How should I connect a new GFCI outlet to a circuit with red, black, and white wires?

Question

I'm trying to replace a normal outlet with a GFCI. I have an outlet where there are two black wires, two red wires, one white wire and ground (copper).

One black and one red are hot wires and other black and red set is neutral.

Appreciate all the help if someone can help me understand which wires will go in Line and Load side.

The pictures are of the old non-GFCI outlet.

Answer 1

"One white" limits possibilities. Provided that you don't have some monkey business downstream it would seem that your circuit is a "multiwire branch circuit" that at the location you are working the halves of the receptacles are fed by separate 120v circuits, and the black and red that have no voltage sensed on them feed a downstream 240v outlet (as allowed by NEC 210.4(C) exception 2). Both of these should be fed by adjacent handle tied breakers or a two-pole breaker.

Your problem is the standard style receptacle has break-off tabs that allow the top and bottom to be fed separately, GFCI receptacles do not have this feature. If GFCI protection is required you have two options. (1)Protect all the involved outlets by connecting the existing style receptacle wired as it originally was, with a broken hot tab, and feed it with a two pole GFCI breaker which would maintain availability of both circuits at this location [that option won't work if somebody has improperly acquired a neutral downstream from another circuit or put some current on the ground], or (2) Pick only use one circuit for use to feed the GFCI receptacle by using a wire connectors to connect the blacks together with a third wire to brass line screw on the receptacle, the two reds together with another wire connector, and the white wire directly to the silver line screw.

The load screws are unusable with either configuration.

A ground wire needs to feed to the receptacle, the feed through to the 240v downstream feed, and to the box if metal.

Answer 2

In a kitchen backsplash this is most likely an MWBC where two circuits on two breakers share a white wire. You have the two circuits connected to a single outlet by breaking the tab that connects the top and bottom half. So you can plug say a kettle in one and a toaster in the other and use them together.

You cannot do that (split the top/bottom) with a GFCI device, so your options if you want GFCI are:

1. Leave this outlet alone, and install a two-pole GFCI breaker. (Expensive).
2. If there is room in the junction box on the wall for TWO double outlets (or if you don't mind sawing tile in place), put two GFCIs in there, one on the red circuit and one on the black. The white serves both. The downstream circuit (another outlet somewhere else fed by the second pair of red/black wires) can then be GFCI or not, by wiring things differently here.
3. Put a GFCI here with the black wire, and use the red wire to feed a different GFCI at the next location. In this box, just connect the two red wires together by themselves. This way, each GFCI outlet is on a different breaker, and you can put your kettle over here and your toaster over there.