Interactive Inverter Interconnections
Inside this Article
The 2014 and 2017 editions of the National Electrical Code provide solar companies with more interconnection options than previous Code editions did. In this article, I offer an overview of the Code requirements and allowances for interconnecting parallel power production sources, such as PV or energy storage systems, to premises wiring supplied by a utility or other primary on-site electric power sources. My goal is to help solar company personnel identify the most appropriate point of connection (POC), which is specific to both the system and the site.
To cover the maximum number of interconnection scenarios in as much detail as possible, I have chosen to focus specifically on distributed generation applications, where parallel power production sources interconnect at utilization voltage levels in properties with on-site loads. I assume that readers have a working knowledge of and access to the NEC, which contains many important definitions and references. In the interest of brevity, I italicize on first use those terms that the NEC defines; if you are unfamiliar with any italicized terms in this article, especially those in Figure 1, please refer to Article 100, “Definitions,” or to the NEC index. I provide Code references in square brackets throughout the article, indicating the 2014 or 2017 revision cycle where relevant.
NEC Article 705 details the basic safety requirements for interconnected electric power production sources. Though distributed PV systems are a common parallel power production source, other sources include on-site generators, fuel cells, wind electric systems and some energy storage systems. Regardless of the power source, qualified persons must install these systems [2014-705.6; 2017-705.8] using approved equipment, such as listed interactive inverters certified to UL 1741 [2014-705.4; 2017-705.6].
The first step when planning a safe interconnection is to document relevant PV system equipment ratings. The essential data for Code compliance include utility-interactive inverter output circuit ratings [690.8(A)(3), 705.60(A)(2)] and the associated overcurrent protection device (OCPD) ratings [690.9(B), 705.60(B)]. Where multiple inverters interconnect to a single POC, it is useful to record individual inverter output circuit currents as well as the sum of these currents wherever you combine inverter outputs.
The next step is to assess the configuration and condition of the existing premises wiring, paying special attention to any equipment or locations that provide potential interconnection opportunities. As shown in Figure 2, the Code allows for two basic types of interconnections: supply-side connections [705.12(A)] and load-side connections [2014-705.12(C); 2017-705.12(B)]. Note that the delineation point between supply- or load-side connections is the disconnecting means for the utility-supplied service; this is an important distinction, as feeders rather than services supply some buildings or structures.
As illustrated in Figure 2, multiple potential interconnection opportunities exist on both the load side and the supply side of the service disconnecting means. Generally speaking, cost and complexity increase as the POC moves from left to right. I have generally organized the following scenarios accordingly, from the most common and least complex options to those that are less common and more complex. In most cases, I provide a formula that you can use to evaluate the Code compliance of different interconnection methods using existing equipment. You can easily adapt these formulas to evaluate potential equipment modifications or upgrades, while that is beyond the scope of this article.
Though I focus here on a few key metrics—most notably, supply overcurrent device ratings, panel busbar ratings and feeder conductor sizes—a thorough site survey is a prerequisite for identifying the optimal POC. Ideally, this survey identifies the locations and ratings of the utility transformer, revenue meter, service entrance conductors, main service panel, service disconnecting means, grounding electrode, subpanels, supply breaker ratings, on-site power production sources and even load breaker ratings. In addition to photographing and taking notes on the general as-built conditions, be sure to take pictures of any electrical equipment labels, as these data will invariably prove essential later.
Note that often a manufacturer-applied label on the panelboard identifies the busbar or mains rating for existing equipment. In some cases, however, you may need to find the original equipment documentation to determine this value. If you are unable to document a busbar rating conclusively, the generally accepted practice is to use the rating of the associated OCPD.
The 2014 and 2017 editions of the NEC provide detailed requirements for making load-side connections to busbars in panelboards or to load-side conductors [2014-705.12(D)(2); 2017-705.12(B)(2)]. The additional load-side connection guidelines, compared to those in earlier Code editions, are beneficial for system designers and AHJs. The most significant change, however, is the directive to use 125% of the inverter output circuit current, rather than the interactive inverter breaker rating, for load-side ampacity calculations.
