I’m considering installing a Shelly 2.5 at a 2-gang wall switch, but that circuit is currently running on a 20amp breaker. My understanding is that the Shelly will only take 14 gauge wire, which is generally not permitted on a 20 amp circuit. Has anyone else encountered this? Should I swap for a 15 amp breaker?
It’s not about wire nor breaker.
Your cable diameter and the breaker both suggesting that your installation is prepped for higher loads
Shelly 2.5 is rated for 2400W (10A at 240V).
as long as device to be controlled by Shelly is within this limitation it’s ok.
Diameter of cable is another story. You must to connect thinner cable to fit Shelly terminals
No because the breaker should trip at 10A[1], if it lets your Shelly get 20A, say in a short circuit situation, it is not doing it’s job.
[1] well a little bit over 10A basically.
@nickrout is right. Circuit breakers are designed to protect the cable or the weakest part of the circuit. If a cable or device is rated for 16 Amps the breaker should trip before that limit is reached. So the breaker should be changed. But get it done by an electrician.
Thanks all, this is helpful. It then sounds like Shellys are designed for 15A circuits, although I can’t find that stated explicitly anywhere. Since both code and general consensus from people speaking as professionals online seem to say 14AWG on a 20A circuit is a non-starter, I think I’ll skip this project for now.
But just to help my basic understanding: this post from Shelly describes that the two L connections are actually connected within the unit (rather than belonging to to their respective channels), so do I understand correctly that there would only ever be 10A running on one of the 14AWG cable going into one of the L connectors (before the 20A breaker would trip)?
No, they are designed for 10A circuits. In the specs
Maximum load 10A per channel
This makes sense, but does this mean that all these people installing Shellys on their 15A circuits are just risking it?
10A Shellys, yes. I don’t know whether they sell 15A products.
People are conflating two different numbers here. The shelly can switch a 10 amp load…that means only 10 amps can flow through the device. They are rated to be installed on a 15-amp breaker.
My GE fan switch is only rated for 1.5 amps. Load limit does not equal circuit current rating. Breakers are sized to protect the wiring, not downstream devices (unless the appliance calls for a device-specific breaker). The Shelly would only see the 15 amps if you pulled 15 amps off the line it switches at which point its built-in overcurrent protection kicks in.
Incidentally it isn’t safe to wire nut 14-guage wire to connect to a 20 amp line here because this device can actually switch 20 amps (two 10 amp channels), but the screw terminals on the device are only rated for 16 amps. https://www.facebook.com/notes/shelly-support-group-english-version/clarification-of-the-purposes-for-the-multiple-l-terminals-on-shelly-1pm-25-and-/2128219340610752
Depends what is meant by 15A circuit I guess. I took it to mean feeding a 15A load (because without a load there is not even a circuit in technical terms).
While you’re correct in terms of what is a “closed circuit” or “completed circuit” it is a circuit whether the load is present or not. A “circuit” in electrical terms is the pathway whether complete or not. In industry parlance a 15-amp circuit means the 15-amp breaker and downstream, as branch circuits are rated by the maximum potential load on the entire branch circuit. (The definition comes from Article 100 of the NEC.)
From the Shelly downstream would also be a circuit but amperage isn’t like voltage. Connecting a device to a 15-amp circuit doesn’t mean that 15 amps suddenly flows through the circuit. You need to increase the resistance to build amperage. From the Shelly down each channel is a 10-amp circuit even when connected to a 15-amp breaker. Similar to a breaker the Shelly itself has overcurrent protection (required by Code).
I guess my main concern is that someone looks at the fuse box. Sees 15A breakers on the circuit. Assumes the circuit is OK for 15A, plugs 15A heater into wall socket that has shelly in the circuit. Boom something goes phut
It wouldn’t go boom. It should gracefully turn off. The Aeotec version of this allows you to configure a zwave message that’s sent when it happens. The amperage limit is usually set to prevent the device from overheating so I’d be willing to be that it would run at 15 amps for a bit before it got too hot and turned off anyway.
