It makes no sense to mix wire sizes.
If the circuit will be on a 20 amp breaker then you must use all #12. If you mix #12& #14 then you can only breaker the circuit at 15 amp. The #12 on that circuit is overkill and wasted money.
Keep the existing #14 circuits all #14, and make the new ones #12.

I don't think the code has anything to say about mixing, except as Petey points out. One reason that one might use larger wire on, say, a 15a circuit would be to prevent excessive voltage drop on a very long run.

Why not put the switches for lights on a 14/2 system with a 15 amp breaker, leave the outlets alone on a 12/2 20 amp system. Or run 14/2 from a box in the infinished part of the basement to a box that already has 14/2 and in the switch box for the outdoor light have the 14/2 in there only for the outdoor light.

The entire issue depends on what you plan to do. 12ga. wire is not "better" than 14ga wire. The curcuits that handle lights and receptacles to plug in a lamp or a TV is fine on 14/2. On the other hand, if you have a laundry in the basement or perhaps some large power tools such as a table saw, you need to have seperate 12/2 circuits for those uses.

OK, I've been told "don't do this" and "don't do that," but what I want to know is, WHY can't I use #14 wire for branch circuits serving lights, on an otherwise #12/20A circuit?

He's my own scenario, and I need to know whether I REALLY need to re-wire some sections or run some completely new circuits. I've run 12/2 for all my outlets, running basically one circuit per 2 bedrooms, or bedroom and bath. I'm coming off any nearby outlet box for my lights. Mostly I have used 12/2 for those, but for a couple of lights on 3-way switches, and for ceiling fans, I've used some 14/3 that I had laying around. In each case, there is ONE LIGHT, or a ceiling fan and a light kit. That's it. Nothing else on the 14/3 "branch".

WHY is this a problem? Don't tell me the inspector won't like it, that's unhelpful. Don't tell me "BECAUSE" and leave it at that, like we do to our children when we don't have a good answer.

How is such a 14/3 branch on a circuit ANY different, or more "dangerous", from a lamp plugged into an outlet with an 18g extension cord? How is the little "branch circuit" that lamp creates ANY different from a light that's permanently mounted in place? Electrically it doesn't matter one bit whether the "load" is plugged into an outlet or a switch, so why is that 18g lamp cord less of a fire hazard than that 14g wire in the wall?

For that matter, what about those little bitty wires that the light fixture ITSELF is connected with? Why don't THOSE have to be #12 wire? What's the difference between THOSE wires and any other wire on that BRANCH?

Sure, I can understand why you would never go from #12 to #14 back to #12 on a 20 amp circuit, but why is smaller wire a problem for a branch circuit? I have NO OUTLETS wired with #14 wire, so there is NO CHANCE that anyone, ever, will extend one of these ciruits to power a microwave oven or a table saw.

I am honestly looking for an answer because if I NEED to replace that 14/3 with 12/3, I will. But I just don't see why it's necessary for any logical rational reason.

I think in hindsight I would have put all the lights on separate circuits, but that's not the way I did it, and I'm not keen on spending a day rewiring all my lights. And again, I don't see why I should feel compelled to do so.

Haysdb,

Circuit breakers protect wiring from overheating and causing fires. If you want to use a 20A breaker, you must use 12 awg (copper) or larger. If your needs are for 15A only, then 14 awg is fine. If you put a 14 awg wire anywhere on a branch circuit with a 20A breaker, you are risking overheating and a fire.

Yeah part of it is protection for a short circuit. If a 14/2 line was shorted, it may not have the proper current cappacity to trip a 20 amp breaker (even though it probably would). This argument was something that electricians got away with 40 years ago. On a 20 amp circuit, 14 wire was often used for switch loops and feeds to lights because the estimated use was way under 20 amps. It's not a bad idea to wire lights with 14 wire, just to keep it legal, run a 15 amp circuit for all the lighting and keeps the receptacles on a 20 amp circuit. Not fun paying the cost o 12/3 for a ton of 3 way switches.

1) A fairly standard practice is to keep lights and outlets on separate circuits. That way if an outlet is overloaded and the breaker trips, the lights are still on in the house.

2) kurto is correct about overheating and fire.

3) If you mix 14 awg wire and 12 awg wire on the same circuit, and use a 20 amp breaker that violates the Wireing Code. If you should have a fire, and the Insurance Inspector discovers that you did this, then they will not pay the claim. You then lose much more than the one days time in rewiring your lights with 14 awg wire.

haysdb, you keep referring to "branch". The WHOLE circuit is a "branch". A "branch circuit".
Why did you use wiring your knew was not sized right for the circuit? IMO, "...because you had it lying around" is NOT a valid answer.

