No, it does not defeat the purpose.

Any wiring that you install must be at least #12 for a 20 amp breaker and #14 for a 15 amp breaker. When considering these requirements dont worry about what size wiring the fixture itself has so long as it has a UL listing and is approved for use in your jurisdiction.

Also, it is a fact and part of the national code that a 20 amp circuit must be at least 12 gauge wire - not simply something that was determined to be a good idea by people on this forum in other posts.

"Why should I run 12-2 wire to the lights when 90% of lighting fixtures use substantially lighter wire (as small as 18-2). Doesn't wiring 12-2 wire to a light fixture with 18-2 wire completely defeat the purpose?"

It's not an altogether unreasonable question, IMO. If a 20-amp breaker is considered to be too large to protect a wire smaller than #12, how then does it adequately protect a short piece of #18 feeding, say, a light socket? (Is that a fair restatement?)

I believe the answer is that the framers and keepers of the NEC, together with the testing laboratories, apply more than just electrical theory; they engage in a good bit of "risk management" as well.

They take into account, for example, that the maximum probable current draw from a light socket, properly installed and used, will not exceed a couple of amps. They also factor in experiential data about where failures have been observed, how heat is dispersed under the limited circumstances of a luminaire whip that will feed nothing but the luminaire itself and the consequences of actual failures (how many house fires have been attributed to "x" or "y" factor?).

Fewer assumptions can be applied to general-purpose circuitry. In the case of a 20-amp circuit, there's no telling how many outlets the circuit will have, what power loads will be placed on the circuit, and so forth. There's simply no basis for assuming a less-than-full-load implementation.

While the requirement to use a larger wire guage on the circuit as a whole may appear to be inconsistent and contradictory with the UL approval of a short piece of 18-guage wire in a device suitable for connection to a 20-amp circuit, both regulatory decisions are made as part of a larger risk management process.

Theoretically, all building codes were designed to protect the next owner (or renter), not you. Laws are meant to protect others from our actions, not to protect us from our own decisions. If all you are ever going to do is plug in a light bulb, any wire will handle that current. What about the next owner though? If they want to add an outlet, they shouldn't have to worry about whether you saved a buck by using 14/2 instead of 12/2. The part that will be visible to them is the 20 amp breaker. If the code is followed, the future owner will know that if they follow the same rules, there will be no safety concerns. However, if you deviate from the code, the future owners could follow code in their new work and unknowingly create a dangerous situation through no fault of their own.

rodney01, I think most people ask themselves this question at one time. billl explains it best IMO, the next person might not know that the circuit has 14 gauge wire in it and make modifications that coule lead to overcurrent on that wire.

Just think, what if you were to change that light fixture you currently have installed with a huge array of track lighting coming out of that same electrical box?

Building codes are not to protect "the next guy" working on your home, they are there to protect everyone.

Also, the original question was not about putting 12-2 on a 15 amp circuit - but rather it was about why 18 gauge wiring is legal inside of a fixture yet branch circuit wiring is required to be at least 14 gauge.

So what if you were to install a huge array of track lighting out of an electrical box? How does this have any bearing whatsoever on the discussion at hand?

But, if you were to install a huge array of track lighting on an existing ciruit, and the circuit is wired properly the breaker will protect the wiring should an overcurrent situation result - thus proving that building codes are indeed there to protect all of us...

Code certainly isn't there to protect "all of us." Most of it protects against danger in fairly unusual circumstances, not everyday usage. There are lots of safe ways to do something in a particular circumstance, but they don't all meet code. Code basically assumes someone is going to do the most extreme thing possible to push the system to failure and then dictates the safety mechanisms and materials based on that.

To the OP's original questions, yes, the 12/2 is more wire than you really need for this particular application if your personal safety was the only concern. If something is going to overheat, it will be the 18 gauge at the fixture. Code, and your local inspector, isn't going to care about that though. It isn't about whether the circuit is safe for your intended usage, but rather all possible uses. Somewhere down the line, you or a future owner might decide that the garage is too cold and that stringing several heatlamps from the ceiling would be great way to fix that. If the wire in the wall couldn't handle that type of current, you could theoretically overheat it before the breaker trips and you could start a fire. Is your little light bulb going to do that though? No. Do you need to follow code to protect someone else down the line? Yes!

jmvd20, you really haven't read much of the thread. It's a shame that you are coming off as such a nasty know-it-all...

