This document provides strategies and tips for taking the NABCEP PV Installer exam, including: - An explanation of the "120-Rule" for sizing overcurrent protection devices and calculating maximum inverter output. - Recommendations for last-minute study resources focusing on code articles and questions/answers. - Suggestions for effective test-taking strategies like circling question types, using provided formulas, and time management to complete all questions in the allotted time. - Details on what to expect on exam day including available materials, security procedures, and focusing amid distractions.

1. NABCEP PV Installer™ Exam
120 Rule & Test-taking
Strategies
Richard Stovall
SolPowerPeople, Inc.
Austin, TX
April 18, 2013

2. Today’s Topics
 120-Rule
 Last Minute Articles &
Resources
 Test-taking Strategies
 Time Management
 What to Expect

3. 120-Rule: Question from
NABCEP 2009 Study Guide

4. Problem 64
 NEC 705.12 (D) (2)

5. Problem 64
 Rating of the buss bar = 225 amps
 225 A x 1.20 = 270 amps
 Rating of the breaker serving the utility = 200 amps
 270 amps - 200 amps = 70 amps is the maximum breaker size for the PV system
 Divide by 1.25 from NEC 690.8
 70 amps / 1.25 = 56 amps is the maximum inverter output Answer (c)

6. Problem 64
 NEC 690.8(B)(1)(a)

8. 70A * .8 = 56A
inverter output max!

9. 70A * .8 = 56A
56A * 240V = 13,440 Watts max!

10. NEC 705.12 (D)(7)

14. What if you had a 200A Main
Breaker and a 200A Main
Buss??

15. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 What would your OCPD max be?
 (200A Buss x 1.2) – 200A Breaker = ?
 240A – 200A = 40A
 40A OCPD maximum!!!

16. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 What would the most current allowable from your inverter?
 40A OCPD/ 1.25 =
 32A current from inverter MAXIMUM!!!

17. What if you had a 200A Main
Breaker and a 225A Main
Buss??
 …AND you needed to put in a 20A OCPD, but the opposite
end of the panel was already filled up?
 Could you do it?

19. Yes!! Total supply to
the BUSS (220A) is
less than the BUSS is
rated for (225A)!!

20. Side note on Fastener
Requirement for OCPD
 NEC 408.36(D):

21. Side note on Fastener
Requirement for OCPD
 Exception to this provided by 705.12(D)(6)

22. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 …AND you really wanted to use that 13KW inverter that
requires a 70A OCPD?
 Could you do it?

23. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 …AND you really wanted to use that 13KW inverter that
requires a 70A OCPD?
 Could you do it? Maybe… There are two ways it can be
achieved.

24. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 …AND you really wanted to use that 13KW inverter that
requires a 70A OCPD?
 Could you do it? Maybe…
 1. Do a load analysis on the facility and if you determine
that you can reduce your Main Breaker down to 150A, then
that gives you 90A of PV supply OCPD head room:
 (200A Main Buss * 1.2) – 150A Main Breaker = ?A
 240A – 150A = 90A

26. What if you had a 200A Main
Breaker and a 200A Main
Buss??
 …AND you really wanted to use that 13KW inverter that
requires a 70A OCPD?
 Could you do it? Maybe…
 OR, 2. Do a line-side tap.

29. Various ways to do Supply-Side Tap

30. How big can I go with a supply-
side tap?

31. In this example with the
service size being 200A,
then the maximum size
line-side tap OCPD would
be 200A.
Maximum current from PV
array? 200A/1.25= 160A
PV Supply!!!
Wattage?? 160A x 240V =
38,400 Watts!!

32. 705.12 (D)(2) Exception
 Now hold on there!!!… The specifications of my 6KW
240V bimodal inverter (grid-tied with battery backup) is
asking for a 60A OCPD in the main panel. I’ve got a 200A
Main Breaker and a 200A which gives me just 40A PV
supply OCPD headroom… I sure don’t want to downsize
my main. Is there any hope for me??
 YES… there is.. It is the exception listed in 705.12(D)(2).

33. 705.12 (D)(2) Exception

34. 705.12 (D)(2) Exception
(rationale)
 Because the charging current required from the grid for the
batteries can be greater than the current supplied by the
inverter and PV array, the size of the backfed breaker must
be larger.
 However, the 120-rule is concerned with the supply currents
to the main buss, not the load currents leaving the buss.
They therefore allow 125% of the inverter’s supply current
to be used instead of OCPD size to determine the buss
loading as it pertains to the 120-rule.

