A safer solution would be to open the AC circuit at the PV back-fed breaker in the building service entrance panel—but only if that breaker can be locked open. However, breaker lock-outs are few and far between and lock-out/tag-out procedures are often not used in residential and commercial electrical systems. NEC Section 690.14(D) addresses this situation and points to a solution: Installing a separate AC disconnect near the AC PV modules or microinverters meets NEC requirements and enhances system safety. A common 60 A, unfused, pull-out air-conditioning disconnect (less than $10) can serve as the disconnect, a place to terminate the AC output cable from a set of microinverters or AC PV modules, and a place to originate the field-installed wiring system to the AC load center in the house.
Since a microinverter and an AC PV module work individually, each inverter extracts the maximum power from its matched module—independent of the other module/inverter pairs in the array. The outputs of the microinverters or AC PV modules are connected in parallel rather than in series, which isolates the performance of one from another.
The outputs are at 240 VAC, with the AC output circuits acting much like AC branch circuits. When utility power is removed at any disconnect in the circuit, all the inverters go dead and do not pose the safety hazards associated with daytime, “always-energized” DC circuits, which operate at hundreds of volts between the modules and a string inverter. If a short circuit or ground fault were to occur in these AC output circuits, the dedicated branch circuit breaker would open and the circuit would be de-energized. Opening the main service disconnect or the back-fed PV breaker will de-energize those PV AC output circuits—a boon to firefighters.
Numerous microinverters and AC PV modules are being installed. They are even being sold in home improvement centers, building supply houses, and electrical supply houses—so the general public is buying them. PV installers are best equipped to design and install these devices and understand the NEC requirements that apply to them.
John Wiles works at the Institute for Energy and the Environment (IEE) at New Mexico State University. John provides engineering support to the PV industry and a focal point for PV system code issues.
Southwest Technology Development Institute • www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/Codes-Stds.html • PV systems inspector/installer checklist, previous “Perspectives on PV” and Code Corner articles, and Photovoltaic Power Systems & the 2005 National Electrical Code: Suggested Practices, by John Wiles