MPPT Controller De-rating Graph

[J.P.M.]

FWIW, I can appreciate both, or all sides of what this discussion turned into. Looking at it from what may be some of SK's perspective, using a 1.25 factor may be first off, safe(r), more practical and less calc. work, but I'd probably still do the calcs anyway if anyone asked for them, particularly if design temps. went below -40, but otherwise, still use wire sizes/components based on the voltage a 1.25 factor gave me. Having a license and potential liability can make one conservative in design philosophy.

As a practical consideration, and in a vendor "do as much the same every time as possible" standardization, any relative $ savings in materials from cutting things close every time can be quickly lost by calc time and engineering charges, not to mention added inventory costs and a somewhat hidden cost of more opportunity for mistakes by using the wrong stuff. All this may seem like overkill, but practical reality (and safety) trumps theory most times.

On the other side, I'd suggest that doing the design calcs is never a bad idea, provided they are understood, as well as the reasons behind them, their necessity and their limitations. One doesn't need to have a P.E. license to understand what's required. Just a brain.

Reading all this got me thinking. If one goal of codes is safety, and that goal has Voltage as f(operating temp) as a parameter, as one example only, one temp. consideration that neither codes (at least not explicitly to my knowledge) nor anyone here has mentioned, is the idea that a PV array can, particularly in the type of climate that may use design temps of -40, have an operating temp. less than -40 due to nocturnal, or more likely, early morning radiative sky cooling. I note that, as a comment to the informational note added to 690.7 (A), that the ASHRAE temp. mentioned as an informational note does not take such radiative cooling into account. Such cooling can depress equipment temps. a few to maybe 10 C. below the ambient temp. Just an example of situational awareness that good engineering requires that codes may miss. There are many others.

Around most situations and in this case about what Voltage to use for design, relying on the most conservative code direction will probably get you by and may even be the most cost effective and common sense approach. But, throwing a code at a situation by using code calcs, or worst case, or not, without understanding what the whole situation can entail, and in so doing getting hung up on what may be written in a code while not looking at what the situation really calls for, maybe even beyond code is, to me anyway, poor engineering.

[Sunking]

Using the 1.25% multiplier might be on the conservative side but it would be accepted 100% of the time by all AHJ's. It isn't a flag indicating a designer who is clueless but someone that understands the code and does not push the envelope to save a few pennies.

Using a lower multiplier could be accepted by a % of the AHJ's but unless there is a critical reason to push the voltage into the CC why take the chance of getting a red flag by the AHJ or over loading the CC?

I am sure you can find a good reason for getting the perfect calculation, but based on the hands on experience per Sunking and myself (on the electrical power side at least) being more conservative makes things go more smoothly and quickly when it comes to project planing and installations.

Yep you get it. Called experience.

[SunEagle]

FWIW, I can appreciate both, or all sides of what this discussion turned into. Looking at it from what may be some of SK's perspective, using a 1.25 factor may be first off, safe(r), more practical and less calc. work, but I'd probably still do the calcs anyway if anyone asked for them, particularly if design temps. went below -40, but otherwise, still use wire sizes/components based on the voltage a 1.25 factor gave me. Having a license and potential liability can make one conservative in design philosophy.

As a practical consideration, and in a vendor "do as much the same every time as possible" standardization, any relative $ savings in materials from cutting things close every time can be quickly lost by calc time and engineering charges, not to mention added inventory costs and a somewhat hidden cost of more opportunity for mistakes by using the wrong stuff. All this may seem like overkill, but practical reality (and safety) trumps theory most times.

On the other side, I'd suggest that doing the design calcs is never a bad idea, provided they are understood, as well as the reasons behind them, their necessity and their limitations. One doesn't need to have a P.E. license to understand what's required. Just a brain.

Reading all this got me thinking. If one goal of codes is safety, and that goal has Voltage as f(operating temp) as a parameter, as one example only, one temp. consideration that neither codes (at least not explicitly to my knowledge) nor anyone here has mentioned, is the idea that a PV array can, particularly in the type of climate that may use design temps of -40, have an operating temp. less than -40 due to nocturnal, or more likely, early morning radiative sky cooling. I note that, as a comment to the informational note added to 690.7 (A), that the ASHRAE temp. mentioned as an informational note does not take such radiative cooling into account. Such cooling can depress equipment temps. a few to maybe 10 C. below the ambient temp. Just an example of situational awareness that good engineering requires that codes may miss. There are many others.

