Interactive Inverter Interconnections: Page 2 of 6

Identifying the Optimal Point of Connection

Load-Side Connections

The 2014 and 2017 editions of the NEC provide detailed requirements for making load-side connections to busbars in panelboards or to load-side conductors [2014-705.12(D)(2); 2017-705.12(B)(2)]. The additional load-side connection guidelines, compared to those in earlier Code editions, are beneficial for system designers and AHJs. The most significant change, however, is the directive to use 125% of the inverter output circuit current, rather than the interactive inverter breaker rating, for load-side ampacity calculations.

CONNECTIONS TO BUSBARS
All else being equal, the simplest and most cost-effective interactive inverter interconnection is to connect to a panelboard busbar by adding a circuit breaker. In addition to providing a Code-compliant POC, this new breaker also provides overcurrent protection for the inverter output circuit and often serves as the PV or interactive system disconnect. The NEC details five different methods or scenarios for interconnecting an electric power source to a busbar, each of which is potentially useful in a subset of real-world situations. Note that while the following examples assume the use of circuit breakers, the Code also allows for the use of fusible disconnecting means.

Power sources do not exceed busbar rating. Where applicable, this is likely the easiest and most cost-effective POC. As long as the busbar rating is greater than or equal to that of the primary power source (the busbar OCPD rating) plus the sum of the parallel power sources (125% of the inverter output circuit currents), the Code does not limit the locations or number of sources or loads connected to a panelboard busbar [2014-705.12(D)(2)(3)(a); 2017-705.12(B)(2)(3)(a)]. Since any inverter OCPD location is acceptable, the Code does not require a warning label adjacent to a backfed breaker in this scenario.

Though opportunities to use the busbar interconnection method shown in Figure 3 are relatively uncommon, they do exist. For example, a site evaluation might identify a residential panelboard with a 225 A–rated busbar but a 200 A main breaker, or a commercial main distribution panel with a busbar rating higher than its main OCPD. In this type of scenario, you can use Equation 1 to confirm that a proposed interconnection is Code compliant:

Busbar ≥ Supply OCPD + (Inverter Current x 125%) [1]

120% allowance. This is the busbar interconnection method familiar to most solar professionals. Since 1987, the Code has included some version of “the 120% rule,” which allows primary and parallel power sources to exceed a panelboard’s busbar rating under certain circumstances. This allowance originally applied only in residential applications, where load diversity prevents overload conditions. Eventually, the Code-Making Panel was able to extend the 120% allowance to commercial and industrial applications by requiring that the primary power source (utility) and parallel power sources (interactive inverters) connect to opposite ends of the busbar, as shown in Figure 4.

Whereas earlier Code editions used the inverter OCPD rating in calculations related to the 120% allowance, calculations under NEC 2014 and NEC 2017 are based on 125% of the inverter output circuit current [2014-705.12(D)(3)(b); 2017-705.12(B)(2)(3)(b)]. You can use Equation 2 to confirm that a proposed interconnection complies with the 120% allowance:

Busbar ≥ (Supply OCPD + (Inverter Current x 125%)) ÷ 120% [2]

Since the physical location of the inverter OCPD prevents any potential overload conditions, the Code requires a warning label to alert someone not to inadvertently move this device in the future:

WARNING:

POWER SOURCE OUTPUT CONNECTION—
DO NOT RELOCATE THIS OVERCURRENT DEVICE.

Limit load and supply OCPDs. This calculation method is unique insofar as it ignores the rating of the overcurrent device protecting the busbar and instead evaluates the total rating of all the applied load and supply OCPDs. In this scenario, a proposed POC is Code compliant as long as the panelboard busbar rating is greater than or equal to the sum of the attached OCPDs, regardless of whether these connect to loads or inverters [2014-705.12(D)(3)(c); 2017-705.12(B)(2)(3)(c)]. Since an overload condition cannot exist in this scenario, the Code does not limit the number or locations of load or inverter breakers, as illustrated in Figure 5. In this scenario, you can use Equation 3 to confirm Code compliance:

Busbar ≥ Load OCPDs + Inverter OCPDs [3]

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