Low voltage and high voltage go into a box - again

[RGK]

I had a task dropped into my lap. Our company is branching into home automation design. My boss told me to figure out whether our upcoming boxes can be wired properly in NEC/NFPA installations, and how that might look.
I’m an EE by training and practice, quite a bit of it in design of power supplies for things that eventually got their ETL certification. I know that I >don’t< know much about how the NEC/NFPA says to wire things up.
The product is to be a hard-wired logger for conditions in HVAC and plumbing installations that do not already have on-board smarts. Some setups will require connecting to the mains wires on in-place equipment to watch voltage, current, phase, etc. I know what to do with the mains voltage conductors inside our box, but the more I read of how to satisfy NFPA, the more I get confused about exactly how this affects everything all the way back to out box’s power supply.
I’ve followed the chain down to 725.136, probably section (I), but I’m also very aware of my lack of experience.

1. Does the monitoring box have to have a physical barrier inside between Class 1 and Class 2 wiring?
I think no, per 725.136(I), as long as the Class 1 and Class 2 are separated by 6mm or more.

2. If the mains voltage being sensed is connected to a PCB inside the box, and the PCB inside provides ETL-standards of isolation (hi-pot, creepage, clearance, contamination class, etc.), must the power cable entering the box meet Class 2 wiring standards if it’s a permanent installation?

3. Does the power cable have to be Class 2 wired from the AC supply that feeds the power supply? Or Class 1 because Class 1 enters the enclosure?

4. Can the power cable be flexible/stranded? I think no, based on reading the code

Sorry for the long-winded question, and thank you for any pointers to things I’ve missed.

 

[tom baker]

You should consider working with UL to get a listed product. This would ensure a safe product that can be sold. In some areas, and certainly in WA, if an AHJ sees a "hard-wired logger" in an AHU (considering they see a lot of AHUs) they will take a close look for the UL or similar listing.
No listing and the job stops. I have seen this happen.
I will try to answer your questions
1. Most equipment has a barrier, for example AHUs
2. Class 2 wiring would not be suitable for the mains voltage
3. The power cable would be a Class 1 non power limited
4, I see no restrictions on flexible cables

How will the mains power be connected? you won't be able to double tap the load side a breaker, that is the biggest red flag stop work for an AHJ
Also I hope when you get a design you get comments from installers, to do us all a favor

 

[RGK]

Thank you for your reply. Yes, we intend to get UL or equivalent listing/certification, as well as FCC/etc given the switching nature of the electronics inside.
On 1: Your answer helps clarify some things in my mind. It now seems to me that the point of the NFPA code is how the actual wires that enter and leave the enclosure are to be insulated, isolated from one another, and terminated inside the enclosure. It seems that once the wiring outside the box is done and terminated inside the enclosure, the UL/CSA/etc. standards govern what happens to the conductors in the circuitry inside the enclosure.
That is, NFPA governs making sure that the wires from outside the box get placed, retained, and terminated permanently to prevent them from moving and causing a hazard or fire. The physical barrier then makes perfect sense. It's to keep the Class 1 wires from contacting Class 2 or even unclassified wiring. Is that right?
On 2: I was not clear in my description. I meant that if the logger enclosure was fed, for instance, 24Vac from a Class 2 transformer, could that low voltage power from a Class 2 transformer source enter the logger on Class 2 wiring?
I was confused by my reading that Class 2 wiring entering an enclosure which also contained Class 1 wiring (in the case of the logger enclosure, the Class 1 sense wiring) might also need to be upgraded to Class 1.
The setup would look like this:

=====Class 1====[ logger box ]===power to logger=====(low voltage AC or DC)==AC mains

My confusion is that "power to logger" section. The logger box needs ETL certification per our plans, and the low voltage AC or DC to power the box from the AC mains on the right need to meet ETL and NFPA codes, but can the "power to logger" be Class 2, or must it be Class 1 wiring only because it enters an enclosure which has Class 1 coming in on the sense wires?

If the "low voltage AC or DC" is an ETL certified, (e.g. UL listed) DC power source, must the power source be labeled Class 2, or can it simply rely on its UL listing and feed Class 2 (or Class 1 if required as above) wiring into the logger to power it?

Thank you for putting up with what must seem like very simple questions to one skilled in the art. I'm skilled in other arts, not this one.

 

[tom baker]

You are asking good questions but outside of my knowledge, and time to answer