Around most situations and in this case about what Voltage to use for design, relying on the most conservative code direction will probably get you by and may even be the most cost effective and common sense approach. But, throwing a code at a situation by using code calcs, or worst case, or not, without understanding what the whole situation can entail, and in so doing getting hung up on what may be written in a code while not looking at what the situation really calls for, maybe even beyond code is, to me anyway, poor engineering.

I agree. The Engineer should first do the calculations and then determine if there needs to be a more conservative answer. But if timing is important and lost time is lost money then sometimes just using a standard (and conservative) multiplier becomes the best answer for the plan.

I have never heard any inspector complain about someone putting too much concrete into a foundation.

[Sunking]

Reading all this got me thinking. If one goal of codes is safety,

Life and property safety are the main objective. The biggest problem most folks with Code is they think it is a How To or Design Guide which it is not. You get yourself in a lot of trouble trying to use NEC as a How To or Design Code as Sensij has done. He is at the point reading things that are not in the Code. The correct way to use the NEC Code is as "What you cannot do".

Around most situations and in this case about what Voltage to use for design, relying on the most conservative code direction will probably get you by and may even be the most cost effective and common sense approach. But, throwing a code at a situation by using code calcs, or worst case, or not, without understanding what the whole situation can entail, and in so doing getting hung up on what may be written in a code while not looking at what the situation really calls for, maybe even beyond code is, to me anyway, poor engineering.

Well said JPM, you understand. For me it is real simple. Can I sleep at night with my designs. My priorities in order is

Is the Design Safe as it can be
Most Cost Effective
Performance and does what the client expects.

As Max brought up when the scale gets larger, Cost effective measures change the landscape. Shaking out that extra panel in series more than offsets the extra cost. In NEC code there are several articles that offer lieninancies if certain conditions are met. 690.7 is one of those lienionancies. If you use 1.25, no questions asked or raise as it is the most restrictive. As WWW said the AHJ cannot ask for more unless local amendments alter the code. In Wyoming goes up to 150% which is even more restrictive. If you want to use something lower than 1.25 to take advantage of the leniencies, you are going to Raise Red Flags. It is at that point all that extra wording in the code takes precedence and now you have to back it up with spec sheets, calculations., and sign-offs. That takes extra time and money. Use 1.25 and I am not required to do anything. Less than 1.25 and you now have to jump through hoops, get questioned, and take the risk.

What Plane do you want to fly in? One where the wings are only rated at 100% of the load, or 125% of the load.



Score Engineers 3, Sensij 0

[J.P.M.]

I agree. The Engineer should first do the calculations and then determine if there needs to be a more conservative answer. But if timing is important and lost time is lost money then sometimes just using a standard (and conservative) multiplier becomes the best answer for the plan.

I have never heard any inspector complain about someone putting too much concrete into a foundation.

Part of my point was knowing what "conservative" actually amounts to in a given application. As you, and any engineer worthy of the title well knows, engineering is much more of an art than it is simply throwing codes and handbooks at a task. I've seen people claiming to be "engineers" overdesign stuff in the name of "conservative design" by misapplication and misunderstanding of what "the Code" was intending to the point the design wouldn't be (as) fit for purpose and perhaps not as safe as possible because of designer ignorance of the intent and the limitations of a code.

The way I learned it in the pressure vessel design business, and with applications to other engineering disciplines: Follow (in my case) the ASME Boiler and Pressure Vessel Code. Live by the code. Die by the Code. And, and more importantly, know what the code is saying, and most importantly, WHY. And of course, the inspector is always right. You may actually know more than the inspector, but the inspector is still right. Do what he says. Argue with the inspector's boss later if you think the inspector is mistaken. I actually had a code case go in my favor once, but that was long after the equipment shipped.

[sensij]

Part of my point was knowing what "conservative" actually amounts to in a given application. As you, and any engineer worthy of the title well knows, engineering is much more of an art than it is simply throwing codes and handbooks at a task. I've seen people claiming to be "engineers" overdesign stuff in the name of "conservative design" by misapplication and misunderstanding of what "the Code" was intending to the point the design wouldn't be (as) fit for purpose and perhaps not as safe as possible because of designer ignorance of the intent and the limitations of a code.

Let's take a step back for a minute and review what we've seen in this thread.

Sunking asserted that "the rules" require 125% adjustment to Voc. I've offered the clarification, by citing code, that there are no rules that say any such thing. Code does not require it, industry literature does not support it, and you won't find it written in the instructions for any equipment produced by a reputable manufacturer.

