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HeatSpring PV Course Blog


Determining low temperature that caused the inverter to go over voltage

Posted on 27 May, 2016 at 2:46 Comments comments (359)


Sean: Thanks for a great course. Two questions: I think I would like some additional training for the PV Installation Professional Certification Exam. Do you have any recommendations?
Also with the addition practice questions, I got really knotted up on questions 74 and 78. On question 78, when you state the temperature coefficient for the module, is this actually Voc x Temperature Coefficient Voc?
Again, thank you for two great classes. I look forward to meeting you in person someday, and perhaps attending hands-on training you might offer.
My best to you


For 74 and 78 we have to work backwards.

We are asked to determine the cold temperature and we are given the number of modules in series, temperature coefficient Voc and the inverter maximum voltage.

We first divide the inverter voltage by the number of modules to get the cold temperature voltage.

78 has a temperature coefficient that is in mV per C, so it is a little confusing, but simple once you see it.

145mV = 0.145V

We then find out the difference between the Voc at STC and the Voc at cold temperature.

Then we will divide that difference in voltage by 0.145V to get our delta T and then figure out how far our delta T is from 25C.

1000V/20 modules = 50V per module

50V cold Voc - 44V STC Voc = 6V is the difference in voltage

6V/0.145V = 41.4 Delta T

41.4 less than 25 is -16.4C

Isn't that cool!

Sounds like you are about done with the class.

This is the best class for preparing for the NABCEP PVIP exam and I think the best thing to do is go over this material a bunch.

I also teach workshops at Intersolar US in SF in July with Bill Brooks and at Solar Power International in September (Vegas) with Bill Brooks right before the NABCEP Exam. That would be a good way for you to get a chance and heckle me from the back row!

Also, there are other classes here at HeatSpring by other instructors, such as Ryan Mayfield

I have a NABCEP PVIP Prep course here too that covers a lot of the same material as this course.


Sean White

Advanced PV Class Review

Posted on 24 May, 2016 at 22:52 Comments comments (247)

Submitted: 05/21/2016
Training: 40-Hour Advanced Solar PV Installer Training / Online / Anytime

How would you rate this course?
What did you like about the course?:
That it continuosly answers questions that I've had or I get in the workplace. I like that Sean uses a lot of examples and does not limit to the short explanation of an answer.
How effective was the instructor's communication during the course?:
What would you change about the course?:
I can't think of anything.

Thanks for the great review!!

Passed the NABCEP PVIP Exam!!

Posted on 24 May, 2016 at 18:35 Comments comments (108)
From HeatSpring Advanced PV Course Discussion Board


I took the April exam for the first time and just found out I PASSED. After the completion of this course I felt really prepared and confident going into it . This course definitely covered enough information to pass the NABCEP exam and much more that I will be able to apply in my future career in solar. Sean White's book is the "Swiss army knife "of study material. Somewhat small but jammed packed with all sorts of useful information. Most definitely will recommend this course. Thanks Sean!


Congratulations on being NABCEP PVIP Certified!!!

I like the swiss army analogy. When I was first in talks with the publisher, I asked them to make a book that was not too big to read in a week and something that is not too heavy to carry around. I also didn't want it to be expensive, so it would reach more people. The plan was to make the book have the information required to pass the exam. Glad it worked!!

Sean White

2014 NEC 690.7 Maximum System Voltage 600V and 1000V Distinctions

Posted on 16 May, 2016 at 22:09 Comments comments (467)


Article 690 - Part II - 690.7(C): Maximum System Voltage. In your lecture you mentioned that the NEC distinguishes between 3 family dwellings (up to 1000V PV max system voltage), versus 1-2 family dwellings (up to 6000V PV max system voltage). Two questions: First, I proudly bought the book, but I do not see this distinction in 690.7. From what I read, it addresses 600V and 1000V. Second, it seems arbitrary to distinguish to maximum inverter size between 3 family dwellings vs. 1-2 family dwellings. Do you think this will change? Wouldn't you want ideally to have one string on one inverter versus breaking strings out onto multiple inverters, or to run the system in parallel?


The rules are the rules. The 2014 NEC changed to allowing 1000V PV systems, however 1 and 2 family dwellings were still limited to 600V.
Personally, I would feel safe with a 1000V system on my roof. If you start looking at buildings that are more than 1 and 2 family dwellings, it is more commercial property.
Often times with smaller PV systems, there is no need to go over 600V since we are always trying to fit PV on a rooftop and it is often difficult to fit 20 modules on a normal house facing all in the same direction.
Also, with the 2014 NEC we are more likely to see electronics under modules on rooftops, so that is another reason we will not be needing to go over 1000V.
I dare you to put a 1001V system on your roof (don't tell anyone).

