CODE CORNER: Load-Side Connections

A PV source interconnection made at a subpanel. The interconnection breaker is at the opposite end from the main feed, and is marked not to be moved.
A PV source interconnection made at a bottom breaker of a top-feed main distribution panel. The breaker is marked as PV input and not to be relocated.

Article 705 in the National Electrical Code covers the interconnection of electric power production sources to a primary power source, typically the utility. This Code article is not specific to PV systems; it applies to any power production equipment that interconnects with the primary source of electricity. In the “Scope” section, 705.1, an informational note indicates that primary power sources can include a utility supply or on-site electric power source.

In 2017, notable changes in Article 705 took place. First, a few definitions have been added with respect to microgrids, defining what a microgrid is in addition to a microgrid interconnection device. Second, the definition for inverter output circuits no longer includes “utility,” given that the inverter can interconnect with a source other than a utility. The term “inverter output connection” has been replaced with “power source” within the text of 705.12(B). While the changes in definitions and section titles likely don’t affect any PV system interconnection, the language within this Article is more inclusive of other systems that may interconnect to a primary source of electricity.

Beyond the definitions and terms, the focus of this “Code Corner” is Article 705.12(B) [705.12(D) in 2014]—load-side connections. The point of interconnection is allowed at the load side of service disconnecting means at any distribution equipment on the premises. Making a connection in the main service panel is not required—downstream subpanels are perfectly acceptable for interconnection points.

To make a connection on distribution equipment, five subsections of the 2017 NEC must be met, one less than in 2014 (Tentative Interim Amendment 14-11 deleted 705.12(D)(6) in its entirety and in 2017 it was omitted). Read each of these subsections carefully—in some cases, the calculations are based on 125% of the power source’s output circuit current; in others, the calculations are based on the overcurrent devices’ ratings.

The first requirement is that “each source interconnection of one or more power sources installed in one system shall be made at a dedicated circuit breaker or fusible disconnecting means.” This does not necessarily require an interconnection circuit breaker for each individual inverter, but rather one for the entire system per the manufacturer’s instructions. For microinverter-based installations, this will result in having multiple microinverters on a circuit; string inverters will typically have a dedicated circuit breaker or fusible disconnect.

The first subsection in 705.12(B)(2) covers connections directly to feeders. In this case, the power source output would be connected to a feeder that is not at the opposite end from the overcurrent protection device (OCPD) that protects that feeder. When this output connection is not on the opposite end of the OCPD, there is a risk of overloading the feeder. Thus, to meet the requirements of 705.12(B)(2)(1), the feeder conductor being connected to must either be sized such that the conductor’s ampacity is large enough to handle the sum of the primary source OCPD and 125% of the power source’s output current, or there must be an OCPD on the power source connection’s load side that is not larger than the feeder’s ampacity. Meeting either of these two requirements will ensure that the feeder conductor does not have an excessive amount of current on it from the two power sources.

The section that follows addresses the typical taps that electricians are accustomed to. When a tap is made on the same feeder that is connected to a secondary power source, Section 705.12(B)(2)(2) requires that the methods used to size the tap conductors—as calculated in 240.21(B)—include both the rating of the OCPD protecting the feeder and 125% of the power source’s output current. As with the feeder calculations in the above section, this ensures the tap conductors installed will be sized for all the power sources present.

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