Why am I getting 120V on both my hot and neutral wires?

You’re likely looking at a classic **switch loop** where that white wire isn't actualy a neutral at all. In older house wiring, it was common practice to run a single two-conductor cable down to a switch. They’d use the black wire as the power coming in and the white wire as the switched power going back up to the light fixture. Technically, that white wire shoud have been wrapped with a piece of black tape to show it’s hot, but most people skipped that step.

The reason you're seeing 120V on both wires is probly beacuse the circuit is complete trough the light bulb in the ceiling. The voltage is traveling through the fixture and back down that white wire to your switch box. Since you don't have a real neutral in that box, you can't just swap the switch for an outlet. An outlet requires a path back to the panel via a neutral wire, and right now, you just have two hots. Another common culprit for this is a **miswired receptacle** elsewhere in the room. Check the outlets nearby. Sometimes people replace an outlet that was originally "half-switched" (where one plug is always on and the other is controlled by a wall switch). If they installed a new outlet but didn't break the small brass tab between the two gold screws, it back-feeds the switch leg. This makes the switch appear useless because the "switched" side is now permanently tied to the "always-hot" side through that metal tab.

If you're dead set on putting an outlet there, you'll need to verify if there are any other bundles of white wires tucked into the back of the junction box. If it's just those two wires coming out of one cable, you’re out of luck without pulling new wire. Whatever you do, don't be tempted to use the bare copper ground as your neutral. It’ll make the outlet work, but it’s dangerous and puts return current onto the grounding system of your whole house.

Wait, so if I unscrew the light bulb in the ceiling, shold one of those 120V readings disappear? I have a similar setup in my guest room and I'm wondering if that's a good way to verify the switch loop theory. Also, I noticed you mentioned the dial switch—if that's some kind of old dimmer or fan speed control, coud that mess with the readings on a digital multimeter? I've heard some meters can get ghost readings if the wires are just running next to each other in the wall.

You're likely dealing with 'ghost voltage' becuase of that Fluke. High-impedance digital meters are notorious for picking up induced readings from wires running parallel in the same Romex. If that white wire is disconnected at both ends, it can still show 120V just by being right next to the hot black wire. Try using a low-impedance tester (LoZ) or even an old-school solenoid 'Wiggy.' If it’s just induction, a real load will pull that reading down to zero instantly. If it stays at 120V under load, then you've confirmed it's a hard-wired switch loop or a serious back-feed issue from another device.

If you're seeing exactly 120V on both and it's not just ghost voltage, check if you're dealing with a multi-wire branch circuit. Sometimes two circuits share a single neutral wire, and if that connection fails or gets loose upstream, you can end up with voltage from one circuit 'bleeding' over into the other through a load.

Try checking if everything in that box is actually on the same breaker. I’ve seen plenty of double-gang boxes where the two switches are on completely diferent phases. If you've only flipped one breaker, the other side of that box could still be live and backfeeding through a shared connection.