Both. 14 for lighting, 12 for receptacles.

In Canada we still use 14ga for everything 120V except kitchen GFCI'd countertop plugs, which are of course 20A/12ga. Bathroom recepts can still be GFCI 15a, but 20A is recommended.

eveerythign in my house is 12. i sort of wish the lights were 14, but they built it with 12 and that can be a pain when you are changing out fixtures.

Just don't put #14 on a circuit fed from a 20A breaker-that requires #12.

A circuit on a 15A breaker can be #14 and as pointed out it is easier to work with. If you have long runs it is beneficial to use #12 on the 15A circuits as there will be less voltage drop (dimming lights when other loads change).

It is highly advisable to use 20A (#12) circuits for bathrooms and to keep each bathroom on its own circuit because of the big loads hair care appliances can take.

If it is a 15 amp circuit it gets #14.
If it is a 20 amp circuit it gets #12.
What the circuit is used for determines the amps required.

The rule is relative simple, use 14 for lighting and 12 for plugs and do not mix them. That keeps the dimming etc at a minimum when large loads start. The breakers protect the wire not the loads so use a 20A for #12 and a 15A for #14. Don't forget to connect all the grounds properly, they protect you. Unless your run is over 150' and has a large load increasing the wire size has virtually no effect, it just costs money, for example 150' of Romex (300' wire) @ 15A will drop 11.5V on #14 and 7.2V on #12, the delta is only 4.3V.

Even with #12 the 7.2 V is 6% of your power going to waste and possible problems starting motors. At the assumed distance of 150 ft, which is longer than most runs in the average house, #14 would be a bad idea for heavy loads and #12 still doesn't quite meet the suggested guideline of 5% drop.

"The rule is relative simple, use 14 for lighting and 12 for plugs and do not mix them."

What rule is that? It certainly isn't the NEC or CEC.

I think a better way to put it would have been: "My rule is relatively simple..."

I may have mentioned that I did a service call to the home of a friend that was having a GFCI problem. The home is new and quite large and nice. The individual who wired the home did everything with 12ga on 20 amp circuits. IMO, this is just fine, however, I think 12 ga is a little overkill on light circuits. We even use 14ga 15 amp circuits on some receptacles such as bedrooms where there is little demand. By code, we use 12ga 20 amp circuits on all receptacles in kitchens and dining rooms and others where we think necessary such as garage circuits and GFCI circuits in bathrooms. Why not use all 12 ga? Think we all enjoy wrestling 12 ga wire.

14 for bedrooms is what I do, and I also agree for things like lights and smoke detectors. It is a bit easier to work with...

BUT...

I have noticed a trend of guys going all 12awg even in places where it is not needed. They all say the same things... "the what ifs", as in preparing for the future.

I am using 12 gauge for everything in my addition -- lights and receptacles. We're going to be in this house for the long haul, and I figured in the future I might want to tap into some power for outside lights, or whatever. And even if I don't grab power from a receptacle in the future, I like the thought of a bigger wire taking care of a relatively light load of electricity. Yeah, it's a real pain in the rear trying to pigtail that crap in a box, but I'm happier with it than if I had used 14 gauge.

Well, as Americans we all know the "bigger is always better" mentality is all around us.
We also know this theory is quite flawed and we are paying the price for it now.

"I figured in the future I might want to tap into some power for outside lights, or whatever. "

Be careful of the code there. My local code says that outside lights or recepts must be on dedicated outside only cc'ts. I can't tap off inside cc'ts.

12 gauge for everything, fer crying out loud.

Guys,, just want say how much I've appreciated this thread. I looked exhaustively for a pros and cons to using 12 AWG vs 14 AWG for all circuits (including standard 15 Amp ones). I was always taught that 14 was for 15 Amp, and 12 was for 20 Amp, but lately I've read about using 12/2, especially for longer runs to compensate for line loss. I'm not so worried about the added cost or even the added difficulty of pulling 12/2, but like most of you, I just hate working with it inside boxes or making pigtails.

I'm building my dream home and just starting on the electrical rough in, so this couldn't have come at a better time. I could see using 12/2 for long runs, and by that I mean typically 70 to 100 feet, but only for receptacles, not lighting.

I live in Ontario Canada, so our electrical code isn't exactly the same as your NEC. If anything, we're usually a little behind the curve (e.g., we still only require AFCIs in new bedrooms where as your NEC requires them in almost every room of a house).

I'm thinking I should probably run 12/2 on 15 Amp breakers on these longer runs, 12/2 on 20 Amp breakers for appliances like fridge, freezer, washer, dishwasher and to all the oulets in my workshop.

Finally, the only point I didn't see mentioned when people argue the "future use" card, is that in our modern society, the trend seems to be moving toward higher efficiency, so I don't think devices of the future should be drawing as much power as they have in recent years. Hopefully these improvements in efficiency should make up for higher usage...

Any thoughts?

I agree devices are getting more efficient in the way they use energy. However, we are using many more devices than in the past and the new devices are often using more power than earlier models (again, even if they use the added power more efficiently). An example is my wife's stand mixer. The new model is quieter and does not get very hot (efficient use of electricity); but uses about 50% more amps than the old mixer.

I err on the side of more circuits and/or 20 AWG circuits for future requirements.

Better to have bigger wire on a smaller breaker than the other way around. Connecting #14 wire to runs that go back to your breaker panel that is set up with all 20 amp breakers would not be a good idea. Play it safe and go with #12.

Linda, connecting #14 wire to a 20A breaker is a violation of the code. When I wired my old house, I used #14 wire/15A breakers for lighting and #12/20A for receptacles. When my new house was built, the electrician used #12/20A for everything (other than high-power devices).

Mtvhike..Linda Didn't say to use 14 gauge.

It is highly advisable to use 20A (#12) circuits for bathrooms

Not only advisable, required by the code if you're somewhere that uses the NEC. Kitchen Small Appliance circuits, bathroom receptacles, laundry receptacles all need to be 20A circuits.