I'm looking to replace (what I thought would be) a simple light fixture and switch.

Upon opening up the box, replacing the cables and doing some testing I have found some strange results. I do believe I'm dealing with a switch loop but I am unsure as to how to rectify my issue

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The white wire in the 3rd picture (wall switch) is HOT (not labeled, but i tested it with a non contact voltage tester)

If I connect the items, how I think they should be connected, i get almost no power to the light (its very dim) and does not get controlled by the dimmer itself.

I also don't understand why there are two tied off white/black tied together in each location (perhaps for some of the outlets?)

Any help would be greatly appreciated!

  • That is a bit odd. I suggest that you get yourself an inexpensive meter for work like this. Voltage testers often create more questions than answers. – isherwood Jan 8 at 14:03
  • OK since I'm new to this - what would a multimeter help me do? Track down live voltage and current? – Timothy Plaza Jan 8 at 14:22
  • Well, voltage is present in everything. Literally. Sometimes you're not seeing line-level voltage but phantom voltage or something else. It helps to read actual to know which. – isherwood Jan 8 at 14:23
  • Funny the comments from Isherwood disappeared when I deleted? So verify your hot and make that goes on the brass screw of the switch this may end up with the wrong colors on the switch but some electronic switches have to have the hot on the black screw or they don’t get the gate voltage to turn on properly. If your switch is not marked just try to swap the 2 wires it may help since the old snap switch worked. – Ed Beal Jan 8 at 14:31
  • With the current setup, should i detect electricity/voltage at the fixture? As of now only the white (unused cable) at the switch is reading voltage. All cables connected to the fixture have no voltage. Is this due to the break in the circuit? – Timothy Plaza Jan 8 at 14:39

Because if power comes first to the light, you need a "switch loop" to the switch. The two necessary wires in a "switch loop" are:

  • Always-hot (so the switch has a source of power to convey onward) preferred color black.
  • Switched-hot (hot when the switch is on, obviously) preferred color red.

On hot wires, any particular color is not required, it simply needs to be not white gray or green. These are mild preferences.

Now if you look at the North American market for cables, you'll find a whole lot of 2-wire+gnd cables that have black and white wires. You will not find any cables that are black and red. So wiring a switch loop presents a cable supply problem.

Rules for re-tasking the white wire

The cure to that problem, post-2011, is to require neutral to be brought on switch loops separately and also, which means you use /3 cable which is black-white-red. Solved.

However, prior to 2011, you would re-designate the white wire to be a hot. We have specific rules for that.

  • White must be neutral if neutral is in the cable. Otherwise
  • White must be always-hot if that is in the cable (but neutral isn't). Otherwise
  • White can never be switched-hot to the lamp. (which leaves "traveler").

So, on a pre-2011 /2 switch loop, white will always be always-hot, and black will be switched-hot. It has to be that way; the reverse would violate two rules.

Because if the were exchanged, both wires to the lamp would be white. How on earth would you tell which is neutral? That would be bad, since electricians notoriously refuse to mark the wires with phase tape.

Now, neutrals must be marked

Also, neutral wires re-assigned to a "hot" must be marked with electrical tape or paint. Yours are not properly marked, but on the other hand that requirement came into Code in the first decade of the 21st century. Feel free to install the marking.

The marking also does the very useful thing of telling you which of these two cables is a switch loop.

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