PV Wire Sizing, Terminal Temperature Limits, 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c), 110.14(C), 110.14(C)(1)(a) and 110.14(C)(1)(b)

Posted on 28 April, 2016 at 12:59 Comments comments (118)


Dear Doc,
A little late question ,concerning article 310 and 110.14 (c),110.14(c)(1)(a) or (C)(1)(b)
When you talk about terminal ratings,I am aware of three areas in solar where wires terminate ,combiner box ,inverter and main panel ocpd.
Are we talking here about, adjustment and correction factors ,dealing with ocpd of the solar breaker in main panel or the equipment shutoff breaker ,I am assuming these articles are referring to the main panel termination or equipment shut off termination.
This is the missing link I need to know to understand.
Appreciate the help


When we are doing any wire sizing, we have to take the terminal temperature limits into consideration, whenever a wire terminates at a terminal.

This does include at a termination at a combiner box, inverter, main panel OCPD, even a wire nut.

When we do the checks for corrections factors for derating of ampacity with tables 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c) (conditions of use derating) we do not take the terminal temperature limits into consideration for that calculation.

We do take the terminal temperature limits into consideration when we are looking at the required ampacity for continuous current, which is 125% of maximum circuit current.

This means that when we check for continuous current, we take the terminal temperature into consideration.

When we check for conditions of use, we do not take the terminals or continuous current into consideration.

Let me say that one more time:

When we check for continuous current, we take the terminal temperature into consideration.

When we check for conditions of use, we do not take the terminals or continuous current into consideration.

Remember that conditions of use are the 310 tables 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c) where we reduce our ampacity due to high temperatures and greater than 3 current carrying conductors in conduit.

Good questions!

Sean White

Rapid Shutdown Retrofits?

Posted on 13 April, 2016 at 23:42 Comments comments (451)


Hi Sean,
The new regulation for the 'rapid shutdown' function, was driven by first responders for their safety, do you think that they may request that the rapid shutdown be installed on existing large commercial systems?


Once something is built, inspected and permitted, the AHJ does not come back and make them change it. I am sure that they could request it.

Insurance companies and owners would probably be the driving force for something like this, seeing how they would like the firefighters to put out the fire, rather than watching it burn out of fear of shock.

Here is an article that includes references to different methods of making ground fault systems safer with retrofits that people are doing:

As for people retrofitting systems with rapid shutdown devices, I have never heard of it. In California where Rapid Shutdown is not required until the 2014 NEC is adopted in Jan 2017, many if not most PV systems installed are not 690.12 Rapid Shutdown compliant.
Sean White

NABCEP Exam Requirements and Cycles

Posted on 11 April, 2016 at 16:22 Comments comments (121)


Uh, did I miss something? I thought this course was until the end of this month. Or is this overlapping cycles? I was planning to take this class as part of the 58 hours I need to even qualify to take the NABCEP test this fall. I plan to return here to bone up after applying the lessons learned here on two more systems I'm planning to build to qualify also as the systems I built before my knee (and other body parts) crashed out in '12 don't qualify any more.
I would ask this as a private message but I didn't see any link to send it just to you and not bother the other students.


This class is ongoing!

I just sent that message about 8-days until the exam for any students that may be taking the NABCEP Certification exams coming up on Saturday.

If you are taking the exam in the future, that is great. It happens every 6 months as you probably already know.
It is fine to ask the questions here, since it gives information to everyone else, but my email address is [email protected]

Also, if your knees hurt, you can also qualify by designing systems without going on the roof.
Here is what it says on the NABCEP website about experience:

The Applicant shall have performed in a decision-making role, which had material impact on the quality and serviceability of the installation. Examples of such roles include, but are not limited to: Lead Installers, System Designers, Project Managers, Site Managers, Foreman, Electricians, System Engineers, and Quality Assurance / Commissioning Agents.

Also, the NABCEP Exam typically adopts the NEC 1.5 years after cycle, so the second exam of 2018 will probably adopt the 2017 NEC.


MPPTs Stacked Series and Parallel with Shading

Posted on 8 April, 2016 at 18:04 Comments comments (93)


You said, "if the entire string is shaded with MPPT, then the PV source circuit would operate as if the entire string was in the shade.." ...
But then what of the other strings still in the sun?
Assuming for this example that there is some flat horizontal object (like a neighboring building) that causes the bottom landscape series string to experience a lower light level (not zero). So therefore the amperage of that bottom string would be less than the other (parrallel) horizontally strung series strings above the lowest shaded string. I say this because your video said that the AMPERAGE for an array of series 160v strings that are in parallel means their amperage is added.
But at SOME point the shading may be sufficient to drop the voltage.
Which brings me to the next question.

