NABCEP Exam Prep

#SolarMOOC Newsletter from September 13, 2012 : NABCEP Exam Prep – Formulas Review Part 2.

Video of presentation, powerpoint slides, and additional resources can be found below. Enjoy!

 

Formulas Review Part 2 NABCEP Exam Prep

 

Presentation Power Point

Voltage Drop

Voltage Drop Problems: Here’s another look at voltage drop calculations with a different formula reviewed by Richard Stovall, SolPowerPeople CEO and Instructor, and Sarah Raymer.  This link includes the video as well as the slide set from the March 2012 review of Voltage Drop.

Conductor Sizing

Installing Electrical Components: Another look at Conductor Sizing with Mike Holt, author of Understanding NEC Requirements for Solar Photovoltaic Systems 2011.

Grounding and Bonding

Grounding and Bonding: Ryan Mayfield offers clarity on Grounding and Bonding requirements in the 2011 NEC Code for Solar PV systems.

A note from Ms. MOOC:

This is an explanation on Voltage drop to clairfy questions that were presented during the Webinar last Thursday, Sept. 13. Please scroll to the bottom to see a list of some of my favorite webinars related to NABCEP Photovoltaic Exam topics!

During the webinar there were a few questions regarding voltage drop, specifically pertaining to the NABCEP 2009 Study Guide questions that were reviewed. To clarify and answer these questions as promised, I have written up an explanation that should clarify and wash away confusion left behind  by Thursday evening’s presentation.

Voltage drop has been something I have struggled with, and still I know I don’t understand it completely- but I will, eventually.  The thing is there is so much more to it than just this basic equation…

When calculating voltage drop in AC conductors, it is standard practice to use the nominal voltage on the AC side.  For example, if you were calculating the voltage drop in a conductor in a 240V residential system, you would use the 240V. However, when it comes to PV there are more variables to consider and even knowing the nominal voltage or array voltage wouldn’t really be completely all inclusive to determine the drop in voltage.  A PV system designer may decide to use the short circuit current [Isc] as a conservative measure in the voltage drop calculation to consider the potential voltage drop under extreme conditions imposed by high currents from fluctuations in irradiance. Also, the designer may chose to consider temperature fluctuations and the possible drop in system voltage due to high ambient heat, using the adjusted high- temperature system voltage on the DC side. 

 For example, when using a voltage drop formula to determine the PV output circuit conductor size in a grid-tied PV System, you would use the array voltage [module Vmp multiplied by  # modules in a series string] and the maximum power current [Imp] of the module to determine the voltage drop potential under standard operation conditions. 

 In the case of #’s 47 and 49 of the 2009 NABCEP PVI SG, we are calculating a stand alone system.  It is stated in these questions what we must consider “that Im is flowing” [that current is flowing] but not that the voltage is that of the modules- therefor we use the nominal voltage of the batteries.  

 When calculating the voltage drop in question #50 of the 2009 PVI Study Guide, we are asked to calculate the voltage drop “under maximum power conditions at STC”. This makes the question different in stating that we are designing for a stand alone system under maximum power conditions which would only be achieved with a MPPT tracker- in which case we could consider the voltage of the array. 

 Confusing? Yes. 

 The first thing I would do if I was taking an exam and I saw this would be to say – “wait just one gosh darned second…”.  I would discern that because of the similarity in these questions and this little differentiation in wording, that something was being asked of me that was ‘specific’ and that there was something fishy.  Something is different in these 2 questions, but you may not realize what was really being asked in the previous questions [47 & 49] until you get to the last one [50]. 

 I hope this clarifies!

Here are a few of my favorite webinars on NABCEP PVI Exam related topics:

Installing Electrical Components: Another look at Ampacity, OCPD, and Conductor Sizing with Mike Holt, author of Understanding NEC Requirements for Solar Photovoltaic Systems 2011.

Grounding and Bonding: Ryan Mayfield offers clarity on Grounding and Bonding requirements in the 2011 NEC Code for Solar PV systems and where to find them in the Codebook.

Verify Mechanical Design: Johan Alfsen explains requirements for rooftop mounting techniques.

Managing the Project: Jeff Gilbert reviews safety guidelines and specific requirements as per OSHA regulation.

Verify System Design: Jeff Gilbert reviews many formulas and their applicaiton in his review of a complex example problem from the NABCEP PV Installer resource Guide Section 8.1.

Verify System Design: Jeff Gilbert– another great look at how to do the neccessary equations.

Trigonometry Lesson and more: The video in this Newsletter is a must watch- Richard Stovall clarifies how to use trigonometry to solve solar calculations on shading.

Expansion Joints: Richard Stovall–  A concise look at an incredible important topic that you must understand.

Check our archive for any other incredible webinars. I would reccoemnd paying close attention to those that were conducted in March leading up to the Spring NABCEP Exam.

 

GOOD LUCK!!!

 

NABCEP Exam Prep

 

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