CONNECTIONS TO BUSBARS
All else being equal, the simplest and most cost-effective interactive inverter interconnection is to connect to a panelboard busbar by adding a circuit breaker. In addition to providing a Code-compliant POC, this new breaker also provides overcurrent protection for the inverter output circuit and often serves as the PV or interactive system disconnect. The NEC details five different methods or scenarios for interconnecting an electric power source to a busbar, each of which is potentially useful in a subset of real-world situations. Note that while the following examples assume the use of circuit breakers, the Code also allows for the use of fusible disconnecting means.
Power sources do not exceed busbar rating. Where applicable, this is likely the easiest and most cost-effective POC. As long as the busbar rating is greater than or equal to that of the primary power source (the busbar OCPD rating) plus the sum of the parallel power sources (125% of the inverter output circuit currents), the Code does not limit the locations or number of sources or loads connected to a panelboard busbar [2014-705.12(D)(2)(3)(a); 2017-705.12(B)(2)(3)(a)]. Since any inverter OCPD location is acceptable, the Code does not require a warning label adjacent to a backfed breaker in this scenario.
Though opportunities to use the busbar interconnection method shown in Figure 3 are relatively uncommon, they do exist. For example, a site evaluation might identify a residential panelboard with a 225 A–rated busbar but a 200 A main breaker, or a commercial main distribution panel with a busbar rating higher than its main OCPD. In this type of scenario, you can use Equation 1 to confirm that a proposed interconnection is Code compliant:
Busbar ≥ Supply OCPD + (Inverter Current x 125%) 
120% allowance. This is the busbar interconnection method familiar to most solar professionals. Since 1987, the Code has included some version of “the 120% rule,” which allows primary and parallel power sources to exceed a panelboard’s busbar rating under certain circumstances. This allowance originally applied only in residential applications, where load diversity prevents overload conditions. Eventually, the Code-Making Panel was able to extend the 120% allowance to commercial and industrial applications by requiring that the primary power source (utility) and parallel power sources (interactive inverters) connect to opposite ends of the busbar, as shown in Figure 4.
Whereas earlier Code editions used the inverter OCPD rating in calculations related to the 120% allowance, calculations under NEC 2014 and NEC 2017 are based on 125% of the inverter output circuit current [2014-705.12(D)(3)(b); 2017-705.12(B)(2)(3)(b)]. You can use Equation 2 to confirm that a proposed interconnection complies with the 120% allowance:
Busbar ≥ (Supply OCPD + (Inverter Current x 125%)) ÷ 120% 
Since the physical location of the inverter OCPD prevents any potential overload conditions, the Code requires a warning label to alert someone not to inadvertently move this device in the future:
POWER SOURCE OUTPUT CONNECTION—
DO NOT RELOCATE THIS OVERCURRENT DEVICE.
Limit load and supply OCPDs. This calculation method is unique insofar as it ignores the rating of the overcurrent device protecting the busbar and instead evaluates the total rating of all the applied load and supply OCPDs. In this scenario, a proposed POC is Code compliant as long as the panelboard busbar rating is greater than or equal to the sum of the attached OCPDs, regardless of whether these connect to loads or inverters [2014-705.12(D)(3)(c); 2017-705.12(B)(2)(3)(c)]. Since an overload condition cannot exist in this scenario, the Code does not limit the number or locations of load or inverter breakers, as illustrated in Figure 5. In this scenario, you can use Equation 3 to confirm Code compliance:
Busbar ≥ Load OCPDs + Inverter OCPDs 
This new method of interconnection is particularly advantageous when you are adding a new panelboard to aggregate multiple inverter output circuits, as might be the case on a commercial project deployed with 3-phase string inverters or a residential project deployed with microinverters. Since this method accommodates load breakers, you are free to add breakers to an inverter aggregation panel to supply power to monitoring equipment or equipment servicing receptacles. You could also use this method to connect an interactive system to a lightly loaded subpanel. Note that you must include a warning label to ensure that the installation remains Code compliant in the future:
THIS EQUIPMENT FED BY MULTIPLE SOURCES.
