Why are there 3 wire conductors on this outlet circuit

Question

Hey there I’m an apprentice electrician and still learning so please forgive me on my lack of knowledge here, but I had a friend who asked me to change out some receptacles in their house for them which sounded like a straightforward task that I've done before, but the issue I ran into is something I haven't seen and experienced yet.

There are 3 wire conductors feeding 3 outlets on this circuit (Red, Black, White and bare copper) and I've never seen a third wire (Red) for outlets before, and unfortunately my friend had removed them all already to do some painting so I was not able to see for myself how they were wired before, so I checked for voltaged where the power comes into the first box to see which wires where hot and I was getting 120v on the Red and 20v on the Black, at this point I asked my friend how the old ones where wired up and I installed the new ones accordingly as shown in my picture diagram (sorry for my poor MS paint abilities) and I powered them up and checked them with my receptacle tester and it said it was wired correctly and worked.

So I'm wondering is this actually correct?

If this is not correct how should it be wired?

And why is there a 3rd wire?

Also is the 20v I got on the black wire just phantom voltage?

I’d greatly appreciate the help and knowledge!

Answer 1

Something is odd there.

Normal reasons for /3 wire on a receptacle run:

• Half-switched outlets (tabs broken, 0V between black&red, or perhaps phantom voltage between the switched conductor and neutral or unswitched if the switch is off)
• MWBC (tabs broken, 240V between black & red.)

In either case, black & red would NOT normally connect to the same terminal. Since the breaker didn't immediately trip (sign of an MWBC without the tabs broken, or with black & red connected any other way), look for a switch that will seem to "do nothing" because the switched and unswitched hots are currently joined together.

If red is always-hot, and no switched outlet function is desired, simply remove black from each outlet, isolate it, and leave them wired as they are otherwise. You can join the blacks to each other (but not the receptacle) in the intermediate boxes or isolate each one in each box as you prefer. While it's more common for red to be the switched hot, that's evidently not the case here, and that is not required by code.