Defined in 705.2 as a “utility-interactive inverter output circuit,” these conductors—between the inverter and its interconnection OCPD—cannot supply power to any other loads, nor can they extend past the inverter to supply loads. They are strictly dedicated to the connection of an electric power production source to the primary source. This point of connection can occur at “any distribution equipment on the premises,” such as a subpanel. There also can be multiple PV systems interconnected through their own separate, dedicated means.
Section 705.12(D)(2)(1) addresses protecting feeder conductors, which are defined in Article 100 as “conductors between the service equipment…or other power supply source and the final branch-circuit overcurrent device.” A common example would be the conductors between the main service panel and a subpanel in a home or building. The concern is that if a PV system is connected to a feeder at some point other than its opposite end, some section of the feeder could be subject to higher, additive currents: from the primary source OCPD supplying the feeder and from the inverter output.
There are two methods for protecting the portion of the feeder which is subject to the combined output of the supply breaker and the inverter. One option is to add another OCPD—rated to protect the feeder conductor and typically the same size as the feeder’s existing primary supply breaker—on the load side of the inverter connection to the feeder. In the other method, the feeder ampacity (as well as the busbar rating, if there is a main lug-only panel) can be increased so that it has an ampacity greater than or equal to the primary supply breaker plus 125% of the inverter output current.
Tap conductors, defined in Article 240, “Overcurrent Protection,” are addressed in Section 705.12(D)(2)(2). Essentially, a tap is a conductor, smaller than the feeder conductor, that is connected to the feeder without overcurrent protection at that point of supply; as such, it is subject to current that could exceed its ampacity. The tap conductor then terminates in a piece of distribution equipment (such as a subpanel) that contains a main breaker. Sizing requirements for tap conductors, which are very common in commercial buildings, are found in Sections 240.21(B)(1) and (2).
Like protecting the feeder conductor, the concern is the addition of a second source of current on the feeder to which the tap is connected. Provided the numerous requirements of Section 240.21(B) are met, a tap up to 10 feet long must have an ampacity equal to or greater than 10% of the feeder supply breaker’s ampacity, and equal to or greater than the ampacity of the OCPD at the tap conductor’s terminal end. For taps between 10 and 25 feet, the ampacity has to be at least one-third (or 33.3%) of the feeder supply breaker’s ampacity (and greater than or equal to the terminal OCPD). When a PV system is connected to the feeder supplying the tap, Section 705.12(D)(2)(2) requires that 125% of the inverter’s output current must also be included in the calculation for the tap conductor ampacity.
Section 705.12(A), in conjunction with the allowance in 230.82(6), permits power production systems to be connected on the supply side of the main service disconnect, typically between the meter and the premise’s main breaker. The system size that can be connected on the supply side is limited by the size of the existing service. For example, a building with a 200 amp (A) service could have a PV system connected on the supply side through a 200 A fused disconnect, which computes to a maximum inverter output current of 160 A (160 A × 1.25 = 200 A).