PV and the NEC

Bill Brooks, Sean White

• Publisher: Routledge
• Year: 2018
• ISBN:
  9781351615655
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Recursos de energía renovable

Used throughout the United States and many other countries, the National Electric Code (NEC) is the world's most detailed set of electrical codes pertaining to photovoltaic (PV) systems.

Photovoltaic Systems and the National Electric Code presents a straightforward explanation of the NEC in everyday language. The new book is based on the 2017 NEC, which will be used widely until 2023, with most of the interpretations and material staying true long after. This book interprets the distinct differences between previous versions of the NEC and the 2017 NEC and clarifies how these Code changes relate specifically to photovoltaic installations.

Written by two of the leading authorities and educators in the field, this book will be a vital resource for solar professionals, as well as anyone preparing for a solar certification exam.

1
Article 690 photovoltaic (PV) systems

Article 690 first came out in a little book known as the 1984 NEC and has been updated and mostly lengthened ever since.

In comparing the original 1984 version of Article 690 to today’s NEC, there are many similarities yet also quite a few differences. Time to dig in!

Let us first list what we are dealing with in Article 690 before we dig deep. This will give us perspective and familiarize us with how to look things up quickly.

The NEC is also known as NFPA 70 and is divided into Chapters, then Articles, then Parts and Sections.

For example, rapid shutdown requirements are found in:

NEC Chapter 6 Special Equipment

Article 690 Solar Photovoltaic (PV) Systems

Part II Circuit Requirements

Section 690.12 Rapid Shutdown of PV Systems on Buildings

Here is what we find in Article 690:

Article 690 solar photovoltaic (PV) systems

Part I general (part)

• 690.1 Scope [Section 690.1]
• 690.2 Definitions [There are more NEC definitions in Article 100, such as the definitions for PV, ac and dc.]
• 690.4 General Requirements [They could not come up with a better title for this category.]
• 690.6 Alternating Current (ac) Modules

Figure 1.1 1984 NEC (a much smaller Code book)

Photo by Sean White

Part II circuit requirements

• 690.7 Maximum Voltage
• 690.8 Circuit Sizing and Current
• 690.9 Overcurrent Protection [Article 240 is also Overcurrent Protection.]
• 690.10 Stand Alone Systems [This has been moved to Article 710 in the 2017 NEC.]
• 690.11 Arc-Fault Circuit Protection (Direct Current)
• 690.12 Rapid Shutdown of PV Systems on Buildings [Big changes]

Part III disconnecting means

• 690.13 Photovoltaic System Disconnecting Means
• 690.15 Disconnection of PV Equipment

Part IV wiring methods

• 690.31 Methods Permitted
• 690.32 Component Interconnections
• 690.33 Connectors
• 690.34 Access to Boxes

Part V grounding and bonding [Article 250 is also grounding and bonding.]

• 690.41 System Grounding [Big changes in the 2017 NEC]
• 690.42 Point of System Grounding Connections
• 690.43 Equipment Grounding and Bonding
• 690.45 Size of Equipment Grounding Conductors
• 690.46 Array Equipment Grounding Conductors
• 690.47 Grounding Electrode System [Experts argue over a lot of this article, which is interesting to observe.]
• 690.50 Equipment Bonding Jumpers

Part VI marking

• 690.51 Modules
• 690.52 Alternating Current Photovoltaic Modules
• 690.53 Direct Current Photovoltaic Power Source
• 690.54 Interactive System Point of Interconnection
• 690.55 Photovoltaic Systems Connected to Energy Storage Systems
• 690.56 Identification of Power Sources [This includes new Rapid Shutdown signs.]

Part VII connection to other sources

• 690.59 Connection to Other Sources [Directs us to Article 705]

Part VIII energy storage systems

• 690.71 General [Directs us to Article 706]
• 690.72 Self-Regulated PV Charge Control

Now it is time to dive into the detail of Article 690.

Article 690 solar photovoltaic (PV) systems Part I general (part)

690.1 scope (section 690.1)

Word-for-word NEC:

“690.1 Scope. This article applies to solar PV systems, other than those covered by Article 691, including the array circuit(s), inverter(s), and controller(s) for such systems. [See Figure 690.1(a) and Figure 690.1(b).] The systems covered by this article may be interactive with other electrical power production sources or stand-alone or both, and may or may not be connected to energy storage systems such as batteries. These PV systems may have ac or dc output for utilization.

Informational Note: Article 691 covers the installation of large-scale PV electric supply stations.”

Discussion: For the most part 690.1 is self-explanatory, however, if we read the 2014 and the 2017 NEC carefully, we will notice that energy storage systems (batteries) are no longer part of the PV system.

