LFMC instead of expansion fitting on long Rooftop EMT conduit Run

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The hazards are different. Arc flash hazards are higher in ac electrical system, whereas PV power circuits are current limited. However, legacy PV systems can experience sustained dc arcing faults. (Systems built to NEC 2011 or NEC 2014 should not present the same hazard.) Sustained arcing faults are not something you are likely to see in an ac electrical system.
 
SolarPro said:
The hazards are different. Arc flash hazards are higher in ac electrical system, whereas PV power circuits are current limited. However, legacy PV systems can experience sustained dc arcing faults. (Systems built to NEC 2011 or NEC 2014 should not present the same hazard.) Sustained arcing faults are not something you are likely to see in an ac electrical system.
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OK, so lets think about this thread.

The thread asked about expansion fittings for EMT rooftop conduit.

The idea was put forward that PV conductors where different and required expansion fittings because of that difference.

I am still not seeing it.

What any electrical installation needs is properly coupled, secured and installed raceways regardless of the source.
 
Free-air PV source circuit conductors are different. Once you are in conduit, there isn't a necessarily difference. Although, a lot of commercial PV systems will need 1,000-volt rated wire.

The difference isn't the conductor. The difference is the application. And if you study failures in PV systems, like the author of article I referenced earlier, you start to see trends. These failure trends result in Code changes, among other things. (In fact, Bill Brooks, who wrote that article, is on CMP-4, which is responsible for Articles 690 and 705; he's also on Mike Holt's videos.)

On these giant rooftop systems, there are a lot of opportunities for conductor damage. One of the things that contributes to damage is differential expansion and contraction. The expansion characteristics for copper or aluminum conductors are different than for metal conduit. In practice, this movement can chew through conductor insulation. This is why a lot of commercial rooftop systems are deployed using cable tray. This practice reduces field failures.
 
electrofelon said:
Do you have data to back that up? ...
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Data to back up that PV source circuits and output circuits don't have overcurrent protection that will shut them down? I think you failed to read my whole post.

Yes with PV, a source or output circuit wont shut off, but you also have the fact that it is current limited where a typical feeder or branch circuit is not.
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This is rather a moot point. An AC circuit with an overcurrent device is not going to sustain an arc at a current level higher than the device. With respect to starting fires, what we're worried about here is a sustained low-resistance arc with no device to shut it down. Old-school PV circuits are very much more capable of providing that than AC circuits.

There an important caveat to this, which is that technology has progressed to make PV circuits a lot safer since the Bakersfield fire. With newer non-isolated inverters PV fault dangers are significantly better detected and less likely to become that situation. And if you have optimizers, then I won't claim those circuits are any more dangerous; if fact they're less so.

Not to mention rapid shutdown. With rapid shutdown, non-isolated inverters, and optimizers, I will no longer claim that PV circuits are more of a fire hazard.

iwire said:
You think a PV circuit presents a much higher danger faulting than say a 100 amp 480 volt feeder to an RTU?
I would like to see some numbers to back that up.
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Again, read my whole post. It's not an assertion that really requires data. We have real world examples of this going wrong in PV systems, one of which has been posted in this thread.

A PV output circuit, BTW, may have the same voltage and amperage numbers, or higher.
 
jaggedben said:
...
FWIW, the author is quite widely respected in the PV world, and does not come off as inexperienced when you meet him in person.
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Maybe so, but I worked many years installing long conduits with conductors in the industrial world and have never seen those issues. Often a short conduit in the industrial world is one under 200'.
 
ADub said:
+1 but it seems the solar crowd will take any opportunity to make themselves feel special


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Actually, it is codified. Check out the title of Chapter 6 of the NEC.

[If you don't have it handy—or memorized like Mike Holt—I'll save you the trip: "Chapter 6 — Special Equipment". :p]
 
jaggedben said:
Again, read my whole post. It's not an assertion that really requires data. We have real world examples of this going wrong in PV systems, one of which has been posted in this thread.
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What real world example?

And yes it will requires some data to back up your assertion if you want it to be taken as a fact.

A PV output circuit, BTW, may have the same voltage and amperage numbers, or higher
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As a 480 volt 100 amp utility feeder?

Please provide an example of such a PV circuit that does not have OCPDs
 
iwire said:
What real world example?

And yes it will requires some data to back up your assertion if you want it to be taken as a fact.



As a 480 volt 100 amp utility feeder?

Please provide an example of such a PV circuit that does not have OCPDs
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PV output circuits are DC. The only OCPD they are required to have is string fusing in a case where there are more than 3 PV source circuits (strings of modules) and that is to prevent two or more strings from backfeeding a string fault. Where there is/are 1 or 2 strings there is no OCPD required at all.

That language in the code is frequently misinterpreted; many folks see "PV output circuit" and think it means the output from the inverter. It doesn't; it means the output of a DC combiner or junction box that feeds the inverter.
 
ggunn said:
PV output circuits are DC.
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Thanks.:D

You are aware I have installed PV jobs, designed the electrical end of PV jobs and have gone to some schooling for it.


The only OCPD they are required to have is string fusing in a case where there are more than 3 PV source circuits (strings of modules) and that is to prevent two or more strings from backfeeding a string fault. Where there is/are 1 or 2 strings there is no OCPD required at all.
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Right and when there is only 1 or 2 strings the current levels are low, the fault current level almost as low. No reason for special precautions in running the conduits due to the source of the power.