Setting the "rules" aside, we can look more closely at what good design practice might be. J.P.M. offered the observation that code may not be taking into account the fact that cell temperature can drop below ambient temperature. If we are going to analyze the calculation required by code to that level, we should also take into account the fact that full Voc won't be generated at very low irradiance, and it takes 200 W/m2 or more of POA irradiance before Voc can approach data sheet values. We also need to keep in mind that Voc is only experienced when the sun is up if the charge controller is not operating, or operating in a coarse PWM mode. If we want to identify a set of conditions where we think code may not be conservative enough, it would require combing through actual environmental data, looking for conditions with sufficient irradiance, ambient temp approaching the ASHRAE minimum, early enough in the day for Tcell to still be below ambient temp due to radiative cooling effects, and the array in open circuit.

Looking just at Tucson, since it is somewhat relevant to the OP, we see that the ASHRAE temp is -3 deg. The minimum temp recorded at Davis-Monthan AFB, per Wunderground, was -8 deg C in 2011, on Feb 3. A flat mounted array would see 200 W/m2 between 8:30 and 9:00 am, with ambient temp still around -6 deg according to the weather history for that day. Is this a corner case in which the code calculation is not conservative enough? We'd still have to go into SAM or something like that to model what Voc might be expected, but the conditions could justify a closer look.

However, I am *not* suggesting this level of analysis is routinely necessary. An *authentic* voice of experience is able to say "when I design my systems per code, I have run into over-voltage conditions, so I prefer to use a more conservative adjustment". That is not the experience that Sunking purports to have. His claim is that performing the calculation per code is a "red flag" for an AHJ, an experience that no other person is able to verify. Is over-voltage a common failure mode, requiring conservative design because code fails us? In the collective experience of this forum, and in my own experience and the experience of those whose systems I follow, this is not the case. Not once have we seen here a cry for help from someone who designed their system per code, and ran into a failure because code wasn't conservative enough in calculating Voc.

I'm all for accepting that commercial expediency may require "conservative" designs that can be deployed into most markets without additional thought. This is not a commercial forum where costs are counted and systems are sold... it is a place we can exchange knowledge and experience related to the requirements, design, economics, performance, maintenance, and troubleshooting (and more) of solar panel systems. Misrepresenting system behavior on the basis of poorly justified rules of thumb does a disservice to all honest seekers of information. If Sunking wants to say... use 125% because I'm not willing to spend more time on this post to calculate it out correctly, so be it. He posts a lot more than anyone else, and I understand that every post can't get A+ effort. Please, let's not call that "good, experienced engineering", and let's not jump down the throats of those who can offer a more detailed response that is grounded in actual requirements and observed system performance.

[SunEagle]

Part of my point was knowing what "conservative" actually amounts to in a given application. As you, and any engineer worthy of the title well knows, engineering is much more of an art than it is simply throwing codes and handbooks at a task. I've seen people claiming to be "engineers" overdesign stuff in the name of "conservative design" by misapplication and misunderstanding of what "the Code" was intending to the point the design wouldn't be (as) fit for purpose and perhaps not as safe as possible because of designer ignorance of the intent and the limitations of a code.

The way I learned it in the pressure vessel design business, and with applications to other engineering disciplines: Follow (in my case) the ASME Boiler and Pressure Vessel Code. Live by the code. Die by the Code. And, and more importantly, know what the code is saying, and most importantly, WHY. And of course, the inspector is always right. You may actually know more than the inspector, but the inspector is still right. Do what he says. Argue with the inspector's boss later if you think the inspector is mistaken. I actually had a code case go in my favor once, but that was long after the equipment shipped.

Another point is that most codes keep changing (sometimes for the better) so what was a conservative value back in 1975 may not be enough for 2017.

I agree that some Engineers hide behind the "overdesign" syndrome because they either are too lazy to do the work or in a lot of cases are put on an expressed time line (not always to their choosing) to get something completed so the next project is started.

It is sometimes hard to determine if the Engineer was just hip shooting (from experience) the value, is under pressure to provide a value so it will be very conservative or is really unqualified to do the calculations properly and just guesses.

[J.P.M.]

Another point is that most codes keep changing (sometimes for the better) so what was a conservative value back in 1975 may not be enough for 2017.

I agree that some Engineers hide behind the "overdesign" syndrome because they either are too lazy to do the work or in a lot of cases are put on an expressed time line (not always to their choosing) to get something completed so the next project is started.

It is sometimes hard to determine if the Engineer was just hip shooting (from experience) the value, is under pressure to provide a value so it will be very conservative or is really unqualified to do the calculations properly and just guesses.

