To Utility

Utility Disconnect

To Utility

Service Entrance:

200 amp

Service Entrance:

200 amp

100-Amp Panel

Inverter subpanel feeding residential service entrance.

breakers suitable for backfeeding are not marked with "Line" and "Load" designations.

Battery Backup, Utility-Interactive Systems—More Complexity

The specifications in Underwriters Laboratories (UL) Standard 1741 require that all utility-interactive inverters cease exporting power to the utility grid when the utility grid voltage and frequency deviate from very narrowly defined values. In blackout situations, the PV system and the standard batteryless utility-interactive inverter cease to operate and will not even supply power to local loads.

In areas where utility blackouts are a concern, some systems are being installed that have a battery-based energy storage system to provide local power during utility outages. The batteries are connected to a specially designed and listed utility-interactive inverter that, in the event of a utility outage, will disconnect from the utility system and provide a set of designated circuits with power from the PV system and the battery. All of these actions are done automatically with transfer devices built into the inverter. The drawing on the next page shows a simplified block diagram of a typical system. Several variations are possible.

Interfacing these systems with the utility grid and meeting 690.64(B)(2) requirements presents challenges for the system designer, the installer, and the inspector. Many of these inverters have internal transfer relays that are rated for 60 amps continuous duty, and that information is presented in the specifications.

This specification leads designers and installers to size the backup load subpanel for 60 amps and to use a 60-amp backfed circuit breaker to connect the inverter to the main load center where the utility connection is made. The use of 60-amp circuit breakers in both positions provides for best use of the internal 60-amp relay and appears to allow maximum loads to be connected to the backup subpanel. Unfortunately, the use of 60-amp circuit breakers poses two problems and code violations.

Inverters commonly used for grid-tied backup systems cannot source these high currents, but NEC Section 690.64 requires the load center to be sized based on the size of

To Utility

To Utility

Utility-interactive PV system with battery backup.

the breaker, not the rated output of the inverter in utilityintertie mode. Even though the inverter may be rated (and can be adjusted) to carry 60 amps, the external wiring and circuit breakers require the normal 80 percent continuous current derating. For a 60-amp continuous current, an 80-amp circuit breaker and conductors rated for at least 75 amps would be required.

Another option that will allow the 60-amp circuit breakers to be retained would be to adjust the inverter to not allow more than 48 amps of continuous current to be handled by these circuits. That adjustment is commonly available on most of these inverters, although there is some question about who has access to the adjustment (qualified or unqualified people).

Second, the 690.64(B)(2) requirements discussed above must be addressed. In a residential installation, a 60-amp backfed PV circuit breaker would dictate that at least a 300-amp main panel be used (60-amp PV circuit breaker + 300-amp main circuit breaker = 360 amps; 1.2 x 300 = 360). Residential load centers rated at 300 amps and above are available but not common. In a commercial installation, the existing load center would have to be replaced with one having at least a 60-amp greater rating than the original rating. In either case, a supply-side interconnection [690.64(A)] might be the more practical alternative. If the full 60-amp rating of the inverter is to be used, then, of course, 80-amp circuit breakers and 75-amp conductors should be used. The use of 80-amp overcurrent devices would require a 400-amp load center to meet NEC requirements.

In all cases, 120 percent of the load center rating must equal or exceed the sum of the main breaker and the 80-amp PV breaker. Some possible combinations would include a 200-amp panel and a 150-amp main breaker. A 300-amp panel could be used with a 240-amp main breaker.

To further complicate system design, many of these systems have an external inverter-bypass switch that is used if the inverter fails. This bypass switch, usually consisting of a pair of interlocked circuit breakers, is used to connect the backup subpanel directly to the main panel when the inverter fails. These circuit breakers are typically also rated at 60 amps and installed in a small 60-amp, three-position (three-phase) load center. Obviously, neither the circuit breakers nor the load center are rated to carry 60 amps continuously. The use of a larger load center and interlocked 80-amp circuit breakers would allow a full 60-amp rating for the inverter-bypass switch.

Some inverters have only 50-amp internal ratings. The ratings of the external overcurrent devices would have to be at least 70 amps and conductors would have to be rated for at least 63 amps. The load center would need to have a 400-amp rating unless a smaller main breaker could be used.


