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)
What if you had a 200A Main Breaker and a 200A Main Buss??
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!!!
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!!!
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?
Yes!! Total supply tothe BUSS (220A) isless than the BUSS israted for (225A)!!
Side note on Fastener Requirement for OCPD NEC 408.36(D):
Side note on Fastener Requirement for OCPD Exception to this provided by 705.12(D)(6)
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?
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.
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
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.
In this example with theservice size being 200A,then the maximum sizeline-side tap OCPD wouldbe 200A.Maximum current from PVarray? 200A/1.25= 160APV Supply!!!Wattage?? 160A x 240V =38,400 Watts!!
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).
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.
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!!!
Chargingcurrents Actual currents with exception factor = 32.5A 32.5A < 40A so we are good!
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
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
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
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.
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
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).
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!!!
“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%
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