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National Electrical Code Articles and Information

Based on the 2014 NEC

by Mark Lamendola

National Electrical Code Tips: Article 690 -- Solar Photovoltaic Systems, Part 10

Only about 15% to 20% of the work of installing a Solar Photovoltaic (PV) System is electrical. But that electrical portion can easily result in disaster if not done correctly. Thus, Article 690 provides requirements for that portion.

  1. You can bet that at some point a PV system will lose power. If that system is interactive with the utility, the inverter must automatically de-energize its output to the connected electrical production and distribution network. [690.61]. It must stay disconnected until that network is restored. Among other things, this prevents dangerous feedback into that network.
     
  2. The same thing applies to ac modules [690.61].
     
  3. While disconnected from the network, the PV system can function as a stand-alone system to support the connected load [690.61].
     
  4. Also see Article 705, if you have an interconnected system.

     

Something many people don't realize is that as the battery systems of PV systems get larger, they begin to resemble the battery systems in data centers. People in the battery industry are very aware of this, as are the relevant NEC CMP (Code Making Panel) members and their counterparts on the IEEE battery committee. Consequently, the battery requirements for PV systems draw from a rich depth of experience, practical knowledge, and empirical data.

  1. Storage batteries for PV systems present an additional safety challenge and thus additional installation and maintenance requirements to meet. In some cases, it makes sense not to provide battery storage because doing so isn't worth the cost and complexity of meeting these requirements. But if the installation is going to be something more than merely a supplemental source then these costs and complexities are worth incurring and addressing. The battery installation must meet the requirements of Article 480 plus the additional requirements of Part VIII of Article 690.
     
  2. There is confusion about what a "battery" is. Many people think of the jar in their car as a "battery" because that's what we call it. An arrangement of such jars in a string for purposes of storing power as in a data center loss of utility power protection system (or for a PV system) is actually a battery. Each jar is typically a cell, and you string these together to make a battery.

    The automotive "battery" is really a string of cells in a single package. So don't visually confuse the power storage jars (cells) with being a battery, unless you're using a nonstandard system wherein the "battery" really is a battery rather than a cell designed to be part of a string that makes up a battery.
     
  3. If you install the PV power source per 690.41, the interconnected battery cells are considered grounded [690.71(A)].
     
  4. For dwellings, connect the storage batteries so they operate at 50V or less [690.71(B)(1)]. The NEC says you can skip this requirement if live parts aren't accessible during routine battery maintenance, but such a setup makes proper battery maintenance impossible. Ignore the exception, because it does not account for the realities of battery maintenance.
     
  5. If the available short-circuit current from the battery exceeds the interrupting or withstand ratings of other equipment in that circuit, then you must install a listed, current-limiting overcurrent device in each circuit adjacent to the battery, making sure it complies with 690.16 [690.71(C)].
     
  6. Many installers now use (Valve Regulated Lead Acid (VRLA) batteries. The battery manufacturers and other battery experts (look up BATTCON and read the presented papers) look at VRLA as a suboptimal choice. They have largely been pushed by APC and others with a legacy background in small point of use systems that depend upon VRLA.

    VRLA batteries have many significant disadvantages compared to their "flooded" counterparts. But they have some advantages (some of which are only perceived and not real) that tip the balance toward chooseing VRLA. For example, they do produce less gas than flooded batteries and thus can be colocated (in small numbers) with actual equipment instead of being located away from the equipment.

    A perceived but not real advantage of VRLA is they are maintenance-free. They are actually harder to maintain, and for this reason do not last as long as properly maintained flooded batteries. This is why you rarely see VRLA used in data center battery rooms.

    For a residential PV system, the VRLA is often a good choice. One reason why is the homeowner isn't going to maintain them anyhow and is unlikely to ever be qualified to do so. With the VRLA, the "maintenance" includes cleaning the terminals and connections, and replacing based on time in service or possibly based on a condition such as internal resistance or terminal voltage.

    That's a huge advantage, though it's costly when scaled to any size because it inherently wastes battery life over what you can get from flooded batteries.

    Another advantage is VRLAs exempt you from the nonconductive case requirement that comes into play when you have more than twenty-four 2V cells connected in series [690.71(D)].

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How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.

 

 

 

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