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There are several previous questions that have been asked on this topic but they're all related to receptacles. I'm seeing that it is against code to use 14 AWG on a 20 amp circuit but when it comes to smart switches, manufacturers don't seem to follow this code.

Looking at 3 switches I have (Lutron, TP-Link, Insteon), none of them are 12 AWG. In fact, the Lutron is using 18 AWG wires. I enlarged the text in the picture to clearly see this.

My question is, how is it any different or worse to connect 14 AWG wires to a switch with the screw-in type fastener to then connect to the Romex? (bottom picture)

Essentially these are pigtails and are not soldered on like the other 3 switches.

wire gauges

wire conecting

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    Those switches are tested to be safe using that gauge wire for that distance. If you spend the money to have a short piece of 14 gauge wire pigtailed onto a 20 amp circuit, you can do the same thing(The testing probably much more expensive than six inches of 12 gauge wire).
    – crip659
    Nov 21, 2022 at 22:31
  • This should be on 15 A CB, not on 20 A CB, you will have all wires melt before the 20 A CB acts to save the day
    – asinine
    Nov 21, 2022 at 22:48
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    +1 for all the photos with details
    – asinine
    Nov 21, 2022 at 22:54
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    The function of a breaker is to protect the device and the wiring. However if the breaker is bigger than device consumption and bigger than the wires it will act too late. Consider changing the breaker to 15 Amp. The #14 works well with 15 Amp. However the #14 will burn up long before the 20Amp breaker saves it.
    – asinine
    Nov 21, 2022 at 23:22
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    @manassehkatz-Moving2Codidact yes, the point is it's hard to get a good solder joint, and easy to get a good screw connection, tight enough to make and maintain a colloidal/metalic bond. So I would not dismiss a factory made pig tail screw connection simply as inferior, temporary or not professional. I think we agree here. And it was just an aside.
    – P2000
    Nov 23, 2022 at 19:59

5 Answers 5

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It is not that the 14 AWG wire will zap you. Properly insulated (just the original insulation, no need for extra tape) will protect you from being zapped.

The concern is heat, because heat leads to fires. A 14 AWG will heat up more than a 12 AWG wire, and if that wire is inside insulation and at a full 1500W then it could be a problem.

But, you may say, I am only using it for 50W of LED lighting? That's true now.

But the next owner (we see it all the time) will ask about connecting a receptacle in the same box. But wait, they may say, I only want to use it for plugging in phone chargers (low wattage, minimal heat).

But the next owner will see the receptacle and decide to plug in a space heater when the furnace is on the blink. Oops, two space heaters because it is a duplex receptacle. That's 24A - 2 x 12A - because a standard US space heater, with or without a fan, ceramic, oil-filled, whatever, uses 12A = 1500W (based on 125V).

24A = 120% of 20A (the circuit breaker) but 160% of 15A (what you should have for 14 AWG wire).

Take a look at a trip curve

Hard to tell exactly, and it is a range because of manufacturing tolerances, aging, etc. But basically it shows that at 120% you could get a trip from anywhere between 100 seconds, which would be fine, and never. The wire sizing rules factor this in - everyone knows a 12 AWG wire could handle a bit more than 20A and a 14 AWG wire could handle a bit more than 15A, but there are limits for a reason.

Now look at 160% (1.6 on the chart - i.e., a bit past 1.5). You get a trip somewhere between 20 seconds and 2 minutes. So on a 15A breaker, that double space heater on 14 AWG wire will trip the breaker in enough time to prevent a fire. On a 20A breaker it might not ever trip, even with everything functioning properly.

And that's how houses burn down.

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  • The OEM used #18 wires, #14 will work well, #12 is overkill, the 20 A breaker is to strong for #18
    – asinine
    Nov 21, 2022 at 23:35
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    @Ruskes The rules are sometimes overkill, but we have to stick with them. #14 on a 20A breaker is perfectly fine until you have 24A of stuff going and the wires inside insulation. Yes, 24A - 2 x 12A (max. continuous on a 15A circuit, so standard for space heaters) space heaters will be enough to overheat that #14 wire but not enough to quickly trip the breaker (because that's only 20% above capacity, compare to 60% over capacity for 15A - see "trip curves").
    – manassehkatz-Moving 2 Codidact
    Nov 21, 2022 at 23:50
  • @Ruskes Have a 240v 5HP motor that uses 18 amps running. The OEM supplied/fitted 18 gauge from the motor to the junction/connection box on the motor. The breaker is sized for starting amps.
    – crip659
    Nov 21, 2022 at 23:58
  • @crip659 you should fix that wiring
    – asinine
    Nov 22, 2022 at 0:15
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but when it comes to smart switches, manufacturers don't seem to follow this code.

No. They follow UL standards, which are what apply to equipment design, and take into account design considerations for equipment. Testing labs such as UL and ETL then certify their equipment as safe.

That is the end of your query, I'm afraid.