The requirements of NEC Section 690.64 can be met in nearly all installations. While the requirements, at first glance, are somewhat complex and sometimes overlooked, attention to these details in the design, installation, and inspection of these systems should help to ensure a safe, durable, and code-compliant installation.


John C. Wiles, Southwest Technology Development Institute, New Mexico State University, Box 30,001/ MSC 3 SOLAR, Las Cruces, NM 88003 • 505-646-6105 • Fax: 505-646-3841 • [email protected]

Photovoltaic Power Systems & the 2005 NEC: Suggested Practices, a 145-page manual can be downloaded from the SWTDI web site • codes-stds/PVnecSugPract.htm

Sponsor: Sandia National Laboratories, Ward Bower, Sandia National Laboratories, Department 6218, MS 0753, Albuquerque, NM 87185 • 505-844-5206 • Fax: 505-844-6541 • [email protected]

The 2005 NEC and the NEC Handbook are available from the NFPA, 11 Tracy Drive, Avon, MA 02322 • 800-344-3555 or 508-895-8300 • Fax: 800-593-6372 or 508-895-8301 • [email protected]


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• Helps you keep current in technology and continue your professional development

Consumer Benefits of Using NABCEP-Certified PV Installers

• Provides confidence that PV system will be installed to manufacturer's specs and in compliance with NEC

• NABCEP Code of Ethics stresses consumer protection, professional conduct and integrity of service

• The installer has met established PV standards and industry requirements through certification process, and keeps current through continuing education

For more information to

independent power providers

Growing Solar

Strategies for RE Success

Don Loweburg

©2006 Don Loweburg

Hailed as "America's Largest Business-to-Business Solar Event," the Solar Power 2005 conference was held October 5-9, 2005, in Washington, D.C. More than 1,300 attendees, and a record number of presenters and displays resulted in the largest conference to date. Julia Judd, executive director of Solar Electric Power Association, reported that attendance was greater than ever (exceeding last year's by 15 percent), and also was the most diverse. Companies represented everything from roof-integrated solar-electric (photovoltaic; PV) systems and domestic solar water heating to inverters and Web-based monitoring systems.

Increasing Solar's Exposure

Certainly, Solar Power 2005 was a trade show, with manufacturers and service providers vying for the attention of the attendees. New players and established manufacturers put on their best face, while information-hungry attendees cruised the aisles, collecting glossy brochures and chatting with manufacturer reps. Perhaps more importantly, though, Solar Power 2005 focused on addressing the question, "What's needed to grow the U.S. solar energy market?" Below is a sampling of ideas companies are investing in to move solar technology forward.

Sponsored Solar Thermal Systems. Solar thermal currently represents a small fraction of the total solar

German company KACO showcased their line of inverters.

German company KACO showcased their line of inverters.

energy business, but this technology offers great potential for increasing the amount of renewable energy (RE) in the United States' energy mix. If the amount of thermal energy collected is quantified in kilowatt-hours (KWH), the installed cost of solar water heating systems is about US$1 per watt—an amount that's considerably less than the price of PV installations. Even accounting for the generous PV rebates offered by several states, solar thermal systems still deliver more energy for every dollar invested. And a new federal tax credit of up to 30 percent of the cost of a solar thermal system may help stimulate consumer interest.

At Solar Power 2005, Jeff Curry of Lakeland Electric, a municipal utility and Florida's third largest public power utility, described an innovative utility program for promoting solar thermal systems. In 1998, Lakeland started placing solar thermal systems on some of their customers' homes and then metering each system's hot water output. But rather than reporting the energy produced in British thermal units (Btu), the amount of energy produced is reported in equivalent KWH using a special meter. Customers are billed for KWH of electricity used and KWH for hot water delivered. The customer can then see what proportion of the total energy is used to make hot water. Jeff says that "customers love it because they get to use solar energy with absolutely no financial risk—no upfront [or maintenance] expenses—because Lakeland owns and maintains the system."

Besides actively increasing the use of RE in their system and creating revenue, Lakeland benefits by owning the tradable renewable energy credits (TRECs) generated. Lakeland can sell these credits, currently at a value of US$0.032 per KWH. This revenue, when combined with the metered charge in KWH for the hot water generated, covers the capital investment and maintenance of the solar hot water panels.

