A GFCI outlet has two pairs of screws on the back, marked LINE and LOAD. Put the wires on the wrong pair and the outlet won’t work, or it won’t protect what you think it protects. Here’s how to tell them apart.

The Quick Answer

LINE is where the power comes in from the panel. LOAD is optional: it feeds other outlets further down the circuit, and anything connected to LOAD is protected by this GFCI too.

One Cable in the Box

If only one cable enters the box, that’s your incoming power. Black goes to the LINE brass screw, white to the LINE silver screw, and the ground to the green screw. Leave the sticker over the LOAD screws in place.

Two Cables in the Box

Now you have to find which cable brings the power in. With the breaker off and the wires tested dead, separate the two cables, cap every wire end, and make sure no bare copper is showing. Turn the breaker on and, without touching anything, hold a non-contact tester to each black wire. The one that beeps is LINE. Mark it, turn the breaker off, and test again before you touch anything.

Then decide what to protect:

• Protect downstream outlets too. Peel off the LOAD sticker and connect the second cable there. Put a GFCI Protected sticker on every outlet it feeds, so anyone can find the reset button later.
• Protect only this outlet. Skip LOAD. Join the matching wires from both cables with a short pigtail under a wire nut, and connect the pigtails to LINE.

What Happens If You Mix Them Up

GFCIs made in roughly the last twenty years won’t reset if LINE and LOAD are swapped. Older designs could reset and still leave the outlet itself unprotected. Never connect the incoming power to LOAD.

Test It

Turn the breaker back on. Press TEST and the power should go off. Press RESET and it should come back. Test your GFCIs once a month.

When to Call an Electrician

• Aluminum wire, crumbling insulation, or signs of heat damage
• A box with more than one circuit in it
• A GFCI that won’t fit in the box

My guide walks you through replacing a standard outlet or a GFCI, step by step.

Get the full guide: How to Replace an Electrical Outlet by Hank Palmer, on Amazon