Interactive Inverter Interconnections: Page 5 of 6

Identifying the Optimal Point of Connection

Example 1: New tap for loads. This option is worth investigating if you want to connect to a feeder but avoid upsizing the downstream feeder and busbar. Instead of adding overcurrent protection at the POC, as illustrated previously, you may prefer to add a circuit breaker or fused disconnect directly ahead of the busbar serving the downstream loads. This approach, shown schematically in Figure 9, essentially converts the downstream portion of the existing feeder, between the inverter connection and the loads, into a tap conductor.

If the tap does not exceed 25 feet and meets Code-mandated minimum size and installation requirements, you can use Equations 6a and 6b to verify that the connection is compliant:

Feeder Ampacity ≥ Inverter Current x 125% [6a]

Load Tap Ampacity ≥ (Supply OCPD + (Inverter Current x 125%)) x 33% [6b]

Example 2: New tap for inverters. This option comes in handy where you would like to locate the inverter overcurrent device some distance away from the feeder, perhaps to make it readily accessible. In this scenario, illustrated in Figure 10, the tap conductors serve the interactive system only.

Where the tap does not exceed 25 feet and meets Code-mandated minimum size and installation requirements, you can make a compliant connection by sizing the tap conductor to the worst-case scenario as determined by Equations 7a and 7b:

Inverter Tap Ampacity ≥ Inverter Current x 125% [7a]

Inverter Tap Ampacity ≥ (Supply OCPD + (Inverter Current x 125%)) x 33% [7b]

The larger of these values determines the size of the inverter tap conductor.

Example 3: New taps for both inverters and loads. This option is worth investigating where an existing feeder is available to serve both a new inverter system and a new load, but you would like to locate these at some distance away from the end of the feeder and avoid adding a panelboard. The strategy here is to make two Code-compliant taps, where one feeder tap conductor serves the inverter and the other feeder tap conductor serves the load. Figure 11 illustrates this two-tap scenario.

To ensure that the connections are Code compliant, size the inverter feeder tap conductor according to the larger value as determined by Equation 7a and 7b, and size the load feeder tap conductor according to Equation 6b.

Supply-Side Connections

The NEC language pertaining to supply-side connections is concise and not overly prescriptive. In short, the Code allows for connections on the supply side of the service disconnecting means provided that the sum of the parallel power source overcurrent devices does not exceed the rating of the service [705.12(A)]. A definition in 705.2 clarifies that power production equipment does not include the utility-supplied service, but rather consists of other sources of electricity, such as generators and interactive systems.

When planning an interconnection on the supply side of the service entrance disconnecting means, it is important to establish or verify equipment ownership and control. Technically, the service point (see Figure 1) is the demarcation point between the serving utility and the premises wiring, per the definition in Article 100. In practice, the location of this demarcation point varies depending on the utility’s policies and the type or conditions of the service. Furthermore, ownership and control do not always go hand in hand. For example, the utility generally controls metering equipment even when customers own some or all of this hardware. In most cases, AHJs want to verify that you are making the proposed supply-side connection in a manner consistent with utility requirements applying to services. As such, it is a good idea to start the planning process by obtaining a copy of the serving utility’s design standards.

Connections to service entrance conductors. The Code allows for splicing or tapping service entrance conductors [230.46] and connecting power production equipment on the supply side of a service disconnect [230.82(6)]. In some cases, you may be able to make a connection inside the existing service equipment; in other cases, the AHJ or utility design criteria may require that you add a new enclosure to make a connection.

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