Difference between two

[codequestion]

What is the difference between PV source circuit and PV output circuit? Also, does anyone have clear schematic or diagram besides the one shown in nec 2014?

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[pv_n00b]

When PV source and output circuits were defined in the early versions of 690 it was to allow the code to specify where OCP was needed and more specifically where it was not needed. AHJs wanted to put fuses in every string at the modules because many people did not understand the current limiting quality of PV modules. To keep AHJs from requiring OCP at the modules PV source circuits were defined and the code specified that OCP was not required. So that's how it started.

It's more complex now. If there is a wiring harness that combines strings in the harness is that still a PV source circuit? If there are DC to DC converters at the module or string level is it still a PV source circuit? If a DC to DC converter takes in several strings and has only one output conductor is that output a PV output circuit? There are differing interpretations. I think at his point making the distinction is an anachronism that just makes working with the PV DC circuits more difficult.

My interpretation is according to a strict reading of the NEC, once two or more PV source circuits are combined at a common connection point that point represents the transition from PV source circuits to a PV output circuit. This still does not deal with the issue of a DC to DC converter that takes in multiple source circuits and outputs one circuit. Is it really a common connection point when going through the electronics? If there can't be any back feed then I would say not.

 

[codequestion]

What you talking about? 690.9 says overcurrent protection required at pv source circuit?

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[jaggedben]

What is the difference between PV source circuit and PV output circuit? Also, does anyone have clear schematic or diagram besides the one shown in nec 2014?

Short answer: it's a source circuit until you parallel it with another source circuit. The output of two or more paralleled source circuits is an output circuit.

What you talking about? 690.9 says overcurrent protection required at pv source circuit?

Not really. See the exceptions. What it comes down to is that overcurrent protection isn't required unless you have more than two paralleled circuits, or different circuit current values. Single source circuits don't require overcurrent protection because PV modules can't realistically produce an overcurrent.

Also: don't be mislead by pv_noob's handle, he's probably the most experienced regular PV poster on this forum.

 

[codequestion]

Short answer: it's a source circuit until you parallel it with another source circuit. The output of two or more paralleled source circuits is an output circuit.



Not really. See the exceptions. What it comes down to is that overcurrent protection isn't required unless you have more than two paralleled circuits, or different circuit current values. Single source circuits don't require overcurrent protection because PV modules can't realistically produce an overcurrent.

Also: don't be mislead by pv_noob's handle, he's probably the most experienced regular PV poster on this forum.

So if I have 3 solar panels, each panel made up of 14 modules lets say and each panel their is y feeder cables going to junction box. Total going into junction box 3*y and other side of junction box same number of cables 3*y come out and feed inverter then which is my pv source circuit and pv output circuit? Do i need overcurrent protection between panel and junction box?

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[ggunn]

Short answer: it's a source circuit until you parallel it with another source circuit. The output of two or more paralleled source circuits is an output circuit.



Not really. See the exceptions. What it comes down to is that overcurrent protection isn't required unless you have more than two paralleled circuits, or different circuit current values. Single source circuits don't require overcurrent protection because PV modules can't realistically produce an overcurrent.

Also: don't be mislead by pv_noob's handle, he's probably the most experienced regular PV poster on this forum.

I couldn't put my finger on the code reference, but I'm pretty sure it allows for the unfused combination of more than two strings as long as the available fault current is less than the rating of the maximum series fuse allowed by the modules.

And what am I, chopped liver?

 

[codequestion]

Okay well i dont know what code section that you are mentioning either. Chopped liver?

I couldn't put my finger on the code reference, but I'm pretty sure it allows for the unfused combination of more than two strings as long as the available fault current is less than the rating of the maximum series fuse allowed by the modules.

And what am I, chopped liver?

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[ggunn]

Okay well i dont know what code section that you are mentioning either. Chopped liver?

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I remember going through this exercise a year or so ago, and I found that it was possible with high voltage, low current thin films modules one could legally combine three strings without fuses.

Found it. In the 2014 NEC it's 690.9(A) Exception (b).

 

[jaggedben]

So if I have 3 solar panels, each panel made up of 14 modules lets say and each panel their is y feeder cables going to junction box. Total going into junction box 3*y and other side of junction box same number of cables 3*y come out and feed inverter then which is my pv source circuit and pv output circuit? Do i need overcurrent protection between panel and junction box?

The wires going to the panels are the source circuits. The combined output connecting to the inverter is the output circuit.

