Practical Application of NEC 2017: Page 4 of 5

Callout F: Overcurrent Protection and Disconnects

Apart from rapid-shutdown requirements for PV systems on buildings, the most substantial 2017 Code changes with regard to system design relate to overcurrent protection devices (OCPDs) (690.9, “Overcurrent Protection”) and disconnecting means (690.13, “Photovoltaic System Disconnecting Means”; and 690.15, “Disconnection of Photovoltaic Equipment”). Designers and installers should read these sections carefully.

Overcurrent protection. System designers and installers should pay particular attention to the revised OCPD requirements for PV source and output circuits in 690.9(C). Whereas NEC 2014 and earlier Code editions required OCPD in both poles of PV systems deployed using non-isolated Type-TL inverters, NEC 2017 requires only a single OCPD, as shown in Figure 3. Designers can place this single OCPD on either pole of the array, provided that all the devices in the PV system are in the same polarity.

Much of the content in 690.9(B) regarding OCPD rating is not new, but the 2017 edition reorganizes it. For example, the CMP moved the requirement that OCPDs in dc PV circuits be listed for the application to this subsection. Note that the CMP added a new allowance for adjustable electronic OCPDs in 690.9(B)(3).

Disconnecting means. Previous Code editions sometimes left system designers and installers at odds with jurisdictional authorities regarding what constituted the PV system disconnect, where to locate it and how to label it. The additions and changes to Figure 690.1(b) are welcome clarifications, as the PV system disconnect location is clearly marked for a variety of system configurations. These diagrams, in combination with extensive rewrites to 690.13, should allow designers to confidently implement NEC 2017 requirements for PV system disconnecting means.

It bears emphasizing that all energy storage equipment, battery-based inverters and loads lie outside the boundary of the PV system. This is a very important distinction that SolarPro has covered previously in some detail. See the article by Bill Brooks, “NEC 2017 Updates for PV Systems” (SolarPro, May/June 2016), for an in-depth discussion of this topic.

System designers should pay careful attention to 690.15, as some of the terminology may be new to many in the solar industry. As an example, the CMP introduced the term isolating device, which in this context is a device that is intended for isolating PV equipment and circuits from the source of power and that does not require an interrupt rating. Note that the allowable types of isolating devices are listed in 690.15(C).

While these devices must be able to provide isolation from all conductors that are not solidly grounded, they are not subject to the simultaneous disconnection requirements that apply to PV system disconnecting means [690.13(F)(1)]. Note, however, that isolating devices alone do not suffice for dc combiner output circuits, or inverter or charge controller input circuits operating over 30 A; these circuits require an equipment disconnecting means. Unlike isolating devices, equipment disconnecting means are subject to simultaneous disconnection requirements [690.15(D)]. Both equipment disconnects and PV system disconnects are allowed in place of isolating devices.

Practically speaking, one of the most significant changes in NEC 2017 is that it requires isolating devices or disconnecting means in both poles of a PV circuit, as shown in Figure 3. The Code requires the provision of these devices as needed to isolate PV equipment—including modules, fuses, dc-to-dc converters, inverters and charge controllers—from “all conductors that are not solidly grounded.” Since the vast majority of PV systems are functional grounded rather than solidly grounded, it is necessary to disconnect both poles of PV circuits.

Practical considerations. It is helpful to think about how NEC 2017 requirements apply to common applications, such as dc combiners, inverter-integrated combiners or inverter input circuits. First, disconnection means are required to isolate fuses from ungrounded conductors. A touch-safe fuseholder itself qualifies as an isolation device for circuits with a maximum current up to 30 A. Second, dc combiner output circuits with a maximum current greater than 30 A require an equipment disconnecting means that is capable of opening both poles simultaneously and is either integral to the equipment, located within sight and within 10 feet of the equipment, or remotely operable from within 10 feet of the equipment. The requirement for equipment disconnecting means also applies to inverter input circuits (>30 A).

In rooftop applications, arc-fault requirements effectively limit dc combiner outputs to 40 A or less; rapid-shutdown requirements, meanwhile, mandate remotely operable equipment disconnecting means capable of simultaneously disconnecting all current-carrying conductors. When specifying equipment for ground-mounted systems with a generating capacity of less than 5 MWac, integrators should be aware that the majority of disconnecting combiners currently on the market are designed to disconnect one pole of the array only. In large-scale applications, 691.9 provides PEs with more design latitude for PV equipment isolation; this allowance assumes that only qualified persons service the array and that they are provided with written safety procedures and conditions, as well as  operation and shutdown procedures.

It is not clear how manufacturers will go about meeting the demands of a fragmented market, given that Code cycle adoption varies across the US. Brian Lydic, senior standards and technology engineer at Fronius USA, elaborates: “Our non-isolated units already required simultaneous disconnects for all poles, so that’s no issue for us. The single-pole OCPD question is more interesting. Right now, installers have the ability to install ‘slugs’ or blanks in fuseholders, which means they can fuse poles or not depending on the AHJ’s adopted Code cycle. To reduce costs, of course, we’d like to eliminate half of the fuseholders as soon as possible. We want to work with industry stakeholders to push the acceptance of the new wiring methods so that all customers, even those in pre-2017 jurisdictions, can enjoy the lowest costs.”

Michael Neiman, an applications engineer at Yaskawa–Solectria Solar, echoes these sentiments: “We designed the dc and ac interfaces of our products with flexibility in mind. Thanks to this flexibility, we are configuring interfaces across our inverter and combiner product families to take full advantage of—as well as fully comply with—the new Code requirements. For example, we can simplify our string combiners by fusing just one dc polarity and not both. This lowers the product cost to our customers.”

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