The Evolution of Solar Photovoltaic Systems

The Evolution of Solar Photovoltaic Systems

Although NEC rules for photovoltaic (PV) systems haven’t changed much over the years, confusion still surround some portions of Art. 690

When it comes to solar photovoltaic (PV) systems, there's a common misconception that this growing technology — and the products that accompany it — are simple to install, modify, and maintain by untrained personnel. In reality, it's quite the opposite.

As this market continues to expand, there's actually a lack of qualified electricians and inspectors performing this type of work. As a result, many installed PV systems are not in compliance with the NEC and local regulations. Let's take a look at some of the problem areas and why confusion still surrounds certain sections of the Code.

History lesson

Starting about 1973 in Braintree, Mass., the solar PV industry began in earnest in the Unites States with PV systems being installed on homes and businesses. The industry was not regulated until 1984 when Art. 690 was added to the NEC, which began addressing some of the different aspects that PV systems have from other electrical systems.

Since that time, the PV industry has rapidly evolved in technology and applications. Presently, the third generation of technology is commonly used (see Solar Solutions below). Although the NEC has changed quite a bit over this time period, a quick look at Art. 690 of the 1987 NEC reads very similar to the same section in the 2008 NEC. Why is this the case?

In the beginning, there was little guidance on how to use PV technology, and many inventive souls were quite creative in finding ways to tap this new energy source. Some early PV systems were as simple as using the DC voltage of the PV arrays to charge or supplement batteries that supplied DC power for lighting or homemade inverters. This gave way to more advanced inverters and eventually to systems that could connect to the electric utility grid. Each new development introduced another set of advantages as well as problems — not only from an installation perspective, but also from a safety standpoint.

Commercial use of small PV systems was sometimes difficult or expensive to install until the adoption of the 2008 NEC. One of the principal reasons for this resulted from language in the 2005 NEC that sometimes required the user to upgrade the main service — often doubling the service size — just to add a meager 2.5kW PV system to a subpanel. The rule change [690.64(B)] now allows the actual PV contribution to be considered at the main service and also includes the same bus loading provision that has been allowed on residential applications since 1987.

Code quandaries

Although the NEC has not appreciably changed with regard to PV systems over the years, a lot of confusion seems to surround how to interpret some portions of Art. 690. One conflict for manufacturers is found between Sec. 110.3(B), which requires listed equipment to be installed in accordance with the UL listing, and Secs. 690.8 and 690.9, which cover circuit sizing and overcurrent protection. In some cases, a PV module's protective overcurrent device rating (marked on the back of the module) is less than 1.56(Isc).

For example, one module's Isc rating is listed as 4.97A, and the series fuse is listed as 7A. Since (4.97 × 1.56 > 7A), if one installs this module, it will not be in compliance with both sections of the NEC. Some experts suggest that some changes within UL testing and listing could clear up this issue.

Another section often misunderstood is 690.64(B)(2), which discusses bus or conductor requirements. Although this section clearly addresses load side connection requirements (click here to see Fig. 1), some Authorities Having Jurisdiction (AHJs) have used this rule to apply to supply side connections (click here to see Fig. 2).

Whether one chooses a load side or supply side connection is often governed by the size of the PV system. If the main circuit breaker plus the PV system circuit breaker(s) are more than 120% of the bus (and conductor) rating, then a service upgrade or supply side connection would typically be required.

For example, let's say a certain house has a typical all-in-one 200A service, and the owner wishes to install a 10kW PV system. The 10kW system would typically be composed of two 5kW systems that have a 21A output, requiring two 30A circuit breakers. So (200A + 30A + 30A) ÷ 120% = 217A (minimum bus size). Therefore, the service would have to be upgraded, or a supply side connection would have to be made.

If, however, the owner chooses to install an 8kW system, then two 4kW systems (which typically have a 16A output) require two 20A circuit breakers. So (200A + 20A + 20A) ÷ 120% = 200A (minimum bus size). Therefore, the service would not have to be altered.

In some cases, it's possible to replace the main circuit protection device with a smaller amperage device. However, this is generally discouraged — some AHJs actually prohibit the practice.

For the case stated above, if the AHJ allows the practice — and the peak load information from the electric utility indicates the service protection device size could be reduced without risking a nuisance trip — then the main could be reduced in the following manner: [(200A × 120%) - 30A - 30A = 180A] maximum device size or a 175A circuit breaker (if available in the same frame and the main gear is listed for the application).

These are just a few examples that help demonstrate common PV installations. Because this niche will inevitably continue to change rapidly in the future, AHJs and electric utilities will likely continue to change their requirements. That's why it's more important than ever for electrical professionals to stay informed, stay safe, and stay on top of the latest changes in this ever-evolving industry.

Slade is a licensed professional engineer in California, Colorado, Oklahoma, Utah, and Wyoming. He can be reached at [email protected].

Sidebar: Solar Solutions

Although several varieties are available, the typical modern photovoltaic (PV) system is comprised of a PV array, a DC disconnecting means, an inverter, a power distribution/disconnecting means, and a grounding system. Additional components may include an energy storage system, such as batteries, an electric utility required service disconnect, transfer switches, solar tracking equipment, and other specialized equipment.

The number of PV systems being installed is increasing at more than 20% nationally per year. These systems may be divided into five types:

  1. Stand-alone PV system without energy storage
  2. Stand-alone PV system with energy storage
  3. Stand-alone off-grid hybrid PV system
  4. Stand-alone grid-tied PV system
  5. Grid-tied PV systems

In recent years, the stand-alone grid-tied PV system became the prevalent type of system being installed. This is largely because of the reduced initial cost associated with various government and electric utility rebate programs.

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