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The markings on the thermostat say "LINE RED" and "LOAD BLACK". Does that mean one red wire on the thermostat goes to hot white in the box and the other red wire on the thermostat goes to hot black in the box, and one black wire in the thermostat goes to load black in the box and the other black wire in the thermostat goes to load white in the box?

In this case, the hot wires are on the right-hand side of the box per voltage tester.

Breaker switch is now in the off position.

Breaker Switch in the Off position

Line and Load markingsLine Pair and Load Pair

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  • OK first, that installation is improper per modern Code. Those white wires MUST be marked with paint or tape to a hot color. Most people have black electrical tape on-hand so people tend to use black... but if you don't mind spending a couple bucks on red tape, it's an ideal choice :) Nov 16, 2022 at 19:48
  • That's how it was wired back in the 1960s or 1970s, previous owners. I'll mark the white hot wire with red tape.
    – mr blint
    Nov 16, 2022 at 19:49
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    Sorry, no, I am mistaken. They're not using colors the way I think they are. Read the instructions for your particular thermostat. Nov 16, 2022 at 22:03

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Cables are made only two ways.

However they are used for all tasks. One way is "/2" with black, white, bare wires. The other is "/3" with black, white, red, bare wires. This is how cables are made. As such, wire colors tend not to reflect the actual job the wire is doing.

Often, neutral is not needed in a circuit because of the way American 120/240V works. In that case, only 2 wires are needed (no neutral). Yet, when you go to the shop and try to buy /2 cable, it only comes black-white, so that's what you use.

Neutral.... white or gray
Ground.... green, yellow/green, or bare
Hot.... all other colors

If neutral is not present, as in here, then white can be re-used for a hot wire. In the 2000s Code was changed to require white wires used as hots to be marked a "hot color" with tape or paint. Any hot color will do. They should be marked at both ends.


As far as how to wire your particular thermostat, you'll need to read the instructions. NEC 110.3(B) requires you to follow the instructions when installing equipment.

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