Article 705: Interconnected Electric Power Production Sources: Page 3 of 3

Intermediate

Inside this Article

Back-fed breaker
This back-fed breaker has been secured with an additional fastener, providing another level of protection against shock hazards.

Section 705.31, new to the 2014 NEC, stipulates that overcurrent protection for supply-side interconnections must be located within 10 feet of the point of interconnection, unless cable limiters (devices that isolate conductors from short circuits, but don’t necessarily provide overcurrent protection) or current-limited circuit breakers are installed. This helps reduce the risk from fault current sourced from the primary electricity source (the utility). For general information on supply-side connections, see “Code Corner” in HP150.

Additional Fasteners

In situations using back-fed breakers, additional fasteners may be required to reduce shock risk associated with pulling an energized breaker off a busbar. Section 408.36(D) in the 2014 NEC dictates the requirements for additional fasteners on back-fed breakers. However, Section 705.12(D)(5) states that plug-in-type back-fed breakers used for load-side connection to the utility, and connected to listed grid-tied inverters, are not subject to the additional fastener requirements. Of course back-fed breakers have to be suitable for the application, i.e., not marked “line” and “load.”

Per 690.10(E), these same plug-in-type breakers, when connected to a stand-alone inverter or the stand-alone output of a multimode inverter, are required to have an additional fastener. The logic is that the back-fed breaker terminals or tabs could still be energized when pulled off the busbar, posing a shock hazard—and an additional fastener provides another level of protection. This is not the case with the output circuit of a grid-tied inverter—if the breaker is pulled off the busbar, the inverter will immediately cease operation. A stand-alone inverter’s output, regardless of inverter type, would stay energized—much like a breaker connected to a fossil fuel generator, and thus is required to have the additional fastener.

The required fasteners are available from most major equipment manufacturers. Another option is to connect the breakers to a main breaker that is fastened in place in the breaker panel, rather than back-feeding a plug-in breaker.

Other Considerations

In many grid-tied inverters, only the ungrounded conductors carry AC output current. The AC grounded (neutral) conductor is used as a reference for “instrumentation, voltage detection, or phase detection.” In this case, per 705.95(B), the neutral conductor in the inverter output circuit only has to be as large as the equipment-grounding conductor (which is typically smaller than the phase conductors). Especially on larger inverters, this can result in cost savings, as smaller-gauge wire can be used, but be sure to verify this with the inverter manufacturer.

New in 2014 is 705.12(D)(6), which requires AC arc-fault protection for grid-tied inverters, with wiring harness (i.e., “AC interconnect cable” or “trunk cabling”) output circuits rated at 240 VAC and 30 A or less, unless the harnesses are installed within an enclosed raceway. This provides added protection for microinverter and AC module systems, which often have more cabling under modules than string inverter systems. This may be difficult to implement given a lack of suitable equipment for the purpose—most AC AFCI breakers are not tested for back-feed. Some jurisdictions may waive this requirement, citing the allowance in Section 90.4 for products that are required but not yet available.

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