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there are some long romex wires in my attic. How can I figure out what they go to? HOw can I figure out if there is power in the wire? I want to add a ceiling fan, but there is no easy rout to the panel. There are nearby wires in the attic above where I would put the fan.

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    But how would you splice to them? You can't just do some hacky hork-a-dork splice. That splice needs to happen inside a junction box which remains accessible, and with the minimum 6” free wire length inside the box. Jun 21 at 0:40

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Get a non-contact voltage tester, such as this one from Klein:

Klein tester

  1. Check each of the cables to see if they are live. Note that a voltage tester like this is not an absolute guarantee - i.e., if it shows no voltage all the time, you need to still take precautions just in case. But assuming it shows voltage, you can then move to step 2:

  2. Turn off each breaker until you find which one turns off power to each cable. To make this easier, you may want to turn off half at a time (binary search).

Once you have matched each cable to a breaker, you need to figure out what else is on the breaker. If the breakers are well labeled, great. If not, see what else doesn't work when the breaker is off. Note that a single breaker can feed lights, receptacles and other devices in multiple rooms.

If a cable does not power any of the code-required dedicated loads, such as:

  • Kitchen countertop receptacles
  • Laundry (washing machine)
  • Bathroom receptacles

then you should be able to tap into it. Note that if a circuit serves multiple locations but should not according to current code, you generally can leave it as is but can't make it worse by adding more devices to the circuit.

If you find anything (e.g., built-in appliances, particularly heaters) on the circuit other than lights and ordinary receptacles, check back here before doing anything as there are some other things to watch out for.

Assuming you have little or no slack in the cable, the usual solution is:

  • Pick one location where you want to connect to this cable. Cut the cable a foot past this location (so that you have some extra cable to use inside the box) and install an appropriate junction box.
  • Pick another location at least a couple of feet away. Install another junction box. Cut the cable a foot before that box (so that you have some extra cable to use inside the box).
  • Add a new cable to bridge the distance between the two boxes (plus a foot on each end). If the circuit is a 20A circuit, you must use 12 AWG (or larger) cable. If the circuit is a 15A circuit, you can use 12 AWG or 14 AWG cable.
  • In the first box, connect the original cable, the bridge cable and the new cable for your ceiling fan.
  • In the second box, connect the original cable and the bridge cable.
  • If your ceiling fan will use a physical switch (instead of just a remote), you will need to add another cable (/3 or /4 depending on whether it switches everything or fan and light separately) down to the switch location.

There are a bunch of other details of the wiring, but those are the basics.

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    And pick a day when it's cool out because the temperature in an attic can be awful high and that's a lot of work.+1
    – JACK
    Jun 20 at 17:43
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    Be sure to avoid tapping current from a 3 way switch wire. (Please don’t ask how I know.) Jun 21 at 0:52

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