From the tone of your post you are simply looking for someone to give you the answer you want to hear. You want justification for doing something you KNOW was wrong.

The reasons for the things you question are for code and safety reasons.
Why can #18 be used for a light fixture? Because Art. 402 tells us we can.
Why can we use #18ga extension cords? Because Art.400, Table 400.5(A) tells us we can.

Care to know more? The code book is full of facts.

NO, you do NOT have to re-do your wiring. Just install a 15 amp breaker.

It doesn't matter whether a "tap" (look the term up) is for a half-amp load, you still have to protect the entire circuit at the ampacity of the smallest wire (other than the light fixture pigtails and the like).

It may not be an actual violation, but IMO it's questionable workmanship to mix wire sizes on a circuit. If the circuit is presently wired with #12 and has a 20 amp breaker, you should extend the circuit with #12.

If for some reason you absolutely have to use #14 (why???), change the breaker to a 15 amp and put a note on its wire saying "Notice, circuit has #14 wire" or similar. That way, some future yo-yo poking round the panel won't think "Duh, look at this #12, the turkey put in too small a breaker" and change the breaker to a 20 amp.

I know it doesn't, but can anybody think of a reason that the code shouldn't prohibit mixing wire sizes in most cases?

"If you should have a fire, and the Insurance Inspector discovers that you did this, then they will not pay the claim. You then lose much more than the one days time in rewiring your lights with 14 awg wire"

Not anywhere near true. Insurance companies cannot deny any claim for what you have stated. The only way they could deny the claim is if it can be proven that a homeowner intentionally caused the loss. As I have stated before you can wire your cell phone charger directly to your bus bars, burn your home to the ground, and they still have to pay.

jmvd20,

Thx for clarification on that. I have been following the posts on this wiring topic with interest. I thought that if any electrical work not done to code resulted in a fire that your homeowners is off the hook to pay for the loss. I see that the key word is intentional, which of course is almost impossible to prove.

Thx again

John

Homeowner's covers you for your own mistakes (even stupid mistakes).
The inspection after a fire (I have been hired to perform them) is to try and hang the fire cause on someone with a license and insurance.

Many thoughtful answers. Even a few GOOD answers. ;-)

What it boils down to for me is that so long as everything is connected properly with an understanding of the total circuit, there is no problem mixing wires. The problem is, not everything goes to plan and sh*t happens, such as a dead short in a light fixture or box, or some bonehead tapping into a light socket to power his table saw. I have seen those little screw-into-a-socket things with a plug in them. I'm sure my dad had one or two around the house. And there are light fixtures with outlets in them, although they aren't supposed to be "legal" any more. Plug a blow dryer in there and...oops, meltdown.

[Continuing my take on why the code is written the way it is...]
The code can't protect against everything, especially what the "consumer" does, but can control what the ELECTRICIAN does behind the walls, and can attempt to mitigate the effects of the most common screw-ups.

Does that about cover it?

I never doubted the code was written the way it is for a reason, I just needed to understand the reasons for a few things. It's beginning to come together for me, albeit very slowly.

I should definitely be using the term "tap" rather than "branch circuit":

"Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)"

A tap is a branch off a larger conductor with a smaller one in order to serve one device, often an appliance such as a range. That's really what I'm talking about doing - servicing one device, in my case a light fixture (or two).

"The basic rule: The ampacity of branch-circuit conductors must not be less than the load to be served. (Section 210-19(a)."

The load to be served in my case being no more than a couple of amps. For the load being served, #14 wire is more than enough, arguably even "overkill."

As for the argument that someone could come along and tap into my lighting circuit, thereby overloading it, "...it is critical to understand that a tap conductor must never be tapped from a tap conductor. In other words, never tap a tap."

To the argument that a short in the box could melt the #14 wire before the 20A breaker is tripped, isn't that less likely than someone pinching an 18 gauge extension cord under the leg of a couch? That 18g extension cord, which is specifically approved under the NEC according to a previous post, becomes PART OF the branch circuit, no? And that's more than a "tap" since there's no telling how many devices, and of what load, will be plugged into that extension cord. My light on the other hand will always and forever present no more than a 1 amp load, unless somone manages to plug in a 2000W light bulb.

I AM modifying my wiring to put my lights on a 15A circuit, because I think it makes sense to have the lights on separate circuits from outlets, so that your hair dryer doesn't knock the lights out. NOT because I believe it represents any sort of "code violation" or even "common sense." Tapping off a 20A circuit for a light is no more "dangerous" than tapping 20A and 30A circuits off a 50A circuit. And it's far LESS dangerous than putting 15A outlets on a 20A circuit. The later is apparently allowed by the NEC on the logic that you can't plug a 20A device into a 15A outlet because 20A plugs won't fit in a 15A outlet. Okaaay...