That language in the code is frequently misinterpreted; many folks see "PV output circuit" and think it means the output from the inverter. It doesn't; it means the output of a DC combiner or junction box that feeds the inverter
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I am well aware of what a PV output circuit is and I am not confused.:lol:
 
iwire said:
Not in the way you are trying to make it sound at all.
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I am not trying to make it sound any way in particular. I believe that the DC conductors in a PV system without rapid shutdown are more of a fire hazard than are AC circuit conductors for the very reason you cite, i.e., that the conductors are energized and capable of sustained arcing in the event of a breach even when the system is shut down. I will continue to put them in conduit that I consider to be safe, and if it takes expansion joints to put my mind at ease, then so be it.

There is no OCPD that can quench a PV DC arc fault. PV modules are current sources and cannot feed more than Isc into a fault, and any and all OCPD on PV DC conductors is designed to pass Isc X 1.25. The only purpose it serves is to prevent backfeeding of a single circuit by two or more other circuits.
 
iwire said:
Thanks.:D

You are aware I have installed PV jobs, designed the electrical end of PV jobs and have gone to some schooling for it.
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Actually, I wasn't. Lots of people post in here who do not understand PV.

iwire said:
Right and when there is only 1 or 2 strings the current levels are low, the fault current level almost as low. No reason for special precautions in running the conduits due to the source of the power.
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Yes, but say you have a system with 30 PV source circuits going into a DC combiner and a PV output circuit going from the combiner to the inverter. There is typically no OCPD on that output circuit. Fault current is 30 X Isc and voltage could approach 1000V. I would be very cautious with that circuit if there is no provision for rapid shutdown.
 
SolarPro said:
Actually, it is codified. Check out the title of Chapter 6 of the NEC.

[If you don't have it handy—or memorized like Mike Holt—I'll save you the trip: "Chapter 6 — Special Equipment". :p]
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And what am I looking for in chapter 6? Where it says that pv system conduits aren't subject to the same requirements as other electrical system conduits?

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jaggedben said:
Data to back up that PV source circuits and output circuits don't have overcurrent protection that will shut them down? I think you failed to read my whole post.
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no this:

the dangers are different and much higher.
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Ill say the dangers are different/unique, but that the dangers are much higher I am not sure. This whole thing started (I think) because it was in some words stated or implied that PV conduits need expansion couplings more than non PV conduits. I think the following:

1. I cant see the lack of expansion couplings damaging conductors.
2. I doubt the lack of expansion couplings (think the picture from bakersfield posted earlier) caused or contributed to that. Poor workmanship, specifically a non fully inserted and/or loose fitting did
3. I understand and respect the dangers of a current limited fault between say a loose or separated fitting just sitting there for hours, however I also dont think a non current limited fault is as black and white as you say it is; its not just necessarily going to trip the breaker right away and leave a little black spot on the conduit. These can be arcing events too and although likely not as long lasting as a current limited fault they have much higher energy and heat generated and can be very dramatic. Lets take a several hundred foot long 480V 100 amp feeder in EMT and do some poor workmanship - some loose couplings, non fully inserted fittings, loose lock nuts at the cabinet, paint not removed - and see if it clears right away :)
4. I would guess that PV systems generally have poorer workmanship than "normal" electrical installations. Many PV workers are not even electricians. I do some work with a crew where out of 9 guys, I am the only real electrician. I think that is typical, at least from my experience.


This is rather a moot point. An AC circuit with an overcurrent device is not going to sustain an arc at a current level higher than the device. With respect to starting fires, what we're worried about here is a sustained low-resistance arc with no device to shut it down. Old-school PV circuits are very much more capable of providing that than AC circuits.

There an important caveat to this, which is that technology has progressed to make PV circuits a lot safer since the Bakersfield fire. With newer non-isolated inverters PV fault dangers are significantly better detected and less likely to become that situation. And if you have optimizers, then I won't claim those circuits are any more dangerous; if fact they're less so.

Not to mention rapid shutdown. With rapid shutdown, non-isolated inverters, and optimizers, I will no longer claim that PV circuits are more of a fire hazard.
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Pretty much responded in my numbered list above. I agree that older systems like at bakersfield with the "blind spot" are more of a risk for fire due to poor workmanship.
 
electrofelon said:
no this:
4. I would guess that PV systems generally have poorer workmanship than "normal" electrical installations. Many PV workers are not even electricians. I do some work with a crew where out of 9 guys, I am the only real electrician. I think that is typical, at least from my experience.
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Okay, we'll how about that as a justification for why the dangers with PV circuits are higher. :lol:

Seriously, I don't have that much of a bone to pick in this fight, but I think that there have been some articles published in PV circles recommending expansion joints because:

1) PV circuit conduits usually must be run on hot rooftops where the issue is most extreme
2) There are unique dangers (I'll withdraw 'higher') to PV circuits which argue for above-code standards.
3) There has been at least one well-known incident in which the lack of expansion joints was thought to have contributed to major damage. (Correct or incorrect, some people who investigated in person evidently said that.)
 
ADub said:
And what am I looking for in chapter 6? Where it says that pv system conduits aren't subject to the same requirements as other electrical system conduits?
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Ever heard of a straw man argument? PV system circuits are subject tosome additional requirements. Expansion joints are not one of them, though.
 
LFMC instead of expansion fitting on long Rooftop EMT conduit Run

jaggedben said:
Ever heard of a straw man argument? PV system circuits are subject tosome additional requirements. Expansion joints are not one of them, though.
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Which is funny bc this thread had nothing to do with pv to start and now look.

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