I'm not saying that this is what 100% what JC is dealing with, but
how else could a single fixture be interacting with 3 breakers? An actual voltage
reading with only one (or two?) breakers off might tell us something. I know that
the voltage will vary by what other loads are sharing the neutral when only one
breaker in a normal 3-wire multi-branch circuit is off. All JC's voltage sensor is
telling him is that some amount of voltage is present.