Can someone explain this wiring configuration to me?

Question

See my previous question for more detail on my household's bizarre wiring. Three of my bedrooms have dimmer switches connected to outlets and ceiling fans, which is electrically unsafe and just horrible practice, so I have been replacing the switches with toggle switches. The first bedroom only had two wires and a ground coming into the switch box, so it was an easy change. In the second bedroom, things are a bit more complicated, as you can see here.

This is probably hard to see, so let me describe it. There are two cables coming into the box, one from above and one from below. Both cables have white, red, and black wires, and a bare ground. The two grounds are wired together, as are the two white wires. Not pictured out of frame is the dimmer switch, which is connected across the black and red wires.

This is what is confusing to me. The black wires and the red wires from both cables are connected together as well, and the switch is connected across the black and red wires from both cables. I would imagine the wiring like this is set up so maybe the overhead lights and the fan can be controlled separately, which would definitely be nice to be able to do, but I can't figure out how that would work.

Let me state that again, all of the wires from the two cables are connected directly together. The switch connects two wires in the same cable. Can someone give me a sense of what might be going on here?

Edit: Doing some testing with my ohmmeter, the black wires are connected to the always on outlets, and the red wires are connected to the switched outlets and the lights in the ceiling fan. I separated out the red wire bundles, and the same wire (from the lower cable) connects to both the switched outlets and the lights in the ceiling fan. So I'm not even sure what the upper cable is for.

Answer 1

They are buses

Hang onto your hat. This is one of the few times wire color codes actually mean something. In this case (not often repeated):

• Black is always-hot
• Red is switched-hot
• White is neutral (well, that actually is fairly common)

All those wires have simply been bussed to all locations. Remember

• Supply needs always-hot and neutral
• Dumb switches need always-hot and switched-hot (but neutral must be present)
• Smart switches need all 3
• Lamps need switched-hot and neutral
• Split receptacles need all 3

Here's a drawing I did of a bussed arrangement for sharing motion sensors. It's exactly the same deal.

Nevermind my error with the ground wires. You get the idea.

Answer 2

Edited: See notes at end.

The following makes sense of what you describe.
Note that this is not the ONLY interpretation - just the most likely (and it matches what Harper advises is a standard US arrangement.)

See "Glossary" at end for terms used.

I'll assume mains voltage is 110 VAC = Vmains.

Connections that would make sense:

Black: Phase. Live mains. Always on.
Red: Switched phase - connected to phase when switch is on.
White: Neutral. Mains return. At ground potential.
Bare: Ground

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TESTS:

Wires: White Red Black Bare = WRBb

• Use meter set to high enough voltage AC range to read full mains voltage.

• Use meter rated for mains measurement use. Cat III or better recommended.

  1. Turn switch off.
     Locate an outlet near to an earthed surface (water tap ...). This is "ground"

Measure & record voltages between each of WRBb and ground
If NO readings read full mains voltage find another "ground" and repeat.

Any wire that returns Vmains relative to ground is (probably) Phase.

2. Measure and record voltages between each wire pair 6 Readings: WR WB Wb Rb Rb Bb

If maximum reading between and any other wire and bare wire is full mains voltage then bare wire is probably neutral or ground (as you'd hope).

3. Again measure and record voltages between each wire pair 6 Readings: WR WB Wb Rb Rb Bb

One wire will hopefully have changed from Vground to Vmains.

4. Is the black wire present in all outlets (switched or unswitched.)
   If not, report wire colours present in each type of outlet.

Report back.
The above should allow a high probability assessment of what has been done - but 1. at the start is most likely.

---

NB:

All care and no responsibility. ME Electrical. 55ish years experience. Still learning :-)

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Glossary:

Phase : Live mains relative to ground or neutral
Neutral : Current return conductor - paired with phase. Usually at same potential as ground. They may be joined at switchboard depending on regulatory system.
Ground : Connected to earth via a grounding connection. May be connected to Neutral at switchboard. Switched phase : connected to phase via a switch contact - live when switch is closed / on. Dead when switch is open / off.

    _________________________

Notes:

1. Edited to remove stupid mixup. As red and black have the switch between them they are obviously phase and switched phase.

2. I removed my 'other unlikely alternatives' as Harper's answer makes it clear that my 1st (now colour correct) choice matches what he says.
   My suggested tests still make sense as they will confirm that things are indeed as expected - and there appears to be a lot of non standard work elsewhere in your system.