In any event lots of devices have amp limits and you can’t change out the breakers for all of them. As I said earlier the GE fan switches are limited to a few amps. This in particular is why I dislike the practice of switching an outlet from within the box versus an actual switch or a plug-in module. It’s less apparent.
RE: that post on the Shelly FB page, I actually read that as saying that the 2 L connections on the Shelly are not where the channels branch, but that they reconnect again internally, and the reason there are 2 of them is because the terminals (and 14-guage) are only rated to 16A, so 1 couldn’t take the full load of both channels at 10A. But if you have both L connectors and they are in fact in parallel, doesn’t that mean one L connector will only ever be carrying 10A (assuming the Shelly manages overload beyond 20 correctly)? So if I’m only drawing 8A on one of those channels (and the other has nothing on it), each L connector will have 4A going through it?
Shelly1 and Shelly1PM are 16A are 16A rated
Some of them have some not.
Shelly 2.5, Shelly1PM … yes. Shelly1 hasn’t
In general Shelly devices which can measure power have such protection. Although I don’t know if its software or hardware. For sure there is option to set the limit in software
Current measuring is different than overcurrent or overheating protection. It’s invisible to the user. I’m skeptical that any device could receive UL approval without such a safety system. At the very least it would be a shunt that destroys the device but saves your house. Often when they don’t have over current protection UL would require the instructions to mandate a dedicated breaker.
2 years later, I was looking at this and I can see nothing but conflicting arguments going back and forth.
If a Shelly Device rated at 10Amps is installed in the Consumer Box/Breaker Box, Fuse Box whatever you want to call it or a wall outlet that can have anything plugged into it and it has a 16A or 20A breaker on that circuit, that is a fire risk. You cannot rely on the software of the Shelly doing a graceful shutdown, as it may keep it going and burn up the relay. Not to mention that this would be against electrical code, and invalidate insurance. Anyone remember the SonOff relays that have burnt up, even slightly under their max load?
If you are installing the Shelly on a single appliance, then go by the appliance rating, and that would be no different to using a lower rated extension cable on the socket.
I know 2 years later this would be long solved, but the banter back and forth ignored the OP.
The original post was - Shelly 2.5 on a 20Amp circuit. This would be a 100% no. The breaker must be downgraded to prevent a fire risk and to comply with electrical code.
There is no mixing up numbers, or any of that, The OP was very clear. Shelly 2.5 on 20Amp circuit at a particular power outlet. This is 100% a no, there is no question to that. There was no mention of using it with a particular appliance.
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No. I was an electrician in a former century and dealt with the code and inspectors more than I liked.
If you have a 20A circuit and you plug in a 100W lamp, you don’t need to change the breaker. You can safely, and legally** use a wire nut in a receptacle box to connect a 14ga pigtail to a 12ga wire, as long as the device you connect to the 14ga wire will never draw more than 15Amps. Sloppy, but legal.
Your main panel breaker is probably 100 to 200 Amps. You don’t reduce its capacity because you add a few 15A breakers. So you don’t change your breaker because you are only drawing 10 - 15 Amps on a 20A circuit.
Breakers and fuses only have one job- to protect upstream circuits. The do not and never will protect the devices they are connected to in normal use.
** But you would risk exceeding the box fill calculation which would put you into code violation territory.
To the OP. NO. Even it if were rated for 20A, I would never expect a Shelly, Sonoff or other smart relay to carry its rated current for very long. I have seen too many melted Sonoff Basics to have any confidence that they have been tested at their rated current for more than a few minutes. In addition, the screw terminals loosen over time. Loose terminals increase the connection resistance, and under enough load, the terminals will get hot enough to melt the device. If you have such a device in use, turn off the power and check the connections- I’ll bet you will be able to get another 1/4-turn on the screw terminals.
If you have a 20A circuit that you need to control, then you need to use a contactor (big, heavy duty relay). They usually have a 110VAC coil or sometimes 24VAC which you could control from a Shelly.
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Just realised what you were saying ,which made my last comment make less sense.
Obviously it makes a difference where you are and what the rules are, but yes you are right, but I would not class it as safe to have a 10amp or even 16amp device hidden in a wall behind a socket that could potentially have a 20amp load put on it 
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