This article explains proper interconnection techniques to be followed when installing sunMAX.
Table of Contents
- Preliminary Notes
- Interconnection at Main Panel
- Interconnection at a New or Existing Premises Subpanel
- Interconnection at a Feeder Tap
- Supply-Side Interconnections
- Visible Lockable Disconnect (or PV System Disconnect)
- Related Articles
It is assumed in this article that the sunMAX PV system size has been selected to comply with the following NEC rule (Article 705.12(A)): “The sum of the ratings of all overcurrent devices connected to power production sources shall not exceed the rating of the service.” (Nowhere does the NEC define ‘rating of the service”. “Rating of the service” is most usually defined by the size of the Main Disconnect overcurrent protection device (OCPD).) In simplified terms, this means that you can’t interconnect a PV system to a premises if the rated output current of the PV system exceeds 80% of the rating of the premises Main Disconnect. An exception to this would be allowed if the PV system OCPD is a 100% rated device, in which case a PV system having a rated current equal to the rating of the Main Disconnect would be allowed.
For premises services having two to six switches or breakers as the service disconnecting means (as permitted in NEC Article 230.71), the rating of the service can be assumed to be the rating of the busbar in the equipment containing the service disconnecting means.
For residential sunMAX PV systems exceeding 12kWac, the serving electric utility should be consulted on their allowed limit to the size of an interconnected PV system.
At the end of this article is a short discussion of the PV System Disconnect (also called VLD, for Visible Lockable Disconnect). The requirement for a VLD is a utility requirement, and is not universal. The location of the VLD is discussed at the end of this article.
In the “sunMAX Branch Circuit – From End Run to Interconnection” Knowledge Base article, various concepts were introduced for bringing sunMAX PV system AC power from one or multiple microinverter branch circuits to the Point of Interconnection (POI). In this article we expand on the various options for interconnecting the sunMAX system output circuit to the existing premises wiring system in an NEC-compliant manner. NOTE: The requirements discussed in this article are based on the 2014 NEC. Where possible, references to earlier NEC versions will be included.
For purposes of this discussion, a sunMAX PV system output circuit takes one of two forms – a single microinverter branch circuit comprising up to fifteen microinverters, or the output circuit from a Solar Load Center installed to collect the outputs of multiple microinverter branch circuits. (NOTE: In both these cases, using one of the four methods below, the output of the multiple inverter sunMAX system will interconnect to the existing premises wiring through a single dedicated circuit breaker or fused disconnect, as required by NEC Article 705.12(D)(1)).
Before choosing the Solar Load Center location, and/or the final destination of the one or more microinverter branch circuits, the installer should select a Point of Interconnection from these options (see Diagram 1):
- The existing premises Main Panel, where the majority of load branch circuits are supplied. (Main Panel busbar rating governed by NEC Article 705.12(D)(2)(3)).
- An existing or new premises subpanel. (Main Panel and Subpanel busbar ratings governed by NEC Article 705.12(D)(2)(3)).
- A feeder tap point on the load side of the premises Main Disconnect. (Feeder ampacity governed by NEC Article 705.12(D)(2)(1).
- A supply-side (or line-side) interconnection between the premises Main Disconnect and the utility meter. (Permitted by NEC Article 705.12(A).
The choice of POI options depends on aspects of the existing premises infrastructure as they relate to requirements in the NEC. Elements of home run circuit length, system output circuit length, need for additional breakers and or disconnects, and adaptability of existing equipment for interconnection all have to be considered to optimize the POI location, and minimize interconnection labor and costs. In most cases, only one option is practical, because of current rating of existing equipment/conductors or because of the impracticality or impossibility of making a supply-side connection in a NEC-compliant and UL compliant manner. This article will not attempt a cost analysis of the various POI options that might be available, but instead will present examples of each of the four POI options.
Interconnection at Main Panel
This is a Load Side connection as described by the NEC in Article 705.12(D), meaning the interconnection is made on the load side of the premises Main Disconnect.
A Main Panel interconnection (except when made at open supply lugs on the panel main breaker, if available*) is a busbar connection, governed by the requirements of NEC Article 705.12(D)(2)(3). There are three scenarios to consider, each with a different limit on the rating of the interconnection breaker added to the busbar.
*SPECIAL NOTE: if the Main Panel includes a Main Breaker that has double lugs on the supply side, and open lugs are available, the solar output circuit can be installed in the open lugs. The solar output circuit must comply with the NEC requirements for either a feeder interconnection or a supply side interconnection, discussed elsewhere in this article.
Returning to the three scenarios for Main Panel interconnection (see Diagram 2):
The interconnection breaker is located not at the opposite end from the busbar’s utility feed. In this case the NEC guards against solar current and utility current adding to a value greater than the busbar rating. “The sum of 125% of the inverters’ output current and the rating of the overcurrent device (OCPD) protecting the busbar [from utility current] shall not exceed the rating of the busbar.”