2017 NEC language:

“may or may not be connected to energy storage systems.”

2014 NEC language:

“may be interactive with other electrical power production sources or stand-alone, with or without electrical energy storage such as batteries.”

It takes some careful analysis of the language, but we see that being connected to batteries in the 2017 NEC is different than with batteries in the 2014 NEC.

So what does this mean for us? Batteries are no longer part of the PV system as of the 2017 NEC and are part of a separate energy storage system that is covered in the new Article 706. Consequently, rapid shutdown and other requirements that are specific to PV systems no longer apply to the batteries.

Figure 1.2 2014 NEC Figure 690.1(a) PV power source

Courtesy NFPA

Next, we see diagrams that will show us the dividing line between the PV system and not the PV system.

Section 690.1 also has some figures that we can look at in order to get a picture of what we are talking about.

Figure 1.2 is an image from the 2014 NEC.

Figure 1.3 is an image from the 2017 NEC.

Figure 1.3 is from the 2017 NEC with the added dc-to-dc converter.

From comparing these images, the main difference here is the insertion of the dc-to-dc converters. The writers of the NEC left the dc-todc converter definition open-ended for your billion-dollar invention. 2017 dc-to-dc converters are usually one per module, rather than three modules per converter in this image. Take note that, as we will learn coming up in Section 690.12 Rapid Shutdown, in 2019 the 2017 NEC will increase requirements for rapid shutdown on buildings and module level shutdown may be one of the only methods to comply. However, new inventions in the meantime could introduce other methods not currently foreseen.

Figure 1.3 2017 NEC PV Figure 690.1(a) PV power source

Courtesy NFPA

It is interesting to note that the solar cells in the diagram have gone from round in the 2014 NEC (really old style) to square in the 2017 NEC (polycrystalline). For someone first learning about solar, it could be confusing to see a solar module with 12 cells and then to see panels made of three modules. It would be even more confusing to have one dc-to-dc converter per three modules that is being connected with fuses to a dc-to-dc converter combining busbar and then off to a dc-to-dc converter output circuit. Dc-to-dc converters being installed in 2017 have a single PV module with a dc-to-dc converter under the module and then a number of dc-to-dc converters connected in series, and then the dc-to-dc converter source circuit is connected directly to the inverter.

Figure 1.4 Interactive system [2017 NEC Fig 690.1(b)]

Courtesy NFPA

Figure 1.5 Ac module system [2017 NEC Fig 690.1(b)]

Courtesy NFPA

Images are good to learn from. Next, we will go over the different images in Figures 690.1(B), paying close attention to the various PV system disconnecting means, which separate the PV system covered here in Article 690 from systems covered in other areas of the 2017 Code. Remember, much of this has changed in the 2017 NEC.

Interactive (grid-tied) inverter circuits are very simple. The inverter is used only for PV power; it has no other purpose and therefore is part of the PV system.

A big question installers have is: “What is the difference between an ac module and a microinverter bolted to a PV module?” The answer is that if the PV module was listed to UL1703 while the inverter was bolted to it and the inverter was tested and listed to UL 1741 while bolted to the PV module, then it is an ac module and we do not consider dc part of the product when installing this module.

If the module and microinverter were not listed together, then we are responsible for applying the NEC to the dc circuit going from the module to the inverter. It is also interesting to note that the word microinverter does not appear in the NEC. The NEC looks at a microinverter as nothing more than a small (micro) inverter.

There is a lot of information in Figure 1.6. First of all, dc coupled and multimode are different things, which can go together. A dc coupled system is a PV system that is typically charging batteries with a charge controller connected to a PV array. The inverter in a dc coupled system will be coupled with the inverter and the charge controller working with dc voltage. In fact, it is possible to have a dc coupled system that does not have an inverter, but most people would like to utilize ac electricity with their dc coupled systems.

Figure 1.6 Dc coupled multimode system [2017 NEC Fig 690.1(b)]

Courtesy NFPA

As we can see in the 690.2 Definitions that we are about to dive into, a multimode inverter is an inverter that can work in different modes, such as stand-alone (off-grid) and interactive (grid-tied). This type of inverter was also known as a bimodal inverter for a time and will have different outputs. One output will go to the stand-alone (backed up) loads and the other output will go to the loads that are not backed up and to the grid. When the power goes down, the interactive output of the inverter will act exactly as an interactive inverter and anti-island (stop sending voltage or current to the grid). No interactive inverter circuit is allowed to be an “island of power” and must disconnect from the grid.

Multimodal vs. hybrid

There is often confusion about multimodal inverters and hybr...