Edited to add a video walkthrough, streamable link at the bottom.
An electrical company, very highly reviewed online, upgraded my panel last week. I had run a few new circuits and asked them to test them, make sure my work was up to code before inspection, and then hook them into the panel, which they agreed to.
Two days ago I installed an exterior light, which was running off one of the new circuits. Today I installed an exterior outlet, run off the same circuit.
I plugged in my electrical outlet tester, which spat back out a “hot neutral reversed” warning. This indicates that the black and white wires are mixed up somewhere on the circuit.
I shut the breaker off and removed the outlet. Then checked my wiring, (black to brass, white to silver, green to ground) All good there.
Here’s a diagram of a correctly wired outlet: https://imgur.com/gallery/gUyYGup
So I got my multimeter out and turned the breaker on. The multimeter showed 123 volts black to white (makes sense) 0 volts black to ground (not good) and 123 volts white to ground (not good). White and black are reversed somewhere in the circuit.
Here’s me testing a different receptacle, also hot neutral reversed, hard to see but the voltage reading is 123 from white to ground: https://imgur.com/gallery/cuB5zB5
So I went backwards down the circuit. Breaker off, light down, breaker on, multimeter out. Still reading as a hot neutral reversed. Breaker off, switches removed, breaker on, multimeter out. The cable going directly to the panel still reading as hot neutral reversed.
Main off, take the cover off of the panel and here we go.
The panel has almost all afci or dual function afci/gfci breakers to keep it current with code.
These afci breakers have the white run directly into them and then run from the breaker to the neutral bar. Except that the electrician had wired an entire half of the panel with the blacks where the whites go and the whites where the blacks go. Major problem.
When the hot and neutral are reversed it can create serious problems with receptacles and devices plugged into receptacles. Electricity can be flowing through them even when it shouldn’t be, leading to serious shock and fire hazards.
The most concerning part is that the receptacles still work. So a receptacle with mixed polarity (hot neutral reverse) can go undetected for years until a shock or a fire.
The only way to know is if you test the circuit and find it beforehand. As far as I can tell, the electricians who upgraded my panel tested none of the new circuits and definitely didn’t double check their own work.
The whole point of this is that it’s worth learning a little about everything, even if you’re bringing a professional in.
A year ago I would have had the electricians run the circuits, upgrade the panel, install the outlets and lights and then never thought about it again. Plugging things in willy-nilly, breeze flowing through my hair and not an electrical care in the world. Now I can do those things, but also with correct wiring in my electrical panel.
You can get an electrical outlet tester for $12 from a hardware store or Amazon.
Here’s a video of me testing various circuits, I misspeak a couple of times. Sorry, I wasn’t planning on putting it online: https://streamable.com/nayzj9