The UL rules require certain considerations such as pigtail wires less than X inches long, or internal component arrangements that might make overcurrent impossible, or other engineer-approved methods to assure a worst-case scenario won't cause a problem.

So, seeing a #18 wire on a UL/CSA/ETL Listed device inside a junction box, is not in any way the same as seeing a #18 wire inside a wall packed in insulation.

The UL standards work because they are subject to engineering supervision. I mean you can do anything you want with engineering supervision. Get a qualified engineer to sign off on it, and you're good to go. The problem is paying for the engineering supervision and nobody wants to do that. To make house building possible, they had to come up with construction rules that non-engineers could follow without requiring review by qualified design engineers. That's the point of the NEC. Simply follow the rules and you are assured of work which meets engineering standards.

Yes, it might be overkill sometimes, but consider the alternative.

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    Can you provide more insight? If a #14 wire cut to the same length and is fastened to the switch then wrapped with electrical tape, how is that connection more dangerous than connecting a #18 to the same wire? Does the plastic enclosure provide extra protection from a possible spark?
    – jpQuick
    Nov 21, 2022 at 22:51
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    @jpQuick It is not necessary less safe, it is it has not been tested to be safe So instead of having people who do not know enough adding 14 or smaller gauge wires to a 20 amp circuit, they use a blanket big NO. You start with six inches of 14G, next you are wanting to add a few feet.
    – crip659
    Nov 21, 2022 at 23:12
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    @jpQuick It's entirely possible that you could get UL engineers to look at your junction box and agree that it is safe that way. However that presumes that "engineering" is an unbounded and free resource, and if you think that, I'm guessing you don't work in recruiting for a tech company LOL. Absent infinite engineering resources, baseline rules must be set. Why should Olympic track runners need to follow lanes? Why can't they choose any arbitrary course so long as (with unlimited engineering resources) it can be proven their path is not shorter?
    – Harper - Reinstate Monica
    Nov 21, 2022 at 23:36
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    Note -- the White Book is just a general resource, it's the UL standards in question that govern. I'll edit.
    – ThreePhaseEel
    Nov 22, 2022 at 3:52
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    In this specific scenario, the engineer WAS paid to sign it off because they want to mass produce this thing and don't want to waste hundreds of thousands of dollars on thicker wires than necessary.
    – Nelson
    Nov 22, 2022 at 7:30
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Other answers here are correct. Considerable thought and testing and experience leads us to the knowledge that your switch with its #16 wire will not set your house on fire but if people use #16 wire on 15A circuits, some of them will set on fire. It's a set of principles that works when everyone adheres to them.

In this answer I'll illustrate just one possible reason these principles apply to your house. But this illustration is not the reason. The reason is pretty much what Harper said: One set of rules for people building switches, another set of rules for people wiring up houses.

The illustration:

  • Your switch is labeled "Max 600W load". That's 5A. #16 wire can handle 5A.
  • The switch might have a fuse or some other gizmo that prevents it from drawing more than 600W. Certainly, if you put 1800W of lighting on it something inside the switch will break before its wires or any other part of it overheat. That's why it got UL listing.
  • You wrote in your question that it's #18 wire. It's not. Which further illustrates that people like you are more likely to make mistakes than people designing switches for UL approval.
  • Now, what if it was ok for you to use #16 pigtails, and you use #18 because you think that's ok, and you sell the house and the next owner removes the switch, installs a receptacle on your pigtails and someone plugs in a 1500W space heater and goes to sleep?
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  • Just anther conspiracy theory, what the next owner might or might not do, that will burn the house.
    – asinine
    Nov 22, 2022 at 0:23
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    Thanks for your insight but your details are inaccurate. In my question I wrote that certain switches are using 16 AWG and 18 AWG which can be clearly seen in the photos. I did not mention using #18 pigtails. I asked about what's wrong with 14 AWG pigtails while the same switch is using a thinner wire in a different spot. I did not have any 12 AWG on hand and wanted to make sense of how manufacturers use thinner wires than the rating.
    – jpQuick
    Nov 22, 2022 at 16:28
  • I hope my illustration does help you make sense of why device manufacturers and home builders are subject to different rules. Because it's an illustration, it should help you even if I got the details wrong. Also, even if I got the details right, with #14 pigtails on a 20A breaker, the illustration would look the same.
    – jay613
    Nov 22, 2022 at 16:51
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This is a dimmer switch. When used with lights and not exceeding the rated spec then it's perfectly fine to use on 20 amp circuits.

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Questions like this may give someone a better feeling about breaking the rules, when others agree, but the underlying problem is not about feelings or justifying your own electrical code. Having said that, it sounds like this is just a case of replacing a pigtail with a larger gauge pigtail. If it was soldered onto the device, I would be tempted to solder the new but non-code wire (same length). Then, it is obvious that it is not part of the structure wiring, if someone opens the box later. However, it's still at risk of not being compliant with UL (and the code, if instructions prohibited any modification or repair). I would inquire with the manufacturer. Many companies have a representative ready to answer most questions, and possibly this one.

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