Retrofitting Non-Isolated Inverters in Legacy Arrays: Page 2 of 3

Non-isolated inverter option. Newer non-isolated inverters clearly provide a far safer PV system. However, differing opinions about system grounding classifications and requirements may complicate this option. Since I regularly work with AHJs, I have posed the following question to them many times: “If an existing PV system experiences an inverter failure and a contractor pulls a permit that includes replacing the failed inverter, would you accept the installation of a safer inverter even though some engineers consider the repaired system ungrounded rather than solidly grounded?”

I get a fairly consistent response to this question. If the new PV system is safer, most AHJs will approve the proposed installation based on the following language in NEC Section 90.4: “By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.”

Via this allowance, even AHJs who believe that non-isolated inverters are subject to ungrounded PV system requirements can approve the use of non-isolated inverters for retrofit purposes. It is also possible to show that ungrounded requirements simply do not apply in this scenario.

Non-Isolated ≠ Ungrounded

In 2005, the Code-Making Panel (CMP) responsible for Article 690 introduced Section 690.35, “Ungrounded PV Power Systems.” A close reading of the language makes it clear that these ungrounded PV system requirements do not apply to systems deployed with non-isolated inverters: “Photovoltaic power systems shall be allowed to operate with ungrounded PV source and output circuits where the system complies with 690.35(A) through (G)” [emphasis added].

When you connect a non-isolated inverter to a grounded ac service, the system is grounded whenever the inverter is operating. Therefore, the proper application of the NEC does not require implementing 690.35 (A) through (G) for non-isolated PV systems connected to grounded ac services. Until recently, most engineers did not recognize this ac service–ground connection as a PV system ground.

As a result, PV systems deployed with non-isolated inverters are widely misidentified as ungrounded PV systems. This is a misnomer. Ungrounded systems operate without a connection to earth; non-isolated inverter systems are connected to earth when operating, but floating in reference to earth when not operating. Unfortunately, this misnomer is also ubiquitous. For several years, most solar professionals and AHJs have diligently, if mistakenly, applied 690.35(A) through (G) to non-isolated inverter systems.

Ungrounded system requirements. The practical requirements for ungrounded PV systems are well known to solar practitioners. Subsection 690.35(A) requires disconnecting means in both poles of the array for ungrounded PV source and output circuits; 690.35(B) likewise requires overcurrent protection in both poles of the array. Meanwhile, 690.35(D) mandates the use of PV Wire for exposed single conductors, which effectively rules out the use of USE-2 conductors in these systems.

This latter requirement had the effect of slowing the adoption of non-isolated inverters in the US. While nearly all PV modules sold today have PV Wire cable whips, this was not always the case. Prior to 2013, very few PV modules were manufactured with PV Wire cables. Since older PV modules are unlikely to have PV Wire cables, some jurisdictions have questioned whether they should allow retrofit installations of non-isolated inverters in legacy PV arrays.

Alternative means of compliance. The NEC has long addressed alternative methods of system grounding for PV power sources. Since the mid-1990s, Section 690.41 has allowed for the use of solidly grounded systems as well as systems that “use other methods that accomplish equivalent protection in accordance with 250.4(A).” Since non-isolated inverter systems fit this description, they are technically not subject to the ungrounded PV system requirements in 690.35. Therefore, it is fully acceptable to retrofit non-isolated inverters in legacy PV arrays, even those deployed using standard wiring methods for grounded PV systems. Since this was not clear to many AHJs and solar practitioners, Code-Making Panel 4 (CMP 4) addressed this as part of the 2017 cycle of revisions.

System grounding in NEC 2017. The most recent edition of the NEC resolves the confusion regarding grounded versus ungrounded system grounding designations. CMP 4 introduced a term used in Europe—functional grounded PV system—which NEC  690.2 defines as having “an electrical reference to ground that is not solidly grounded.” An informational note clarifies that both PV systems with fuse-grounded inverters and those with non-isolated inverters meet the definition of a functional grounded PV system. In addition to adding this new system grounding definition, CMP 4 eliminated Section 690.35, “Ungrounded PV Power Systems,” in its entirety.

These changes mean that all PV systems are subject to the same installation requirements under NEC 2017, regardless of inverter topology. As detailed in Figure 2, these unified installation standards are as follows: overcurrent protection is required in one leg of a PV circuit only [690.9(C)]; disconnecting means are required in both legs of a PV circuit [690.15]; and both USE-2 and PV Wire are allowed as single-conductor cable in a PV array [690.31(C)].

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