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I have been replacing my light switches with zwave switches but some of them are wired a bit differently and am having some difficulty understanding them.

  1. This one is a single light switch to control one light in the room. The white wire seems to be the hot one.

    enter image description here

  2. This one is a two-way light switch. The second switch controlling this switch is in the other room. The red and white seem to be running hot all the time while there are two wires (black and white) in the black connected with a wire nut.

    enter image description here

Could someone please help me understand what's going on here?

  • The black wire in the bottom photo is not under the screw head very well. – mike65535 Feb 27 at 14:17
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By US conventions, black should be hot, white should be neutral, and red should be alternate hot. Normally.

When you see a switch connecting white and black directly, this is a "switch loop", where hot was brought to the fixture first, then down to the switch; the switched hot is brought back up to the fixture. If they were competent, the electrician should have put a black mark r tag on that white wire to indicate that it too is hot.

The second example is apparently also a switch loop, but for a "three-way" switching circuit; see other answers for descriptions of how using two double-throw switches and both hot and alternate hot permits either of two switches to turn the light on or off.

(Don't ask why we call it three-way when it has two on states or four states total...)

  • Three-way because it has three posts so I guess it's to make it "simple". Single-pole double throw is more precise. ;-) – mike65535 Feb 27 at 14:15

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