The OP said, and I quote: "My home has a garage outlet that is fed by 12-2 wire, however, the 12-2 line to the outlet enters a junction box where 14-2 wire branches off to feed the lights." He later went on to explain that there was a 20 amp breaker protecting this circuit.

Altho there is no immediate danger in his current situation because there is only a light fixture installed that doesn't draw much current (and only uses 18 gauge wire), it is still dangerous for future situations.

If he or the next owner were to instal a huge array of track lighting that drew more than 15 amps, the 14 gauge wire could heat up and cause a fire since the circuit is protected by a 20 amp breaker.

Do you understand? Let me know, and please do it with some maturity this time, thanks.

fotostat, are we reading the same thread???

I see no where that JMVD is being nasty or immature. I see YOU getting offended for no reason other than someone disagreeing with you.

[quote]"jmvd20, you really haven't read much of the thread. It's a shame that you are coming off as such a nasty know-it-all... "[quote]

Actually I read all of the thread earlier and again just now everything that I stated is accurate and spot on. If you think I come across as a know it all then so be it - I choose to answer questions on an electrical forum to try and help people as best as I can and it is certainly not to impress someone such as you...

[quote]"Altho there is no immediate danger in his current situation because there is only a light fixture installed that doesn't draw much current (and only uses 18 gauge wire), it is still dangerous for future situations. "[quote]

Again  the internal fixture wiring has absolutely NOTHING to do with the size of the OCPD and the size of the branch circuit wiringÂ

[quote]"If he or the next owner were to instal a huge array of track lighting that drew more than 15 amps, the 14 gauge wire could heat up and cause a fire since the circuit is protected by a 20 amp breaker. "[quote]

However, he also stated that he removed the 14 gauge wiring to "fully utilize" the 20 amp breaker.
Now the question is do you understand?

The short answer to the question that was asked is that the breaker in the panel protects the branch circuit wiring; it is not intended to protect what's connected to that circuit. Anything that is connected to the circuit, and isn't capable of sustaining the full rated capacity of the circuit, is expected to provide its own protection if it is deemed necessary. That's why most electronics and small appliances have their own fuses, breakers, or fusible links, often rated substantially below the capacity of the branch circuit.

Lighting has historically been cut a bit of slack in building and electrical codes. That's because there isn't really that much that can go bad wrong with an incandescent lighting fixture. (Well, at least since the days of the screw-in outlet adaptors...) So they usually aren't required to have overcurrent protection. The light bulb generally serves as its own protection; if it shorts internally, whatever part is shorted melts and opens the circuit. If it somehow does managed to become shorted (say, some kid drops a penny in the empty socket), usually the 18-gauge zip cord melts somewhere internally, often at the plug end, before the whole thing gets too hot. I've seen that happen.

And I should point out that it's not a requirement in the U.S. for an appliance to be UL listed in order to be legal for sale. There's a huge number of desk and table lamps on the market that are not UL listed.

The minimum wire size in the light fixture is based on its ability to carry a fault current and trip the OCD without becoming the fuse in the circuit,

Even a #18 wire can carry enough current to trip a 20 amp OCD.

While you could work hard and try and overload a #18 fixture wire, it to take som e real effort.

Higher wattage bulbs have 'mogul' bases to prevent use in typical fixtures.

I do have to ponder here a bit on this example about the future change on the light fixture with the higher load wired into say this 14 gage wire that's on a switch to light on the 20 Amp circuit. When wiring the light in with the larger size wire to the 14 gage wire one should notice that, and how about the 15 amp switch or a 15 amp outlet on a 20 amp braker?

I always was taught. Brakers protect wiring not the load. So fixture whips, loads plugged into outlets are sized to their load. A short trips the braker immediately through a magnetic or other function.

Why not change the breaker to 15A?

HU's post (above) makes an excellent point, why are all the switches in my house 15amp rated when on a circuit with 20amp breakers using 12awg wiring? I clearly understand the purpose of the wiring code after reading the good post herein, i.e. future owners modifications, etc. but it seems as if the switch has a possibility of being the weak link and possible fire. Another thought overlooked in previous post is; when selling your house and a house inspection is required and the 14awg is spotted under a wall plate, then you may have some re-work to complete the sale.

Ps. After reading this article I may change my thinking of running 14awg wire from a 15amp switch, 20amp breaker, to a series of 4 LED mod can lights (8.5w, .078amp each) in my bedroom.

The question i ask would be , in the event of a fault in a 2500 watt heater connected to a 20 amp circuit can the #14 wire inside the heater unit withstand the current necessary to open the heating circuit .