Btw, the code definition for 'panel' does not describe a real world object these days. Sorry for the confusion. We arrange modules electrically into strings, which are the same as source circuits (unless you have optimizer or microinverters). What the code calls modules is what lay people call panels.

Overcurrent requirement protection depends on further details, see other posts.

 

[jaggedben]

I remember going through this exercise a year or so ago, and I found that it was possible with high voltage, low current thin films modules one could legally combine three strings without fuses.

Found it. In the 2014 NEC it's 690.9(A) Exception (b).

I would say if you've got modules that have a fuse rating that's s bit more than twice their short circuit current rating, you could probably do 3 strings with no fuses. I'm not so surprised you found a thin film module like that, but I'd be surprised to see a silicon module that qualified.

And yes, you're chopped liver. :lol::lol: My excuse is I did say 'probably' re pv_noob. You and he can throw down if you want.

 

[pv_n00b]

What you talking about? 690.9 says overcurrent protection required at pv source circuit?

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Not at the modules though. If you parallel source circuits together and there is more fault current available to a faulted source circuit than it is rated for, either the conductor or the module, then OCP is required in the DC combiner, or in the harness if that is being used to parallel source circuits. OCP is usually required whenever you parallel more than 2 source ckts, but it was not always that way. There was a time when the series fuse rating of some modules was much larger than the Isc value. At that time you could parallel 3, 4, or more source circuits without OCP. Those days are gone though.

 

[pv_n00b]

Also: don't be mislead by pv_noob's handle, he's probably the most experienced regular PV poster on this forum.

I defend my total and complete n00bness. I have no idea why people listen to me.

 

[pv_n00b]

So if I have 3 solar panels, each panel made up of 14 modules lets say and each panel their is y feeder cables going to junction box. Total going into junction box 3*y and other side of junction box same number of cables 3*y come out and feed inverter then which is my pv source circuit and pv output circuit? Do i need overcurrent protection between panel and junction box?

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Passthru junction boxes do not combine the circuits, so PV source ckts in, PV source ckts out. A PV system is not required to have a PV output ckt, it can be source ckts all the way down.

 

[ggunn]

Not at the modules though. If you parallel source circuits together and there is more fault current available to a faulted source circuit than it is rated for, either the conductor or the module, then OCP is required in the DC combiner, or in the harness if that is being used to parallel source circuits. OCP is usually required whenever you parallel more than 2 source ckts, but it was not always that way. There was a time when the series fuse rating of some modules was much larger than the Isc value. At that time you could parallel 3, 4, or more source circuits without OCP. Those days are gone though.

Are you sure? Last I checked there were some high V low I thin films modules with max series fuse ratings high enough to allow you to combine more than 2 strings.

 

[pv_n00b]

Are you sure? Last I checked there were some high V low I thin films modules with max series fuse ratings high enough to allow you to combine more than 2 strings.

I have not run across on in a while, feel free to point me toward any you know. Thin film did have the highest series fuse verses Isc ratios in the past. The thin film modules I have seen lately though have low series fuse ratings. Take the FirstSolar series 6, it has a series fuse rating of 6A. So to put three strings in series the Isc of each string would have to be less than 6/1.25/2= 2.4A and all the series 6 Isc values are 2.54A or more.

I'm not sure why this is the case. The series fuse value is given to the NRTL by the module manufacturer and the NRTL tests that the module can support that amount of reverse current. It's not like the NRTL determines the fuse value through testing and gives it to the manufacturer. So the module could be designed to support a higher reverse current. Most likely it's cheaper to design to the lowest reverse current possible.

 

[ggunn]

I have not run across on in a while, feel free to point me toward any you know. Thin film did have the highest series fuse verses Isc ratios in the past. The thin film modules I have seen lately though have low series fuse ratings. Take the FirstSolar series 6, it has a series fuse rating of 6A. So to put three strings in series the Isc of each string would have to be less than 6/1.25/2= 2.4A and all the series 6 Isc values are 2.54A or more.

I'm not sure why this is the case. The series fuse value is given to the NRTL by the module manufacturer and the NRTL tests that the module can support that amount of reverse current. It's not like the NRTL determines the fuse value through testing and gives it to the manufacturer. So the module could be designed to support a higher reverse current. Most likely it's cheaper to design to the lowest reverse current possible.

I haven't seen one lately, but then, I haven't looked. I just wanted to point out that the NEC doesn't specify that more than two parallel strings must be fused, just that the available fault current in paralleled unfused strings must not exceed the maximum module fuse rating.