TOTAL RATING OF ALL OVERCURRENT DEVICES
EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE
SHALL NOT EXCEED AMPACITY OF BUSBAR.
Center-fed panels in dwellings. During the 2017 cycle of revisions, the Code-Making Panel introduced a new busbar interconnection method that applies specifically to center-fed panelboards in dwellings. With a center-fed panelboard, the main breaker is located in the middle of the busbar, rather than at the top. This center-fed configuration makes it impossible to locate the utility and inverter supplies at opposite ends of the busbar as required to comply with the standard 120% allowance. Due to the diversity factor that applies to residential loads, the Code-Making Panel determined that it is safe to apply the 120% allowance (see Equation 2, to center-fed panelboards in dwellings, provided that the inverter POC is located at only one end of the busbar [2014-TIA 14-12; 2017-705.12(B)(2)(3)(d)]. In Figure 6, for example, you could connect a parallel power source to either the top or the bottom of the busbar, but not to both ends.
Solar companies that encounter center-fed panelboards will welcome this new interconnection method. Since center-fed panelboards are relatively common in California, it is not uncommon for solar customers there to incur $2,000–$3,000 service upgrades in order for system integrators to interconnect even small residential PV systems. The new 120% allowance for center-fed panelboards in dwellings eliminates these expenses where they are otherwise unnecessary. In August 2016, the National Fire Protection Association issued a rare Tentative Interim Amendment (TIA), 14-12, which retroactively adds the center-fed panel allowance to NEC 2014 as 705.12(D)(2)(3)(e).
It is a good idea to speak to your AHJ prior to making this type of connection under NEC 2014. Though this is an official change to the 2014 Code edition, the revised language will not appear in hard copy of the Code, which could cause some confusion. Code does not specifically require a warning label, but it is advisable to add such a label alongside the inverter breaker to ensure that the installation remains compliant in the future. This warning label might read:
POWER SOURCE OUTPUT CONNECTION—
DO NOT RELOCATE THIS OVERCURRECT DEVICE.
DO NOT ADD SOURCE AT OTHER END OF BUSBAR.
Multiple-ampacity busbars. Panelboards with multiple-ampacity busbars are primarily found in industrial applications and do not fit neatly into any of the previous categories. Since there is no practical limit to as-built conditions, it is necessary to evaluate each situation individually to ensure that a proposed POC is safe. To make a Code-compliant connection to a multiple-ampacity busbar [2014-705.12(D)(3)(d); 2017-705.12(B)(2)(3)(e)], a supervising engineer must evaluate busbar loading and available fault currents.
CONNECTIONS TO CONDUCTORS
Although connections to conductors are less common than connections to busbars, the NEC allows them under certain conditions. This method of interconnection is perhaps most common when a suitably sized feeder is significantly closer to or more accessible from the proposed inverter location than a suitable panelboard is. In such a scenario, connecting to the feeder conductor results in meaningful savings.
When evaluating a conductor’s suitability as a POC, several general rules apply. Where you are making an inverter connection to a feeder or tap, the ampacity of the conductor must be equal to or greater than 125% of the inverter output circuit current [705.60]. Inverter output circuit conductors must be protected in accordance with Article 240 [705.65], and the number and location of OCPDs must provide protection from all sources [705.30]. Any feeder or feeder tap conductor supplying loads must have adequate ampacity to supply the loads [215.2(A)(1)]. Conductor ampacities must account for actual conditions of use, including ambient temperature and conduit fill [310.15]. Note that the formulas in this section will determine the minimum conductor ampacity before the applicable conditions of use.
Provided that the system meets these general criteria, the Code allows for direct connections to feeders or indirect connections via tap conductors [240.2].
