I just replaced a mechanical timer switch (noisy) with a digital switch. The old switch had red, white, and black wires and the new switch has red, white, blue wires. I simply connected the new blue wire as if it were a black wire. Now I have a bathroom fan (in a different room than the switch) that responds to the timer. This fan was not controlled by the old switch, so I wonder if the old switch was faulty (we’ve only owned the house for a year). Or, did I do something bad by wiring the blue wire?

Note that I actually do not know what the old switch controlled. We were told it was important for ventilation but we never actually heard any fans running until now.

  • Blue is normally remote switch in electronic wiring basically a secondary switch to another unit that works with an initial switch elsewhere, normally the main unit. Lets say you have a light, you switch it, the blue is connected to the fan and when you switch the light on it switches the fan on as well, if that is also in an on position ie. low/med/high. With that in sounds like the other fan is in a series with the switch and you may have used the blue as a secondary switch by connecting it to the black. – norcal johnny Dec 6 '20 at 16:30

With the exception of white == neutral and green == ground, wire colors mean basically nothing in the US/Canada electrical system. Typically black = hot and red = switched hot, but it can vary. Blue? Could be hot or switched hot.

Generally speaking, the best way to do things if it is not a clear, exact replacement, is to figure out what you have. Sometimes you can figure that out from the old connections, and if you can't understand it from the old connections, if you can upload a picture before disconnecting wires, we can often figure it out for you.

In addition, with wires disconnected but safely capped, it is generally safe to turn the breaker back on and use a non-contact tester to determine which wire(s) is hot. Note that sometimes white will be hot rather than neutral in a switch loop, so that is not necessarily an error but actually a clue - because then black is switched hot.

The instructions on the new device should detail which wires are which - e.g., in this case what red and blue mean. In a simple switch (not 3-way, 4-way, smart, dimmer, motion detector, etc.) there is no difference between "hot" and "switched hot". But with anything more complex, you need to know what wires are hot, switched hot, traveler, etc. If the manual doesn't make it clear, that is not a good sign.

  • 2
    Thank you. Your response helped tremendously. I misspoke. The mix up is the new switch has a blue in place of red. Looking at the manual I see the black goes to the power (assume this is the line) and the blue goes to the fan (assume this is the load). So it appears I have them in the right place. If I had the line and load mixed up I assume the fan would not run at all. – Jellybeard Dec 6 '20 at 3:48

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