I replaced a light fixture in the kitchen of an old, funky house. The wires were a mish-mash of old, cloth-insulated Al wires and new, CU wires with plastic insulation. I marked the wires coming out of the ceiling to be sure I would install the new fixture the same way as the old one. There was a red CU wire, which was hot when the switch was turned on. This red wire was wire-nutted to the black wire of the old fixture. There were also a bunch of white wires, some AL and some CU, all wire-nutted together with the white from the old fixture.

At some point in the installation, I got zapped. I determined that one of the white wires was always hot, even though the switch was off. I got smart and turned off the main breaker for the rest of the installation.

My question is: why does the new light work, even though it has two hot wires going to it? I always thought that in order to have a circuit only one wire should be hot.

Keep in mind that the old fixture was wired up this way for years with no problems.