How to identify type of wire running in attic?

Question

One of my guest bedrooms in 1977 home has no ceiling fixture/wiring. I want to add wiring for a ceiling fan.

I know I have 20amp circuit breakers and I know I will have to splice to an existing wire in the attic in a junction box.

In the attic, however, all the existing wires are either round brown/black or flat white.

I'm pretty sure the white wire is 14/2 romex. What would the round brown wires be? (Inside the jacket are just a black, white and ground wire)

From my research, I should only splice like wires together, but its my understanding that with 20 amp circuits, I should be using the yellow 12/2 romex wire. But I am afraid to splice it to the white or black cables in the attic if they aren't the same gauge.

Is there a way I can verify the gauge rating of the cables and just use the same gauge wire to the new fan? Or should I be more concerned that I am potentially using 14/2 wire on a 20 amp circuit?

Answer 1

To reliably determine the gauge of the copper wire inside your brown unmarked jacketed cable from 1977, use a wire gauge that comes with wire strippers like this one:

You'll need access to the copper core, which means you need to get into a junction box or other outlet box that the cable leads to, and disconnect the black or white wire at a nut or screw.

Then first close the stripper and then feed the copper wire core (without insulation) through the stripping hole. If it clearly doesn't fit the wire is larger, if it rubs through it has the gauge of the hole, and if there's wiggle room... well it's under gauge.

It's good to straighten the wire if it's bent or crooked from the screw or cap; it's even better to cut off the exposed part of and re-strip a fresh end since bending and straightening can weaken the core.

If you are on a 20amp breaker all wiring must be AWG12 (or bigger gauge). The breaker protects the wire from overheating and starting a fire. If part of the wiring is AWG14 you have to downsize the breaker to 15amp.

Answer 2

To answer one of your questions, using the "same size" wire as you already find on a circuit is not the rule. It's a clue.

• There may be #12 wire on a 15A circuit simply because the installer had available a piece of #12 the right length for the job. Nothing wrong with that, but you don't have to match it, you can use #14 wire.
• OTOH there may be #14 wire on a 20A circuit. That's wrong, you ought to fix it by changing to a 15A breaker and assessing what loads you have on it. If you don't fix it, you certainly should not extend the circuit with more #14 wire.