Experimental power-limited solar installation?

[tonykuphaldt]

I'm designing a very low power (under 100 Watt), low-voltage (21 VDC) solar-based power "grid" system for my Electronics Engineering Technology students, the purpose of this system being to provide them with a "test bed" for incorporating power-conversion, regulation, and protection circuitry they will design, build, and test. The system will be inherently safe, in that it cannot shock anyone or overheat any of its conductors due to the intrinsic limitations of the solar panels. Any protection modules students build will be unnecessary, only serving an educational need and not a real safety need. So far the plan is to use cable tray to route all the conductors (16 AWG TC-ER cable) along the lab room walls from the panels to NEMA-1 junction boxes where student's experimental circuit modules will reside. The entire system will be indoors, the solar panels mounted up against a large south-facing window.

Ideally this system would fall under NEC Article 725 as a Class-2 power-limited circuit, but so far I cannot find where a low-wattage solar panel is listed as a Class 2 power source as per NEC 725.121(A), nor listed for parallel interconnection as per NEC 725.121(B). At this point I'm thinking it will have to be regarded as a regular Article 690 installation simply for lack of appropriate listing of the panels.

In the not-too-distant future I'll approach my region's AHJ for approval of our plans, but would like to have as much supporting documentation as I can prior to that meeting. Does anyone have experience with power-limited solar electric installations?

Thanks in advance!

 

[GoldDigger]

Depending on how you run your wires, they may not be considered "building wiring" subject to the NEC.
You also need to consider that even a normal glass window will reduce the yield of your panels (reflection loss), and if there is any kind of coating the effect may be large.

 

[tonykuphaldt]

Depending on how you run your wires, they may not be considered "building wiring" subject to the NEC.
You also need to consider that even a normal glass window will reduce the yield of your panels (reflection loss), and if there is any kind of coating the effect may be large.

Thank you for your reply.

That's good to know about the "building wiring" classification. In this whole design process I'm trying to err on the side of safety, interpreting the NEC as conservatively as I can. I suppose my AHJ will be able to make that call?

Your point is well-taken on glass window transmittance. Part of the experimental nature of this system will be how much we must de-rate the panels for losses such as this. Fortunately we have no need to reach any specified power output, because nothing will actually depend on this system for energy!

 

[don_resqcapt19]

While I doubt that the solar cells will be listed as a Class 2 power supply, your AHJ may consider them as such as long as under the worst case the power output cannot exceed those specified in Chapter 9, Table 11(B).

 

[tonykuphaldt]

Thanks -- that makes sense, and hopefully the AHJ sees it as such. We're using that table in chapter 9 as our guide: the system will source no more than 8 Amperes total current (all panels parallel-connected into a dead short at full sun) and no more than 100 VA at peak power.

 

[tortuga]

I'm designing a very low power (under 100 Watt), low-voltage (21 VDC) solar-based power "grid" system for my Electronics Engineering Technology students, the purpose of this system being to provide them with a "test bed" for incorporating power-conversion, regulation, and protection circuitry they will design, build, and test.

Tony your installation sound like it might fall under article 590 Temporary installations if its not intended to be in place for more than 90 days.

 

[tonykuphaldt]

Thanks @tortuga -- it is actually possible to design our system so that all sources are disconnected from the transmission and distribution lines between semesters, and if we lay the cables in open trays as opposed to conduit or surface raceways it would not be that difficult to remove them and roll them up for future re-use. If we use it to power some decorative lights, it might even qualify under 590.3(B)! Then there's 590.3(C) allowing temporary power installations for "tests, experiments, and developmental work" although I'm assuming the idea here is that the experiments aren't being done on the power installations.

A typical semester is 75 weekdays, or about 115 calendar days counting weekends and mid-term breaks, so we could not leave it connected for the entire semester, but maybe for the better part of each semester?? Your idea is definitely worth proposing to the AHJ for their judgment.

 

[tonykuphaldt]

Depending on how you run your wires, they may not be considered "building wiring" subject to the NEC.
You also need to consider that even a normal glass window will reduce the yield of your panels (reflection loss), and if there is any kind of coating the effect may be large.

@GoldDigger , are you aware of any specific criteria for what constitutes "building wiring", as opposed to wiring for some non-building-related circuit that happens to be inside of a building? I don't see anywhere in the NEC where this is explicitly defined. Is there a different NFPA code that covers experimental electrical systems not associated with building infrastructure?

 

[GoldDigger]

Sorry, no.

 

[tonykuphaldt]

I do not quite understand the legislative nuances, but I want to say that you are a great fellow that you make such stands for your students. I am a student who is interested in this, and I can say that we studied only the theoretical part.

Thank you, @bobocat. One of my goals with this system is to expose my electronics engineering students to the world of code, even if the system by its own limitations may not require compliance. It's really important for any student of engineering to see the "big picture" in terms of how their design decisions may affect others, particularly when it comes to safety. Honestly, it's been an education for me as well since I am not a licensed electrician!

 

[tonykuphaldt]

Hi Everyone,

First, thank you for all your input. It was all helpful and thought-provoking.

Today I was able to speak with the state's Electrical Program Manager who is the AHJ for the college campus where I teach, and he told me so long as we build the system so that it does not attach to the building structure nor to the building's electrical system, this will be classified as a "lab project" and not an electrical installation. So, we won't be mounting cable trays or other raceways to the walls, and we won't be mounting enclosures to the walls either. He's fine with us using molded-rubber cable protectors laid on the floor to route wiring so as to avoid trip hazards, and attaching all enclosures to the lab workbenches rather than to the walls.

We will still run the design by the college's Physical Plant manager for insurance purposes, but I've shown him our plans so far and he sees no risk.

Again, thank you for all of your input. This has been an educational experience for me!

 

[tonykuphaldt]

@tonykuphaldt I have a question for you. You, as a teacher, could you check [REDACTED] and see a paper on renewable energy sources. Interested in your opinion. And I would also be grateful to you for any useful literature on this topic. I want to deepen my knowledge of renewable energy and in particular the sun a little. Thank.

I think it would be best to move this conversation out of this forum. Contact me on LinkedIn: https://www.linkedin.com/in/tony-kuphaldt-7913136/

Mod Note: The link posted was to a commercial "dissertation farm". Questionable ethics as well as an attempt at prohibited advertising. The poster has been banned.