AE 868
Commercial Solar Electric Systems

Interconnection Methods

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The point of connection is the location at which the DG source including a PV system can be interconnected with the electric utility grid. Since adding power at that point is beyond the initial intended design of the existing electric system at the point of connection, all service equipment, such as main power distribution panel disconnects and conductors, must be sized and rated to allow this addition according to NEC 690.64.

NEC 690.64 permits the output of the inverter to be connected to either load side (customer side) or supply side (utility side) service points depending on the size of the PV system and marginal power available at that point. In large a PV system, the available service might not have enough capacity to handle the added power and, in this case, a separate service may need to be installed. A backfeed circuit breaker is a circuit breaker that allows current flow in either direction. The backfeed circuit breaker provides overcurrent protection of the branch circuits from the inverter, and the panel’s main service circuit breaker provides protection of the entire PV and load system from the utility. Regardless of the interconnection type, NEC 705 requires that a permanent directory be placed at each service location showing all power sources for a building.

Load-side interconnection

Common in small PV systems, the main service disconnect at the customer facility has enough margin to handle the extra capacity added by the PV system, and that allowed an interconnection at the load side.

NEC permits that type of interconnection providing the following conditions (we will only mention the technical-related issues):

  1. In case multiple power sources are to be interconnected, each added power source (inverter in PV case) must have a dedicated circuit breaker or fused disconnect unless their outputs are first combined at a sub-panel.
  2. In the 2011 National Electrical Code (NEC), the language in 705.12(D)(2) is straightforward. Fulfillment of the 120% rule that states that the sum of the rating of the OCPD in all circuits supplying power to a busbar or conductor must not exceed 120% of the rating of the busbar or conductor to prevent overloading conditions. This only applies to breakers that supply the load center with power including the main utility fed circuit breakers and any back-fed circuit breakers from PV sources (load circuit breakers are not considered)

    Here is what NEC 2014 - Article 705.12(D)(2) code states:

    “Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120% of the rating of the busbar or conductor.”

    In the 2014 code, this straightforward sentence has been revised to include several paragraphs with different scenarios. The meaning might look the same, however, and once you understand the philosophy of the simpler 2011 version of 705.12(D)(2) you will be able to understand NEC 2014’s more sophisticated version. It really is the designer’s knowledge to correctly interpret the code since NEC 2014 provides more flexibility to allow more PV capacity for the same circuit size.

    Here is what NEC 2014 - Article 705.12(D)(2) states:

    • The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
    • Where two sources, one a utility and the other an inverter, are located at the opposite ends of a busbar that contains loads, the sun of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220.
    • The sum of the ampere rating of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity if the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar.
  3. Interconnection point to be on the supply side of all ground fault protection equipment.
  4. A back-fed circuit breakers in the panelboard shall be positioned at the opposite end from the main circuit breaker and marked with a warning label at the back-fed breaker from the PV system.

Note:

More requirements are listed on NEC 690 through the NFPA free access page, and designers are encouraged to read further.

Reflection

Assuming you have a service panel rated at 200A (maximum current) and the main circuit breaker is rated at 200A, what is the maximum allowable current that can be back-fed to this panel?

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ANSWER: Applying the 120% rule, the 200A panel can only handle 240A current (1.2 x 200= 240A). Given the main circuit breaker is 200A and considering the rule that states that the sum of the current supplying power to the service panel cannot exceed 120% of the panel rating, or 240A. Then the allowable current from the additional current is the 40A (240 - 200= 40A). In other words: The maximum allowable back-fed current = 1.2 x 200A (panel rating) - 200A (main breaker rating) = 40A.

Note:

In some cases and based on load electrical study done by professional engineers, the main breaker can be taken down to a lower rating that will in return allow additional current to be back-fed to the panel.

Supply-Side Interconnection

For larger installation or in case the load-side strategy doesn’t provide the required capacity, a supply-side interconnection is the second resort for PV systems. NEC article 230 requires any additional new service to have disconnect and OCPD. That said, the supply-side interconnection must include another service in parallel to the existing one with an additional OCPD and disconnect. The equipment and conductors must be rated to accommodate for that additional power coming from the PV system. The interconnection requires tapping the service entrance conductors, and that is done between the existing service panel and utility meter. A new meter might be needed when the service type cannot establish the tapping strategy. The added disconnect must meet local utility standards in terms of accessibility, interrupting rating, and visibility. The service conductor must be sized for at least 125% of the continuous load current as stated in NEC article 230.