Like I said hays, all you are looking for is someone to tell you "it's ok". Sorry, the majority of us here are licensed pros and you will NOT get the answer you are fishing for.
Keep convincing yourself that what you want(ed) to do is "safe".

NONE of your logic is...logical.

And NO my friend, an extension cord absolutely DOES NOT become "part of" the branch circuit. A branch circuit ends at the outlet. Be it a lighting outle or a receptacle outlet.
And NO, 15 amp rated receptacles on a 20 amp circuit is FAR from unsafe. It is code and it is logical. Once again, your logic about what can be plugged in is NOT the basis of that part of the code.

That 18g extension cord, which is specifically approved under the NEC
The NEC stops at the outlet. It has no authority over what you plug into it.

Tapping off a 20A circuit for a light is no more "dangerous" than tapping 20A and 30A circuits off a 50A circuit.
Who said connecting smaller wires to a 50a circuit wasn't dangerous? The breaker should be sized according to the smallest wire on the circuit.

And it's far LESS dangerous than putting 15A outlets on a 20A circuit.
How is this dangerous at all? 15amp outlets are rated to handle 20amps.

"Tapping off a 20A circuit for a light is no more "dangerous" than tapping 20A and 30A circuits off a 50A circuit.
Who said connecting smaller wires to a 50a circuit wasn't dangerous? The breaker should be sized according to the smallest wire on the circuit."

This is allowed for separate cooktop/wall oven installations. Not the best way, but still acceptable per NEC.

I think what this all boils down to is that the NEC is designed as a safe, consistent way to install electrical service. Could things be done another way? Yes, electrical installation is done differently and safely in other countries. Could things be done more safely? Yes, the code is updated every 3 years with safety in mind.

Rules like "...it is critical to understand that a tap conductor must never be tapped from a tap conductor. In other words, never tap a tap." are great in the Rules by Haysdb, but I don't think too many electricians have that book in their library.

For general safety's sake here in the US,we must all sing from the same NEC songsheet.

"NONE of your logic is...logical."

Explain what's not logical about it. WHY is it not logical? Other folks here have given me very logical and reasonable reasons why things are done the way they are done, and I have modified the way I am doing things. You have spouted complete nonsense...

"an extension cord absolutely DOES NOT become "part of" the branch circuit. A branch circuit ends at the outlet. Be it a lighting outlet or a receptacle outlet."

OK, how does the electricity know that? Talk about not being logical. Does electricity differentiate between a hard-wired connection and the plug on the end of an extension cord?

"a 15 amp rated receptacles on a 20 amp circuit is FAR from unsafe. It is code and it is logical."

And it's also perfectly "logical" for me to plug a 3-way splitter into that 15A outlet and connect a miter saw, an air compressor, and a power strip into which a number of smaller devices are plugged, and very easily draw more than 15A out of that outlet.

But here, I will make it easy for you. The REASON that 15A outlet is safe is because it's fully capable of handling a 20A load, per UL requirement. The reason it's labeled a 15A outlet and not a 20A is the configuration of the receptacle. There, THAT is how you explain WHY 15A outlets are safe on a 20A circuit, not by saying "because the NEC says they are." I BELIEVE the NEC, it's just that memorizing a bunch of rules doesn't help me understand the reasons behind them, which is what I'm after. Armed with the knowledge of WHY certain things are done the way they are, much will become "intuitively obvious," and maybe I won't NEED to memorize everything in a two hundred page technical document.

Rules like "...it is critical to understand that a tap conductor must never be tapped from a tap conductor. In other words, never tap a tap." are great in the Rules by Haysdb, but I don't think too many electricians have that book in their library.

Sorry, I pulled that quote off a webpage explaining something about the sizing of ground wires. It was explaining an NEC rule, so I assumed something of the sort (never tapping a tap) was explained in the NEC. I assume electricians have a copy of the NEC in their library.

I also have to admit ignorance of how a kitchen is wired. Maybe there's a dedicated circuit for every appliance, and taping a 50A circuit (with a 50A circuit breaker) with one 30A and one 20A outlet is not common practice. Or maybe each of those devices must be equiped with their own 30A and 20A breakers. But if not, I await an explanation for how this scenario is logically different than taping a 20A circuit with multiple 2A circuits.

