I am concerned that the 16AWG conductors included in the power cable introduce a number of electric code concerns when installed inside the panel and connected directly to a circuit breaker.
First, I am aware of no circuit breaker manufacturers that sell 15A (or larger) residential breakers listed for conductors smaller than 14AWG. Terminating the 16 AWG conductors directly on these breakers as identified in the installation guide would be a violation of the breaker’s listing.
Second, article 240.4(D)(2) of the NEC requires that 16AWG conductors be protected by no-larger than 10A of overcurrent protection, except in cases of tap conductors, transformer secondary conductors, and other specific applications. I am unable to find any exceptions for other specific applications that would encompass the Sense installation, even in Article 750 which addresses Energy Management Systems and monitors.
Is it possible that I am interpreting this incorrectly, or is there a permitted exception in the NEC that I’m unaware of?
I’m not an electrician, but it sounds like your concerns have merit. Have you checked with your local authority having jurisdiction? NEC is subject to some interpretation and AHJ typically overrides all other rulings.
I brought up these exact same concerns when mine arrived. I kind of got the feeling support felt I was a nutcase at the time.
The power harness should be at least 14 AWG. In fact the official installation video shows a Square D breaker being used, the exact kind in our house, which does not officially support #16 AWG.
A pigtail with #14 inside the panel is probably the least crappy option.
@scorp508 I’m glad I’m not the only one who’s caught this and given it thought. I highly agree with your first bullet point that they should discontinue manufacturing a power cable with 16AWG and instead include one with 14AWG.
I ended up landing mine on its own 2-pole 15A Square D Homeline breaker, and knowing that the breaker isn’t listed to accommodate 16AWG wire, I had to use my judgement to make sure the wires were secure. Pigtailing to 14AWG was an option, but for my install, not doing so was neater. Plus, neither address the fact that 16AWG wire could theoretically fail before the breaker trips.
I installed my Sense as part of a service change and whole-house rewire, and my village inspector didn’t notice the issue and passed the install.
When I contacted Sense Support originally, I received the following response:
We appreciate your concerns as you are in fact correct. The installation guide says “Sense draws less than 0.1A, so you should use the smallest 240V breaker available”, this is what we recommend.
I think the 10ft tap rule applies here. Article 240.21(B)(1). The Sense conductor is under 10ft, the ampacity of the Sense 16AWG wire is not less than the load of the Sense and the ampacity of the Sense 16AWG wire is not less than rated ampacity of the sense unit. In my opinion it meets the NEC requirements.
The original post referenced 240.4(D)(2) which states that the 16AWG conductor must be protected by no larger than 10A of overcurrent protection except in cases of tap conductors which I would classify this as a tap conductor.
I think I understand what you are saying. So the tap rule definitely covers any concern in my opinion for overcurrent protection. The issue would then be that the terminals on the breakers require a minimum of a 14AWG conductor on the terminal, not for overcurrent protection but for terminal size capacity. I just looked up the spec on a 15A GE breaker and it does give a 14AWG min and 8AWG max. The other issue could be double tapping a circuit breaker. Square D is the only manufacturer that I know of that allows 2 wires under one terminal. I don’t see any issues with double tapping in this case but technically it may be violating the manufacturers listing of the breaker.
@tomandjengillis I agree with your opinion that interpreting the Sense power cable as tap conductors meets the requirements for overcurrent protection. Thanks for pointing that out!
The next question is whether there truly is a problem with wires smaller than #14 on terminals - e.g. wire can end up beside the screw rather than under it - or whether the minimum is simply based on the NEC minimum of #14 on 15A circuits, with no one guessing that a breaker might be used to power a short tap to an in-panel device. I would assume that on a screw terminal designed for #14, #16 would also be safe - the diameters are not that different.