You said to avoid paralleling strings that have different voltages. But due to variation in manufacture etc., no two panels are exactly the same so I've seen strings of 3 that put out 93v or 92v or 94volts ...
But what about when there is greater variance than that between strings because the light level has dropped ENOUGH? At what variance level does it make a significant difference?
I suppose at some point the power per panel will increase to meet the decreasing cost of MPPT charge controllers until every string has its own charge controller. A kind of "Micro String" inverter...


A perfect MPPT inverter that is connected to multiple will operate at the voltage that will make the maximum power.

If you have one string in the sun and another in the shade, the voltages will be similar, because the level of light between direct sun and shaded PV does not effect the voltage much. The string in the shade will contribute less current than the sunny string and the currents will add. This is similar to having an east facing and a west facing string on the same MPPT. The currents will just add.

If you have a situation where there are multiple strings and one of the strings has a shaded module or two, then the array will still do best operating at the voltage of the many unshaded strings. The string that has the shading will be stuck operating at the same voltage as the other modules and will operate as if it were shaded. Kicking in the bypass diodes in this situation will not help. However, when you have an inverter that has multiple arrays on a single MPPT, it is often part of a large ground mount and large ground mounts are typically not put in shaded places.

If we have 2 strings on an MPPT, which is common with commercial rooftop systems that are also commonly 1000V systems with about 20 modules in series, then if a single module is shaded, it would be like having a string of 19 and a string of 20. The question here is what would be the Maximum Power Point where the most power is made. Since the difference between 19 and 20 is about 5%, then meeting in the middle would have the MPP of the 2 strings only 2.5% off each, which is not a big deal, like if there were 6 in series.

For most residential systems these days that are connected to string inverters, each string has its own MPP, so it will just bypass the shaded module and the other modules will work at full power.

Just remember, the MPP should work at the voltage that makes the most power and whatever point makes the most voltage will cause the bypass diodes to kick in or not.

Visualize multiple IV curves stacked up in series and parallel for each solar cell. Some people want to make MPP cells. Probably too expensive. ET solar is selling MPPT modules, which is interesting, but not mainstream.

Interesting stuff.

Expect in future code cycles to see MPPT at every module.

Sean White

Message for those of you here preparing to take the NABCEP PV Installation Professional Certification Exam, which is in 8-days!

Posted on 8 April, 2016 at 17:18 Comments comments (172)

Message for those of you here preparing to take the NABCEP PV Installation Professional Certification Exam, which is in 8-days!

Now is time to study hard. No procrastinating. Learn the material, but do not get overwhelmed with a concept that is difficult to understand. Remember that the exam is a numbers game. There are many very difficult questions and nobody gets all of the questions correct. Budget your study time wisely and plan to not study hard the night before. The night before I recommend reading the index and the table of contents of the NEC until you get sleepy enough to fall asleep early. It is also a very good idea to have the index and table of contents on your short term memory, because it can be very helpful during the exam.

If you do not regularly get up early, start getting up early now, so that you will not be tired by a quick change in sleep schedule on exam day.


Sean White

NABCEP Resource Guide Battery Question Explanation Hot and Cold Lead-Acid Battery Charging

Posted on 3 April, 2016 at 21:53 Comments comments (89)


Hi Sean,
NABCEP includes the following practice question in the NABCEP PV Installation Professionals Resource Guide:
  1. The battery bank for a backup utility-interactive PV System is located in a harsh environment with temp and humidity extremes. The system does not have a provision for temp compensation. What is the MOST LIKELY result on the battery state of charge?
c.) Overcharged in cold weather and undercharged in hot weather.
d.) Undercharged in cold weather and overcharged in hot weather.
The correct answer is d according to NABCEP. I thought hot batteries charge less efficiently. Why in this case does that not lead to undercharging?


When it is cold, chemical reactions slow down.
When it is cold, batteries take more voltage to charge.
When it is cold, there is less available energy in a battery.
When it is cold, batteries self discharge less (why grandma keeps them in the freezer).

When it is hot, everything is opposite.

To get specific with the question, if you are charging at a voltage meant for a medium temperature, when it is cold, you will have less than optimal voltage and when it is hot, you will have more than optimal voltage.

You may notice in this situation that your batteries may require regular addition of distilled water in the summer, since the overcharging will cause the battery to off-gas.

You may also have a greater tendency to get led sulfate crystals on the plates in the batteries in the winter, since you are not reaching a full charge.

Equalization, a controlled overcharge can help cure the batteries from lead sulfate crystal buildup.

Thanks for the good question!!

Sean White