Agree with all. My experience is (was) that a lot of national standard codes reissue every 3 years or so with annual updates. Staying current with codes, especially those in your area of proficiency was always a requirement.

Finding the time and dealing with deadlines while producing safe and good designs is one of the reasons why P.E''s make bigger bucks. There are many ways to get there and still produce safe, workable designs. Prioritize with safety as the uncompromisable top design priority.

My experience was mostly that unqualified personnel were soon weeded out of organizations by others who are qualified. That's not to say the real world and reality don't modify best efforts more than we'd all like.

[Sunking]

Let's take a step back for a minute and review what we've seen in this thread.

Oh please lets do that.

AZR66 asked what a graph meant. The answer was simple and complete. Keep the Panel Vmp below 115 volts, if not the Controller will Clip Power. I then said follow the Rules and the problem takes care of itself.

Then you come along looking for trouble. You found it and yourself on the ropes taking it from three people. You stuck your foot in your mouth, then cannot read what is being said

Sunking asserted that "the rules" require 125% adjustment to Voc. I've offered the clarification, by citing code, that there are no rules that say any such thing. Code does not require it, industry literature does not support it, and you won't find it written in the instructions for any equipment produced by a reputable manufacturer.

You cannot even read or understand code correctly or what i said. For some reason to you this does not mean what it says

the rated open-circuit voltage shall be multiplied by the correction factor provided in Table 690.7.

What part of that do you not understand Go to that table and 125% is right in there. You also have the option to use a lower number from the table using lowest possible Ambient Temp, or using actual manufacture data sheets and low Ambient Temp. You can use whatever 3 options you want. What I am telling you and everyone else is if you intend to use one of the leniencies offered in the Code comes with conditions that must be met. If you intend to use something less than 125%, you are going to be required to prove it and certified in wrriting. If you use 125%, is the most stringent conservative number no Inspector can question, nor would any peer review board if something goes wrong like it got colder than you calculated, you will have to explain and justify why you used a Correction Factor lower then required. Use 125% and you exceed any requirements unless you are in Alaska, Canada, or one of the Arctic Poles. You are CYA down to -40.

As JPM knows well the 2 Golden Rules of getting along with your Inspector and Wife:

1. Wifey and Inspector are always right?
2. When Wifey or Inspector is in error, refer to rule 1

So now it is time for Inspection, the Inspector notice you have 6 panels in series on a 150 Voc limit Controller. So the Inspector ask what did you use for the Correction Factor. You answer 1.10 Sir. Inspector says wrong answer boy, in my book it gets colder than that and you shall use 1.18. You failed Inspection, call me after when you are ready and pay an additional Inspection Fee. If you had used 125%, the question and resulting issue would have never happened because you met or Exceeded the Inspector INTERPRETATION which brings us back to JPM's and my point. point.

ts a game out there, and experience teaches us how to play the game using our own rules to make things easier, less expensive, safer, and perform as good or better than expected. Electrical Code Rules are very minimum, and just there for Safety only. Does not guarantee equipment operation or any performance level. So when you look at a code for voltage you look at the most Conservative rating, in this case is 125%, there is nothing more than the code can ask for. Inspector cannot say 126 or 130% unless as WWW pointed out can be Amended in Local Codes. No wif you choose something more lenient like 115%, yep the Inspector can ask you to use 120% or 125% and you must comply. He cannot say a word at 125%

That is what you do not understand Senji. I do not know, do not care, and do not want to see panel specs. It is not required because the NEC gives me several options. Option one without knowing the panel Specs i opt to use Table 690.7 and use 125% There is no tougher standard. It Exceeds manufactures minimums and code minimums. . For such a small scale application (150 Voc ) there is nothing to gain using anything less than 125%, The worse thing that can happen is you end up using 2 parallel strings.

All this crap because you wanted to pick a fight. Perhaps you might understand it this way. We are playing Poker and I draw a Royal Flush Spades against your Full House with Kings and Aces. Who knows they are going to win no matter what you have. An Inspector cannot ask more than 125%, but they can demand you use 125 or something higher than you used. You win every time at 125% No questions asked.

[sensij]

Code does not give options on this. 690.7 states

When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum PV system voltage as required by 110.3(B) instead of using Table 690.7.

Do you understand what "shall be used" means? Do you understand what "instead of" means?

Look at a snip from San Diego's PV plan submittal form:

san diego2.JPG







.

