Ungrounded PV Power Systems in the NEC: Page 12 of 12

Wiles explains: “Transformerless inverters do not connect one of the dc circuit conductors in the PV array to ground (as allowed by NEC 690.35) and have no internal bonding jumper. Therefore, there will normally be no terminal to connect the GEC, and the NEC does not require a dc GEC.”

A GEC is not required for a non-isolated inverter, and if the inverter adheres to the UL 1741 requirements, it will not even have a GEC terminal. According to Wiles, “The UL standard requires a grounding electrode conductor terminal … only when there is a bonding jumper in the direct current (dc) side of the inverter.”

Note that auxiliary dc-grounding electrodes and conductors may be installed for other reasons, such as protection against lightning surges. In addition, an ac grounding-electrode system is still required for the utility supply to which the ungrounded PV system is connected.

Permitting and Inspecting

Ungrounded PV systems are more similar to grounded PV systems than they are different from them. The specific differences in equipment selection and usage are based on fundamental distinctions found in the NEC and UL 1741. Provided that system designers and installers understand these technical distinctions and can discuss them with the AHJ, permitting and inspecting ungrounded PV systems can go smoothly.

“Most AHJs will have questions once they see that the inverter is transformerless and that the PV system is ungrounded,” observes Mync at KACO. He recommends preparing for those questions: “Educate yourself on the NEC sections that relate to the installation of transformerless inverters, and be prepared to discuss these in an intelligent manner with the AHJ.” System integrators also need to be proactive and contact the AHJ before installing an ungrounded system within its jurisdiction for the first time. By definition, the AHJ approves equipment, materials, installations and procedures; see NEC Article 100.

Berdner at SolarEdge notes: “Some AHJs and plan checkers are unfamiliar with the Code provisions for use of ungrounded arrays.” He recommends that engineers and system designers make specific reference to Section 690.35 in their plans. He adds, “I would also include a datasheet or manual for the inverter that indicates the presence of a listed GFDI device.”

“The main concern inspectors have is to ensure the safety and Code compliance of any electrical installation,” reminds Advanced Energy’s Ruberti. He encourages integrators to provide the AHJ with copies of applicable listings, such as the UL 1741 certificate of compliance. “The best thing you can do is be patient and prepared to educate them,” he says. “Explain why the new transformerless technology is not just safe, but safer than what they have approved in the past.”

In addition to educating plan checkers and inspectors, system integrators also need to educate their designers and installers. “Most installation errors can be avoided with team meetings and proper training,” KACO’s Mync says. “A change in technology can slow things down at first, but these impacts are minimized when installers know what they are supposed to do and why.”

CONTACT

Jason Fisher / SunPower / Charlottesville, VA / us.sunpowercorp.com
David Brearley / SolarPro magazine / Ashland, OR / solarprofessional.com

RESOURCES

Mead, John, “Ungrounded DC Systems,” SolarWorld Engineering Bulletin 1002–2010, solarworld-usa.com

Wiles, John, “More Questions from Inspectors: Numerous PV Systems Pose Issues,” IAEI magazine, March/April 2012, International Association of Electrical Inspectors, iaei.org

Wiles, John, “Ungrounded PV Systems,” Home Power magazine, October/November 2010), homepower.com

LISTED NON-ISOLATED INVERTER MANUFACTURERS

Advanced Energy / 800.446.9167 / advanced-energy.com
Chint Power Systems / 972.761.3992 / chintpower.com/en
Eaton / 877.386.2273 / eaton.com
Exeltech / 800.886.4683 / exeltech.com
Growatt New Energy / 832.615.5047 / growattusa.com
Ingeteam / 262.240.9850 / ingeteam.com
KACO new energy / 415.931.2046 / kaco-newenergy.com
Power-One / 805.987.8741 / power-one.com
REFUsol / 408.775.7744 / refusol.com
SMA America / 916.625.0870 / sma-america.com
SolarEdge / 530.273.3096 / solaredge.com

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