Just found the setup in the photos below. This circuit was run with 3 wires + ground. White is neutral, Black and Red are on the same phase. There's a single pole 15 amps breaker feeding it. This is not a MWBC.
Then they are both on the same breaker.
If they are different breakers then they are an MWBC which is dangerously misfired, and should be promptly placed on a 2-pole 240V breaker. (not a tandem).
Each outlet has the neutral bridged between the two plugs, but each plug is fed by either the Red or black wire.
Code requires a light switch in each room to operate a light. An exception allows a switched receptacle instead, presuming you'll plug in a floor lamp and that will be the room light.
Such wiring is typically done like this. Black is "always-hot" and red is "switched-hot". Each receptacle is connected either to black-only, or the tab is broken and one socket is connected to each, making one of the sockets switched with the switch.
The wiring in this socket is appropriate either for that, or a MWBC.
The deciding factor is -- Is there a switch in this room which seemingly does nothing? If so, most likely this is a switched circuit for the light, but some nitwit defeated the switch by wiring a receptacle just like the above but failing to "break off the tab". The improper tab is bypassing the switch, rendering it inoperative. Find that tab and break it.
Why would someone install it this way ? Is there any upside I'm missing ?
... having two wires on the same screw connector.
You can't ever do that. That's just amateur hour / "saving your way to the graveyard". 2 wires on a screw is just illegal and stupid. (unless the device's UL-approved instructions say otherwise). The right way to do this is either
- Pigtail. Join the two to a third pigtail on a wire nut... then land the third on the screw.
- Use receptacles listed for 2 wires per screw, and that means almost any "spec grade" receptacle (cost $3, come in a box instead of loose in a bin). Their instructions specify how to fit 2 wires per screw.
Note that if this is a multi-wire branch circuit (MWBC) then the neutral wire must be a pigtail -- it cannot cannot splice on the receptacle, because it must remain continuous if the receptacle is removed. (just like ground).
Would this make it easier to separate this into two circuits in the future?
No, installers never do things "for the future" unless Code requirements put a gun to their head (neutral in switch loops). They're generally wiring new houses by the dozen, profit margins are paper thin, they don't spare anything.
What you can use it for is entirely decided by where the /3 cables go right now. If to a switch, you can use it for switched receptacles. If to the panel, you can use it for a (corrected) MWBC.
