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I would like to use a smart relay like Shelly 1 UL to control a couple of hardwired light fixtures. The load is at most 3.5 A and the relay is rated for 15 A.

Question 1: Is it okay to use the 15 A smart relay even though the circuit is protected by a 20 A breaker?

Homework: We use regular 15 A light switches ("snap switches") on 20 A circuits all the time. I traces that back to 404.14(A), which says that it's enough for the switch to be rated for its load. The Shelly is not a snap switch, but I found the following in 404.14(E), so I think the answer to my question is yes:

(E) Dimmer and Electronic Control Switches

General-use dimmer switches shall be used only to control permanently installed incandescent luminaires unless listed for the control of other loads and installed accordingly. Other electronic control switches, such as timing switches and occupancy sensors, shall be used to control permanently connected loads. They shall be marked by their manufacturer with their current and voltage ratings and used for loads that do not exceed their ampere rating at the voltage applied.

Question 2: The terminals on the Shelly 1 UL are too small for 12 AWG wire. Is it okay to use 14 AWG pigtails to connect the Shelly to the 20 A circuit, given that the circuit also has duplex receptacles (not downstream of the Shelly)?

Homework: Section 210.19(A)(2) says that

[c]onductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.

Section 210.19(A)(4) has some exceptions. Individual non-receptacle outlets are an exception, but I don't think the Shelly with downstream luminaires qualify as a non-receptacle outlet. Instead, it's part of the branch circuit and needs to be 12 AWG, so I think the answer to my question is no and my only option is to swap out the breaker for a 15 A breaker.

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  • LOL I love how the Chinese builders get all excited when they finally EARN a proper UL listing. I'm surprised they don't make the device shaped like the UL logo and just squeeze the other writing in where they can! Well, they earned it. Huzzah! ...although unless it is on the device proper, then it doesn't count. Anyone can put it on a brochure.
    – Harper - Reinstate Monica
    Sep 6, 2022 at 20:01

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Really? The Shelly terminals won't accept one (1) 12 AWG wire? Not even stranded?

If your plan is to attach two 12 AWG wires to the Shelly, using it as a splice block - that's not allowed unless the UL-approved instruction sheet (which is approved as part of approving the device) says you can do that. I generally expect 1 wire per lug terminal of that style.

Otherwise yes, you will be pigtailing. If you need pigtail wires, most family-owned lumberyards and hardware stores will cheerfully sell you 1 foot of NM or THHN (including stranded) for a few coins. Stranded THHN works great on lug terminals, and also plays well with solid wire on wire nuts. (just give the stranded wire an extra 1/8" when lining up the wire ends).

The entire "thru route" needs to be 12 AWG. You are relying on an exception in Code that allows 15A switches on 20A circuits assuming the design ampacity of the light won't be exceeded.

This is NOT the "tap rules". 240.4(D) still has full force, so all wiring on the light branch must be #12.

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  • That's right, not even a single 12 AWG wire will fit (and stranded is even thicker than solid). The Shelly was made for the European market, and then the UL version was adapted from that. The Europeans use 230 V, so they don't need as thick wires as we.
    – Vebjorn Ljosa
    Sep 6, 2022 at 21:37
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    And the UL-approved instruction sheet is silent on this case? That seems like a troubling oversight untypical of UL. UL could override 240.4(D) and allow #14 pigtailing to the device, but they would state that on the instructions. UL approved instructions are covered with disclaimers, and don't use any Chinese or badly-kerned Roman fonts.
    – Harper - Reinstate Monica
    Sep 6, 2022 at 23:04
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Yes it is "okay" to attach a load less than the circuit's ampacity. This is the same as attaching a lamp. The lamp doesn't have to be rated 20A.

No, it is not okay to use 14 ga pigtails on a 20A circuit. Exceptions are made inside of fixtures and appliances, but generally not for the circuit wiring.

I read through the info on the shelly.cloud website and didn't see anything about wire size. If 12 ga wires wont fit in the device then I think you'd have to get a bigger relay or a smaller circuit breaker.

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