Lakeland's program of metering hot water production in KWH has an important additional benefit. Making solar hot water has been traditionally regarded as an energy efficiency measure or load reduction technology, but neither has had much impact on the widespread adoption of solar water heating in the United States. By explicitly treating solar hot water production as an energy generation technology, it may be possible to shift the public and bureaucratic mind-set.

Maximizing PV Performance. Bill Brooks of Brooks Engineering presented findings based on a PV system's independent power providers performance-testing program—the BIPV Testing & Evaluation Project—funded by the California Energy Commission. Motivation for this project was based on consumers having too little to rely on besides the manufacturers' literature for judging performance and suitability of various PV products. And the literature often does not paint an accurate picture of performance because it doesn't factor in the interaction between the individual components. The end result of these sometimes overly optimistic reports can be a disappointed enduser, which can have negative consequences for the installer and for RE as an industry.

If high-efficiency components are mismatched or improperly installed, system performance will suffer. This project evaluated several professionally installed commercial and residential systems that used various inverter-module combinations. (The live data and project details are available on the Web—see Access.)

The project yielded two general conclusions. First, array string voltage should be as high as possible for any given inverter. Inverter manufacturers specify an input voltage window, but make the window as wide as possible to accommodate many module configurations and types. However, inverter performance is better when the array string voltage is well above the inverter's minimum operational value.

Second, the data showed that over time, sloppy or overly broad module power ratings will be the source of disappointing system performance. This point makes the case for more accurate module power ratings. Many modules sold in the United States have power ratings that are plus or minus 10 percent, while European and Asian markets often require tighter tolerance ratings.

Short-Supply Solutions. No modules, no problem? Well, not exactly. In his presentation, Matt Lugar of Sharp Solar presented several recommendations for weathering the solar-electric market. He says that due to a silicon shortage that may last another two years, PV modules are increasing in price and modules will be in short supply.

With the manufacturers in the driver's seat of this "seller's market," Matt's advice was directed primarily at installers and sellers of PV systems. He suggested that PV purchasers prepare for price increases, and advised installation companies not to get locked into specific module sizes, but to sell watts instead. That way, when "volatility" restricts the availability of a particular module, the installer-seller may substitute another module, while still maintaining the specified system watts. Matt also counseled installers to make monthly projections of future requirements (in watts) so that manufacturers can allocate production, and stressed the importance of communication with the module suppliers. He also emphasized the importance of keeping all accounts current and, at times, to consider prepaying for product.

Matt suggested that folks looking to break into the business focus on promoting solar hot water system installations, energy audits, and other energy efficiency measures. New companies can establish strong customer relationships this way and prepare for PV when the market improves. He also encouraged new business owners to use the current period of module constraint as an opportunity to get additional training, licensing, and certification to enhance their professional and competitive standing.

Future Forward

These innovative ideas and many others will be instrumental in moving solar energy into the U.S. mainstream. And this year, by taking advantage of the federal tax credits and state rebates for solar-electric and solar thermal systems, you can help renewable energy gain momentum too. (For more information, see the Database of State Incentives for Renewable Energy Web site at


Don Loweburg, IPP, PO Box 231, North Fork, CA 93643 • 559-877-7080 • [email protected]

BIPV Testing & Evaluation Project •

Jeff Curry, Lakeland Electric, 501 E. Lemon St., Lakeland, FL 33801 • jeff.curr[email protected]

Matt Lugar, Sharp Electronics Corp., Solar Energy Solutions Group, 5901 Bolsa Ave., Huntington Beach, CA 92647 • 714-903-4679 • [email protected]

Solar Power 2005/2006, 805 15th St. NW, Ste. 510, Washington, DC 20005 • 202-682-0556 • [email protected] • Solar Power 2006 will be held Oct. 15-20 at the San Jose Convention Center, San Jose, California. A CD containing all the presentations made at Solar Power 2005 is available for sale on the Web site.

Suntech, a company based in China, exhibited their PV modules.

Qualify Solar Energy Products

Qualify Solar Energy Products

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