I get that you want to make an economic case that 1.25 is a good factor to use. Maybe it is, maybe it isn't, I'm not arguing that point here. The point at that there is no "Rule" that says 125% is the right value to use, and code certainly does not support it. In this forum, factual inaccuracy is not tolerated, you know that as well as anyone, and your argument that the Rule is 125% is factually incorrect. If your skin is too thin to be on the receiving end of correction, maybe you should consider plying your trade somewhere else.

[Sunking]

Code does not give options on this. 690.7 states



Do you understand what "shall be used" means? Do you understand what "instead of" means?

.

You lost the argument. 125% exceeds all requirements. Exact same as a 20 amp breaker, you had better have at least a 12 AWG wire on it, or an engineers motor calcs if 14 AWG. If the Inspector sees a 10 AWG on that 20 amp breaker could care less because you exceeded the requirements. He knows you went with performance specs rather than code minimum.

Like Sun Eagle said, he has never seen or heard of an Inspector questioning more cement than required. 125% exceeds any code requirement. What part of EXCEED Do You Not Understand? Dude I sat on NEC code panels for 6 years, personally know John Wiles who wrote 690 and worked with him, teach code, and I am also a moderator on Mike Holt's Code Forum. That makes me an expert. What are your credentials?

You started this and I will finish this. It is all on you.

FWIW try this. Pull any manufacture sheet out you want for 36 60 , and 72 cell panels. Pull as many as you want and run the temp calcs at -40. None will be greater than 125% At most in my experience is 123%. What you cannot get through your head is Code is minimum requirements. Engineering and Design Practices exceeds code minimums. A Inspector cannot say or do anything if you give him/her more than they require. If your buddy owes you $15 and gives you $20 and says keep the change, did he exceed your expectation and requirements? Heck yes he did, you put the $20 in your pocket and smile.

To Max's point there are times when you can use the lienancis of the code. Yep you can use a manufacture spec sheet to make the numbers work in your favor to squeeze that extra panel in. Because if you used 125% it does not work. Yo uhave to make a compromise and jump through the extra hoops to get away with it. You only do that when economics turn the tide.

[max2k]

...To Max's point there are times when you can use the lienancis of the code. Yep you can use a manufacture spec sheet to make the numbers work in your favor to squeeze that extra panel in. Because if you used 125% it does not work. Yo uhave to make a compromise and jump through the extra hoops to get away with it. You only do that when economics turn the tide.

I actually didn't have a choice- OC template permit application has that multiplier printed as 1.12. I verified it works and code compliant but I wouldn't take liberty and correct it to 1.25 for 2 reasons:
- AHJ is always right
- it is unlikely SoCal is going to get -40 C weather any time soon

[Sunking]

I actually didn't have a choice- OC template permit application has that multiplier printed as 1.12. I verified it works and code compliant but I wouldn't take liberty and correct it to 1.25 for 2 reasons:
- AHJ is always right
- it is unlikely SoCal is going to get -40 C weather any time soon

Max they could not have said anything about it because 1,25 is more restrictive and exceeds their minimum requirement. Now if you were to use 1.10, you would fail. Think of it like wire size on a breaker. A 20 amp breaker requires a minimum 12 AWG wire. If mr Inspector sees a 14 AWG wire on a 20 amp breaker, you fail. If a Inspectors sees a 12 AWG or larger wire, you pass because you met or exceeded the requirement. You can use a 750 MCN conductor on a 20 amp breaker and there is not a dang thing the Inspector can say about it.


Stop and go back to your first reply. You stated if you used 1.25 you would not have been allowed to use 13 panels in series, and limited to 11 in series correct? You had to use a conditional Leniency provision using 1.12 to make the numbers work for you.

t is great your local jurisdiction tells you what the lowest Correction Factor you can use. It means you can use 1.12 or higher. You cannot go any lower. That is the difference between Practice and Code. A well written Practice will meet and most times exceed Code requirements. Using Voltage Drop is a great example of a Practice vs Code. Back to the 20 amp breaker. Code says 12 AWG shall be used on a 20 amp breaker. Well that is not good enough in low voltage. You can run at 16 amps continuous on a 20 amp beaker. If you're operating voltage is say 12 volts with a 16 amp load, I sure hope that load device is just 3 short cable feet away because once you get more than 3-feet, voltage loss becomes significant. At 10 feet becomes unusable. Easy to work around, use larger wire to limit voltage drop. You are using a Performance Practice, not code.

Remember what I said, Code is minimum safety requirements. Code does not ensure a working system. A design Practice ensures it works and will meet and/or exceed minimal code requirements.

[Mike90250]

And Code in one area/state/county is going to be different than another. One more permissive, one more restrictive. NEC covers basics, but local jurisdictions always play with it.