35. Sooo… In our example…
 6000W 240V inverter will produce a maximum of 25A.
 1.25 (per the 705.12(D)(2) exception) x 25A= 31.25A
 (200A Main Buss x 1.2) – 200A Main Breaker = 40A
allowable.
 31.25A< 40A so it is GOOD!!!

36. Charging
currents
Actual currents with
exception factor = 32.5A
32.5A < 40A so we are good!

37. Last Minute Resources
 Home Power Magazine: Code Corner Articles, Mehalic Article
on Load Side Connections, and the one on Line-Side
Connections
 Solar Pro Articles
 Hren/Mehalic – Issue 4.3 “Code Red: Notable Changes to 2011 NEC”
 Leblanc/Yates – Issue 2.4 “Can We Land?” load vs. supply side
interconnection
 Q & A Sections – All issues
 SolarABCs.org – Expedited Permit Process Workbook
 Cross – reference to the Code Book as you prepare

38. Drilling Down
 Web Reader iPhone App – reads
articles to you!
 Listen passively to article voice-
overs. Good for driving, chores,
etc.
 NEC Code Book – Know where
things are and where related
information is!!!
 Tables
 Index
 Definitions, Article 100

39. Test-Taking Strategies
 Core Dump upon arrival
 Write out formulas you know
 I-V Curve, PIE chart, write it down before you get going
 CIRCLE what the question is asking you to figure out!
 Multiple Choice – the right answer is there!!!
 Look for clues in other questions

40. How to Attack the Questions
 For a question requiring no calculations that you have
complete confidence in the answer you’ve provided put a
star next to it.
 For a question that you are not sure about the answer
you’ve provided, or involves a calculation that you want
to double-check later, put a circle.
 For a question with a circle that gets revisited later and
you are confident of the answer, put a check in the circle.
If you don’t think you need to visit again, put a star.
 For a question that you skip, put a circle.
 BE CAREFUL: When filling out the answer sheet that
you don’t mix up your answers or put them where they
shouldn’t be.

41. Time Management
 4 hours, 70 questions (60 count, 10 are pilot questions)
 First time through – Recommend NOT to spend more than
3 minutes on each question!
 Need to be able to get through the test at least one time and
have time to go back

42. Next Test Insurance
 Do your best! BUT, no matter what, this is a window into
the NEXT test!
 Stay the FULL 4 hours, NO MATTER WHEN YOU
FINISH!
 Do you best to memorize as many questions as possible so
that if you fail this time, you can use those questions as a
study aide for the next test!! (just keep them to yourself).

43. NABCEP Code of Ethics

44. Be sure to buy the Casio fx260
 Buy the Calculator:
Casio fx260 – about $5-
10.00
 Get familiar with the
function keys
(especially trig and
arctan)

45. Why arctan??
 Arctan is the function you use to convert a rise over run
value to an actual value of pitch of the roof in degrees.
 For example: What if you had a roof with a 2/12 pitch?

46. First we must understand what
2/12 means?

47. It refers to the rise over the run
2”
12”

48. For every 2” of rise, there is 12”
of run
2”
12”

49. So how do we solve for the
angle?
2”
?°
12”

50. We know that the tangent of the
angle is equal to the opposite
over the adjacent
Opposite
2”
?°
12”
Adjacent

51. Therefore, 2 divided by 12 is the
tangent of the angle which is
0.1666
Opposite
2”
?°
12”
Adjacent

52. How to solve
In order to convert from the tangent of an angle to
Opposite the actual angle value use the arctan function
2”
?°
12”
Adjacent

53. Arctan function
Press shift
or 2nd key

54. Arctan function
Arctan is the
tan-1 key

56. What to Expect
 Scratch paper is either the exam cover sheet – limited
 They supply pencils, you can ask for more sharpened
 They supply a copy of the NEC 2011
 Tight Security – varies by location, but you can be sure you
will not be able to bring anything into the exam
 A lot of page turning, distractions: FOCUS!!!

57. “You Got This!!!”
 Yes, this is a difficult test.
 26% pass rate (rumor has it)
 Experience helps, but don’t
count on it (solely)
 Code, Code, Code
 49/70 = 70%