Another consideration is whether the wire will carry enough current to trip the breaker in the event of an internal short circuit. (The wire may overheat with a continuous 15A load, but there is no risk of that with a dedicated single device on the wire - only a short.) #16 wire has a resistance of .004 ohms per foot. By my calculation short circuit current on a length of ~8ft of #16 wire is in the thousands of amps, meaning that any size of household breaker would trip in the event of an internal short, well before the wire got hot enough to start a fire. I assume this is why smaller-gauge taps serving individual devices are permitted in the NEC.
My opinion is that although there may be a listing/code compliance issue with the 16AWG conductor in normal square D breakers, there’s no practical usability or safety concern.
Anyone who’s connected Sense to a normal QO, etc. breaker knows that the breaker terminal holds the cable securely. Attached is a picture of an 18AWG stranded cable landed in a QO breaker. It’s very secure.
Even if one were to end up with a loose connection at their breaker terminal, the tens to hundreds of mA drawn by Sense would be completely insufficient to cause enough resistive heating to be a safety concern. Any heat generated would be easily dissipated by the cable and the breaker lug. It would cause an annoyance at most.
The gauge wire Sense uses to connect to a circuit breaker is smaller than the rating of any breaker I have ever seen. Most 15a breakers are only rated down to 14 gauge wire. While the Sense clearly only needs a few watts of power (so wire gauge does not need to be large), the issue is that the screw terminal / clamp on the breaker is not rated for that size. Most people I am sure just do it anyway (and it works just fine), but it is technically not code compliant. To make it code compliant an installer would need to pigtail wire it into the breaker.
Also, I think I would prefer if Sense used solid core wire over stranded wire. Solid allows for prettier installations as you can make nice 90 degree bends and kind of form the wire to go where you want it. Stranded just flops all over the place. After installation the wire never should get moved so there should not be an issue with wear on the wire from bending.
The sense unit is designed to be installed using an existing (already populated) breaker, so to the contrary the wire needs to be thin in order to fit in with the existing wire.
I could be wrong, but IIRC, the installation instructions specifically mention using an existing populated breaker, not a standalone breaker all it’s own for the Sense unit.
As for cosmetics, I couldn’t care less - when the cover was reinstalled on my breaker panel it’s all hidden anyways.
The install docs say a free breaker is preferred of available, and many breakers are not rated for multiple conductors per terminal.
I brought this up when I installed my Sense and support mostly wrote me off as a loony who must not know what I’m talking about. Even the breakers in the Sense ads and physical displays are Square D Homeline breakers which require 14 GA or larger, and are the same model in my own home.
Sure you can jam them into an occupied breaker and they feel mostly snug on a breaker of there own, but when it comes to electricity I’d rather not risk a conductor coming loose and causing an arc. You can pigtail them to some 14GA and then into an unoccupied breaker or into one rated for two conductors per terminal.
I stand corrected then, but it wasn’t an option for me (every slot full) so I piggybacked on an existing breaker, worked fine - everything was snug and tight again afterwards.
My opinion remains that even if there is a listing issue with minimum 14 AWG conductor rating on the breaker, the reality is that a 16AWG conductor is held very securely by every 15A molded case circuit breaker I’ve ever seen. And even if a loose connection were developed, there is insufficient current draw to cause any appreciable resistive heating and certainly nowhere near enough to cause an arc to be struck.
Now, double-lugging a 16AWG with another larger current carrying conductor in an inappropriate terminal could get one into trouble. But in my experience, even in this case resistive heating of the terminal will cause the thermal mechanism of the circuit breaker to trip before it starts melting the cable assembly.
I suppose if you’re concerned about it, a crimped butt connector and a 14AWG pigtail would be the way to go. But I think the proof is in the pudding: there’s no wide scale issue with electrical inspections being failed due to these issues, and it’s likely because from a practical sense there is no real problem.
Regarding the OP’s question about solid wire: the PCB-mount molex connector used for the power lead connections would likely not tolerate the mechanical loading applied by a solid conductor cable. Also I don’t think there are terminals designed for solid cable available for those connectors.
