Interactive Inverter Interconnections: Page 6 of 6

Identifying the Optimal Point of Connection

While the Code does not explicitly state that you must treat the wiring on the line side of the inverter disconnect as a set of service entrance conductors [see 230.40, Exception 5], it is generally considered a best practice to install this wiring in accordance with the long-established Code requirements pertaining to service conductors [Articles 230, 250.92, and so forth]. This is consistent with the revised language in NEC 2017 [690.13(C)]: “If the PV system is connected to the supply side of the service disconnecting means as permitted in 230.82(6), the PV system disconnecting means shall be listed as suitable for use as service equipment.” Understand, however, that a new disconnect for parallel power production equipment does not meet the Code definition of a service disconnecting means [Article 100]; therefore, the inverter disconnect does not count as one of the six switches allowed per set of service entrance conductors [230.71(A)].

As part of the 2014 revision cycle, the Code-Making Panel added a new section limiting the length of unprotected conductors in a supply-side connection. Specifically, it now requires overcurrent protection within 10 feet of the POC [705.31]. An exception allows for the use of cable limiters at the POC if you cannot locate overcurrent protection for power production source conductors within 10 feet of the connection point.

Connections to Other Equipment

The preceding examples intentionally assume a relatively generic set of circumstances, as my goal is to provide high-level guidance for making Code-compliant connections. In the real world, you will encounter a great deal of variety in terms of service types, equipment configurations and as-built conditions. Some facilities will provide multiple opportunities for a safe connection; others will present many obstacles. In some cases, you will need to upgrade the service or some of the existing electrical equipment to connect interactive systems in a way that satisfies the AHJ and the NEC. Though it is beyond the scope of this article to consider all of the methods and opportunities to connect at existing equipment, some common scenarios and challenges merit discussion.

Connections to subpanels. The NEC does not restrict your ability to connect to a panelboard based on its location or hierarchy in the premises wiring. Any panelboard fed by feeder conductors is a potential POC, provided that you evaluate any busbars or feeders between the primary power source and the inverter interconnection according to the calculation methods detailed previously. Pay special attention to breaker location and labeling requirements, as these also apply to upstream equipment. There should no longer be any confusion about what ratings to use in upstream calculations, since the default value is now 125% of the inverter output circuit current rather than the backfed breaker rating.

Adding lugs to busbars. The NEC does not specify how to make mechanical connections to busbars. Where it is not possible or practical to add a circuit breaker for this purpose, you may be able to add lugs to accommodate an inverter connection. When adding lugs, you must do so in a way that does not violate the product listing.

To add lugs, you do not simply make a mechanical connection wherever there is room to do so. Drilling a hole in a busbar to accommodate a mechanical connection removes conductive material. This type of field modification could violate the product listing or result in unintended consequences, both of which increase liability exposure. Moreover, many AHJs will not approve a modification that the manufacturer does not specifically allow or that was not designed under engineering supervision.

Some manufacturers identify approved locations and methods for adding lugs and may even provide hardware for this purpose. Feed-through lugs are perhaps the most common example of an opportunity to add lugs to a busbar using manufacturer-provided hardware. At sites with larger, custom-built panelboards, it may prove more challenging to add lugs to a busbar. Engineering supervision and field labeling may be required where the equipment vendor does not have instructions and recognized hardware kits for this purpose.

Adding lugs to other equipment. On either side of the service disconnecting means, it may be possible to add lugs or studs to existing equipment, including disconnects, meters, meter sockets, connector blocks and so forth. Many of these options are highly site specific, based on the equipment and jurisdiction. Relatively recently, equipment manufacturers and even utilities have begun to offer meter socket adapters or solar-ready panelboards specifically designed to provide the capacity and termination points needed to make a Code-compliant connection. While equipment upgrades are unavoidable in some cases, an increasing number of vendors are developing listed solutions for making a Code-compliant interconnection at existing equipment.

Adequacy of existing equipment. When planning interconnections, it is important to evaluate the adequacy of the existing equipment or service. As-built conditions could prove unsuitable for an interconnection where equipment is damaged, perhaps due to a previous overload condition, or where it is not rated for the environment. You may need to repair or replace equipment due to poor workmanship. In some cases, you may encounter equipment that is subject to a recall or is generally known to be faulty.

Most AHJs grandfather existing conditions to some extent, meaning that you do not have to upgrade everything to the most recent Code requirements to perform a limited scope of work, such as adding a power production source. However, a grandfather clause does not automatically extend to existing equipment that you plan to modify or use as a POC. Especially in older dwellings, it is not uncommon to encounter legacy wiring methods or electrical equipment that AHJs will ask you to upgrade before making an interconnection.

Also, keep in mind that the Code addresses minimum safety requirements only. Once you touch the existing equipment, you own it—certainly as far as the customer is concerned. Every veteran contractor is familiar with this complaint: “Everything was working fine before your crew worked on it.” If you spot a potential reliability issue with the existing equipment, you should either create a budget to fix it, or bring it to the customer’s attention and have that customer sign off on leaving it as is.


Jason Fisher / Solar City / Charlottesville, VA /

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