The previous installment of “Code Corner” focused on NEC Section 705.12(A), which covers supply-side connections for grid-tied PV systems. However, many residential and small commercial systems are net-metered and connected on the load side (typically via a back-fed circuit breaker in an AC panel) of existing service equipment.
Section 705.12(D) establishes the requirements for connecting the output of a parallel power production system on the load side of service equipment. It allows this interconnection point to be at any distribution equipment on the premises, which makes it possible to connect to the utility grid in a subpanel rather than in the main AC service.
Section 705.12(D)(1) requires that each interconnection be made at a dedicated circuit breaker or fused disconnect. Section 690.15 allows for a single disconnecting means for the combined AC output of more than one inverter. This strategy is commonly used on larger systems that are connected on the load side: a dedicated AC breaker panel holds back-fed breakers from several inverters. The panel’s combined output is interconnected on the load side of the AC service through a single, larger breaker.
Section 705.12(D)(2) states that the sum of breakers supplying power to a busbar or conductor must not exceed 120% of the busbar or conductor’s rating. Both the overcurrent protection device (OCPD) between the grid and the service panel (typically the main AC breaker) and the OCPD from the inverter(s) are considered power supplies.
For example, an AC service panel with a 200 A main breaker and a 200 amp-rated busbar could have up to 40 A of inverter output-circuit OCPDs installed (200 A + 40 A ≤ 200 A × 1.2). Since Sections 705.60 and 690.8 require that the minimum inverter output-circuit OCPD be 125% of the inverter’s rated maximum output current, the inverter or inverters in this example could have a total maximum rated output current of 32 A (40 A ÷ 1.25 = 32 A).
In some service panels, the busbars have a higher rating than the main OCPD, which allows installing more inverter output OCPDs (see table). In new construction, installing a service panel of this type will allow for a larger PV system to still be connected on the load side. And, though it is not always possible, in some cases, downsizing the main breaker is an effective strategy. For example, replacing a 200 A main breaker with a 150 A one would allow 90 A of inverter breakers to be installed (150 A + 90 A ≤ 200 A × 1.2). Load calculations must be performed to ensure that the new, smaller main breaker is large enough for the existing loads, and finding a smaller main breaker may be difficult or expensive. Additionally, replacing the main breaker in the main AC service requires shutting down the electrical service. In other cases, a service equipment upgrade may be the only option for a load-side connection and its cost should be weighed against the cost of a supply-side connection.
New to the 2011 NEC is an Exception to the 120% rule for grid-tied with battery backup PV systems. The inverter’s battery charging and pass-through capability in these systems require the interconnection OCPD to be much larger than is required for a batteryless system. For example, a 3,000-watt, grid-tied battery backup inverter can draw 60 A from the grid for battery charging and pass-through to AC loads, but is only rated to deliver 25 A back to the grid. A 60-amp interconnection breaker can’t be installed in a typical 200 A residential service without violating the 120% rule. Instead of basing busbar-loading calculations on the larger breaker size, the Exception now states that 125% of the rated utility-interactive current from the inverter be used. In this example, that would be 31.25 A (25 A × 1.25), which allows the inverter to be interconnected in a 200 A service panel.