[Yes, I am "beating a dead horse" because I've already committed to doing things "the right way." I'm just still not completely "happy" with all the logic involved in why something is OK to do HERE, but NOT OK to do THERE. I see logical inconsistancies in the code, and inconsistancies bother me because it's a sign that some of what I THINK I know is wrong. If that's the case, then I need to find and correct my mental database.]

Gee, thanks for the lesson on "how" to explain things. I feel better now.

If you need all this explanation maybe it's better you didn't do this work. You obviously only care about how YOU interpret things.
I learned this stuff and make it my business to know the what and why. Sorry if I don't have the patience to explain every detail to you so you can do a job yourself, or make you more comfortable with doing something the wrong way. I will let you know what is legal and what is not. If I can explain it briefly I will. If you need to know more, go to school.

"OK, how does the electricity know that?"

Now that's funny!

There are several things at work here. First, outlets are part of the permanent distribution system. The wire between the outlets and the breakers can be buried in walls, or covered with insulation that will limit their ability to dissipate heat. The only safe installation is one that assumes that all of the wires in the walls will be able to handle as much current as the circuit breaker will allow. Yes, there is some over-engineering in the NEC requirements to add a margin of safety.

On the user's side of the outlet, of course, the consumer can do many dangerous things. Extension cords and adapters that allow multiple devices to be plugged into a single extension cord are particularly risky.

But I don't believe that the fact that users can cause a fire by overloading an extension cord should cause any changes in the installation of the permanent wiring structure.

BTW, one of the reasons that individual devices can use smaller wire gauges internally is that they use wire with a higher temperature rating. This wire is more expensive, but in the limited lengths included in a device, sometimes the expense is worth the savings in space.

All of the rules in the NEC have their basis in safety. If you have questions about the reason why any rule is in place, we'll try to explain it. I'll be the first to agree that there are some rules where the reasons for them are not self-evident. But size of wire and circuit breaker capacity are not in this category.

Petey may be a bit cranky here because we DO get a fair number of people here who want us to bless their code violations as some kind of special case for which an exception is justified. There are a few regulars here who sometimes do that where it makes sense and where there is reasonable precedent - just check out this thread. However, I agree with Petey that this isn't such a case. The code is clear-cut and rational on this point. It shouldn't be a significant burden to comply. In Jeremy's case it just means a little more fishing. To me this is, if nothing else, a matter of good workmanship and pride.

Some of the code requirements for ranges were relaxed during the second world war when saving copper was a national priority. That's why the code allowed only 3 conductors instead of 4 for ranges. However, that rule persisted for 50 years after the war! Finally somebody noticed. ;-)

I do recall seeing the NEC allowance for connecting both a cooktop and a wall oven to a 50 amp circuit, instead of having separate circuits for them, but I don't remember that the code allowed a smaller tap for each. I could be remembering wrong though, and I don't have the code here to check. But in any case I'd suspect that this ruling was originally another copper-saving measure - which is now no longer really needed. (I know, copper's scarce and pricey right now, but we have aluminum instead. ;-)

I don't mean to suggest that I speak for Petey here; he can certainly speak for himself. Nor do I mean to impugn Jeremy's motives. I'm just explaining why some of us (including myself at times) now and then get annoyed with posters who come here for advice (which, I remind you, is free ;-), and then get sore and complain when they don't get the answers they had hoped for. After a while maybe some of us can get a little touchy about this, that's all.

You are fine David. You just explained how I (we) feel, only with a bit more tact than I.

I believe "spencer electrician" had the most informative answer. Many older electricians know of the #14 wire on the switch loop, as do those of us taught by them. And - still working in localities or municipalities that still allow, or rather stipulate all branch circuits, both lighting and general use receptacle must be of 20amp and run in number 12. Excepting switch loops where the load is less than 15amps. Even going so far as to limit the #14 switch loop to 1 or 2 luminares only.

As cost goes, the difference is really negligable. In new installation or open renovation I would just go ahead and buy #12, actually cheaper considering the cutting and consideration. etc... Easier overall and better piece of mind.

#14 comes into consideration most often(and only where approved) due to hard pulls or "fishing". Possible box fill calculations and on some certain "calls" what you have on hand that will work most effeciently.

Watch / check your codes as most sizable cities have adopted the NEC pat.

No offense, Mr Clean (sorry, couldn't resist ;-) ...

I appreciate the history lesson, but I'm wondering why you resurrected this old, dead thread over 7 months later. It's a pretty safe bet that by now the original poster has long since wandered away.

I keep breaking the pull string on the basement lights. I want to put in a light switch. What guage of wire would I use to go from the switch to the light?