Connections to feeders. Solar professionals routinely connect PV systems to the end of a feeder, opposite the primary source OCPD. The Code also allows for a connection to other locations in a feeder, provided that the conductor on the load side of the inverter output is protected [2014-705.12(D)(2)(1); 2017-705.12(B)(2)(1)]. System integrators have two options for protecting this portion of the feeder.
Option 1: Make sure that power sources do not exceed conductor ampacity. The first protection option is based on the logic that the downstream conductor is protected as long as it is rated to carry power from all sources. In other words, the connection is compliant as long as the sum of the primary power source (the main OCPD rating) and the interactive power source (125% of the inverter output circuit current) does not exceed the ampacity of the feeder, specifically between the POC and the loads [2014-705.12(D)(2)(1)(a); 2017-705.12(B)(2)(1)(a)]. Figure 7 illustrates this schematically.
Note that this conductor connection method effectively assumes two different feeder ampacities. The ampacity of feeder A, which is upstream from the POC and protected by the primary supply breaker, needs to be greater than 125% of the inverter output circuit currents. Since there are loads at the other end of the feeder, however, the ampacity of feeder B and any downstream busbars must account for both the primary and the parallel power sources. You can use Equations 4a and 4b to verify Code compliance in this scenario:
Feeder A ≥ Inverter Current x 125% [4a]
Feeder B ≥ Supply OCPD + (Inverter Current x 125%) [4b]
Opportunities to take advantage of this feeder connection option are relatively few and far between, simply because it is uncommon to come across oversized conductors and busbars in the field. Generally speaking, it is cost prohibitive to upgrade the downstream feeder conductor unless its length is short and the downstream panelboard already has an oversized busbar.
Option 2: Add an OCPD on the load side of the feeder. The second, and generally more practical, option uses an overcurrent device to protect the downstream feeder. In this scenario, the POC is compliant so long as the ampacity of the feeder is greater than or equal to the OCPD rating on the load side of the inverter connection [2014-705.12(D)(2)(1)(b); 2017-705.12(B)(2)(1)(b)]. Figure 8 shows a connection with a breaker added to protect the downstream feeder and busbar.
Note that the size of the OCPD on the load side of the inverter POC must also take the downstream loads into account. One way to install an OCPD in the feeder is to add a new panelboard at the POC to enclose the inverter breaker and the load breaker. Alternative methods could use wireway with fused disconnects. Either way, this interconnection method likely involves splicing and extending the feeder with the possible addition of tap conductors, which are subject to unique Code requirements (discussed next). You can use Equations 5a and 5b to ensure that this type of connection to a feeder conductor is Code compliant:
Feeder Ampacity ≥ Inverter Current x 125% [5a]
Load-Side Breaker ≤ Feeder Ampacity [5b]
Connections involving tap conductors. The ability to connect to feeders using tap conductors offers solar professionals additional flexibility when optimizing site-specific interconnections. The Code provides multiple allowances, based on tap length or location, for tapping feeder conductors without overcurrent protection at the tap [240.21(B)]. New language in Article 705 clarifies how these general tap rules apply where inverter output connections use tap conductors. Specifically, the Code requires that you base the OCPD rating used to determine the ampacity of tap conductors per 240.21(B) on the sum of the source OCPD and 125% of the inverter output circuit current [2014-705.12(D)(2)(2); 2017-705.12(B)(2)(2)].
The following examples illustrate how to apply tap conductor rules where you are using taps for downstream loads, inverters or both. These specific examples assume that the tap conductors are not longer than 25 feet and that some portion of the tap conductors is located indoors. Moreover, some general rules apply that merit reviewing. You are allowed to tap feeder conductors but not other tap conductors [240.21(B)]. You are generally not allowed to tap branch circuits [210.19]. You are not allowed to tap inverter output circuits [240.4(E), 705.12(D)(1)]. You must size any conductors serving loads, including taps, to supply the load [Article 220, Part III]. You must provide overcurrent protection for panelboards connected to tap conductors [408.36].
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.
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.
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 / solarcity.com