Replacing 2-prong outlet - existing wiring has two neutral wires, one hot?

Question

This is going to sound like a duplicate question but it's actually reversed.
We live in a house built in the 1960's (in the US). We have metal conduit throughout and surprisingly a ground wire that is terminated at each electrical box. Many of the outlets were 2-prong, and I've been replacing them with 3-prong.
I came across one outlet today that confused me. Instead of the standard 4 wires coming in, I only saw 3. And to make things stranger, it was 2 neutrals (white) and one hot (red) attached to a 2-prong screw terminal duplex-X receptacle (all 4 holes are T-shaped). Also it is a switched outlet.
Has anyone ever seen something like this? Is there a way to convert this to a 3-prong receptacle?

JACK · Accepted Answer

That's a combination 120/240v outlet that could be wired for 120 volt or 240 Volt. You're wired for 120 Volt so just turn off the power, remove the old outlet and get your regular grounded outlet and hook the red, switched hot, to the brass screw and the two whites to the silver screws. A better way to work with the two neutrals would be to pigtail both together with a 8" piece of white wire using a wire nut. Then connect the single white wire to the outlet. Make sure you use the right wire size. Since you have a bare ground wire, hook it to the ground screw on the new outlet.

Correct answer by JACK on December 26, 2020

Harper - Reinstate Monica · Answer

First, the outlet is just a red herring; it is just an older style of outlet.
There’s more going on in this box than you know
Note the extra black wires that you haven’t discussed at all.
What’s actually happening is that one black-white pair is delivering power from supply. The other black-white pair is delivering power onward to another point-of-use.
The red is delivering power to this outlet, with the white as neutral.  That means one of (at least) two things:

The outlet is, or was, switched.  And the red wire is (or was) coming back from the switch to deliver switched-hot to the outlet.  Perhaps it’s been wired to bypass the switch and be permanently on.

The circuit is a Multi-wire branch circuit or MWBC, with this outlet receiving one 120V “leg” of the circuit, and the black onward wires are the other 120V “leg”.  However there’s at least one defect with this setup: in an MWBC you are not allowed to use a device to splice the shared neutral.  It must be pigtailed.

It’s normally allowed to use the 2 screws on a (non-tab-broken) outlet as a splice point.  However that is not allowed in MWBCs.
Fortunately, you don’t need to care.  Just use a red wire-nut to join the 2 existing white wires to a short pigtail, and connect that to the new outlet.

Replacing 2-prong outlet - existing wiring has two neutral wires, one hot?

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There’s more going on in this box than you know

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