String Sizing and Array Voltage Calculations

#SolarMOOC Lecture Featuring Rebekah Hren

Rebekah Hren gave an incredibly concise explanation of how to do photovoltaic array string sizing and voltage calculations during the Thursday night lecture on April 4th.

Rebekah also reviewed variables that must be considered and located in order to do these calculations, where to find them, NEC 2011 ™ sections that pertain to these calculations and outline the requirements, and array to inverter ratios.

This was an incredible lecture presented by Rebekah Hren, and if you missed it be sure to watch the video. This is need-to-know material for installers, or for anyone preparing to sit for an entry level or advanced level NABCEP ™ exam.

Resources – homepage, temperature data page: Rebekah mentioned as a place to find ASHRAE ™ temperature data for array voltage calculations.  This website is an incredible resource for many reasons, not just temperature data. Take some time to explore it.

In the 2011 NEC ™ Article 690, section 690.7(A) an informational note states ASHRAE ™ data is acceptable to be used for low temperatures that are used for maximum voltage calculations.

Example for review from NABCEP 2012 Installer Resource Guide ™ Case Study Example 1, page 151:

The inverter maximum recommended STC watts is 9600W STC- what is the maximum number of modules that can be installed on this inverter?

Modules are 230 Watt modules.

9600W / 230W = 41.74.

The NABCEP ™ IRG says the answer is 42, but considering that the NEC ™ is all about safety, I was thinking the answer should be 41, with rounding down.

The answer in the IRG is 42.

The allowance for inverter/array rations to allow the array to be sized slightly larger than the inverter as discussed in Rebekah’s lecture, could explain allowing it to be a round up scenario.   But this is something that is provided with the inverters anual which we do not have. We can also guess that this tiny fraction wouldn’t cause us any problems, especially since we are unlikely to have the array operating at prime STC circumstances.

Still- the questions is: why are we rounding up when all we have been taught about following NEC ™ rules tells us to be safe, and when doing maximum voltage calculations to round down when sizing for maximum voltages?  It makes sense for a real life system design to go ahead with the 42, but for a test question?

Note in the IRG states:

Final answer: Pick your answer- whether it’s 41 or 42, and then make a comment on this question on the provided comments page from NABCEP ™ that you receive during the exam.

Ultimately, this also changes the answer to the next question (number 6), if you go with the conservative 41, so that you must have 3 strings of 13 rather than 3 strings of 14.

What would you do?

Please enter your comments or questions in this Newsletter at the bottom or in the #SolarMOOC FORUM.

Another thing to point out about this example (Case Study Example 1, page 151-154):

Question number 8 uses 240V as nominal voltage in a voltage drop equation for a 600V DC circuit that runs to the inverter.  I have convened with some industry professionals and we are in agreeance that this is a mistake in the IRG ™  and should be changed. The calculation has 600V running through it, and therefore should use that as the voltage in the equation rather than 240, as we are on the DC side.

Again, please enter your comments or questions in this Newsletter at the bottom or in the #SolarMOOC FORUM.

For further review of this topic, check out:

Verifying System Design: Case study Example 8.1 Review w/ Jeff Gilbert

Pssst… Did you miss last week’s lecture by Kelly Larson on Energy Storage Systems?  Catch a crystal clear look at battery charging, battery bank sizing, and more in this lecture.

Rebekah Hren and Kelly Larson are both Solar Energy International ™ trainers, as is Jeff Spies– our lecturer for next week(Register Early here!)

So a great big thanks to SEI and it’s fabulous trainers – you guys ROCK!

Check out their free intro course to renewable energy at

Home Power Magazine™ articles by Rebekah Hren:

Understanding PV Module Specifications

PV Rack Innovations

A Peek Inside PV

Modern PV Roof Mounting

Pitched Roof Mounting

SolarPro ™ articles by Rebekah Hren:

Code Red: Notable Changes in the 2011 NEC (with Brian Mehalic)

Commercial PV System Data Monitoring, Part One (with Kyra Moore)

Commercial PV Data Monitoring, Part Two, (with Kyra Moore)


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