Example: A single MI branch circuit of 15 microinverters, having an output current rating of 15.75A.
125% of 15.75 is 19.7A
For this branch circuit to connect to the Main Panel through a 20A circuit breaker not located at the opposite end of the busbar from the utility feed, the busbar rating must exceed the rating of the busbar OCPD by at least 20 amps. A 100A busbar protected by a 100A Main Breaker does not comply. A 125A busbar protected by a 100A OCPD does comply.
To connect to a 100A busbar in this example, a two-pole breaker space has to be created at the bottom (opposite end) of the busbar by moving existing breakers (see Scenario b, below). A second option, reducing the size of the utility OCPD to provide the 20A overhead required, is not available for a 100A busbar, because the NEC mandates a minimum 100A rating for residential services. For a 150A busbar protected by a 150A OCPD, the OCPD can be reduced to 125A, and the 20A MI branch circuit breaker can now be installed anywhere on the busbar.
Note: the AHJ will probably require a panel schedule or historical load data to show that the reduced rating OCPD is sufficient for the connected loads.
The interconnection breaker is located at the opposite end of the busbar from the utility feed. In this case the NEC recognizes that supply currents do not cross on a busbar (due to Kirchoff’s Law), but still guards against overheating at breaker tabs by limiting total busbar current at any point to 120% of the busbar rating. “Where two sources, one a utility and the other an inverter are located at opposite ends of a busbar that contains loads, the sum of 125% of the inverters’ output circuit current and the rating of the OCPD protecting the busbar shall not exceed 120% of the ampacity of the busbar.”
Using the previous example’s 19.7A of (adjusted by 125%) inverter current, a 20A two-pole breaker can be located at the bottom of the 100A busbar that is protected by a 100A utility OCPD, and still comply with NEC requirements. If the Main Panel has an empty two-pole space at the bottom of the busbar, no more work is required. If the Main Panel has an empty two-pole space in the middle of the busbar, it may be possible to move the load branch circuit breaker now at the bottom of the busbar into that empty space, creating space at the bottom of the busbar for the inverter breaker. If no space exists, either a new subpanel will have to be installed to receive existing load breakers and provide busbar space, or a supply-side connection or feeder connection (if possible) will be required.
Scenario C (applicable only under the 2014 NEC)
If the sum of the ratings of all load and supply breakers (the PV system breaker is a supply breaker) on a busbar (excluding the OCPD protecting the busbar from utility current) does not exceed the rating of the busbar, there is no limit to the rating of the PV system interconnect breaker connected to the busbar, nor is there any restriction on the interconnect breaker location. In this scenario the rating of the OCPD protecting the busbar from utility current cannot exceed the rating of the busbar. The NEC recognizes that current on the busbar in this scenario can never exceed the busbar rating at any physical point.
While this scenario is unlikely to occur on a Main Panel, where the sum of load breakers often approaches 250% of the busbar rating (due to load factor considerations beyond the scope of this article), it is not impossible. Furthermore, this scenario is quite likely to occur both on existing premises load subpanels, and on new load subpanels installed specifically to provide space for a PV system interconnect breaker.
Note: each 120V busbar in a split phase residential panel has to be considered separately in this scenario. The sum of load/supply breaker ratings connected to each busbar must not exceed the busbar rating. Single pole breakers will count against only one busbar, while double pole breakers will count against both busbars.
Example: A 200A Main Panel busbar with 200A OCPD contains load breakers (including existing subpanel feeder breakers, if any) summing to 100A on each of the two 120V busbars. In this case a 100A PV system interconnect breaker can be installed anywhere on the busbar. In no case will current on this busbar exceed 200A under any conditions of load and/or PV supply.
Note: caution must be exercised throughout the lifetime of the PV system that additional load breakers are not added to this Main Panel resulting in the sum of load breakers and the PV interconnect breaker exceeding 200A, unless the conditions of Scenario A or Scenario B are met.
Interconnection at a New or Existing Premises Subpanel
Scenarios A, B, and C above apply to the busbar rating of a subpanel exactly as they do to a Main Panel busbar.
Also, if a new subpanel is installed to receive some existing load breakers and make space (either in the Main Panel or subpanel) for a PV system interconnect breaker, the rules in Scenarios a and b apply not only to an inverter breaker located in the subpanel, but also to the location of subpanel feeder breaker in the Main Panel, if the subpanel contains an inverter breaker. As for the 100% or 120% limit for inverter current on the Main Panel busbar coming from a subpanel with inverter breaker, it is still the 125% of the inverters’ output current that is used in the calculation, not the rating of the subpanel feeder breaker.
Example: A PV system with 39.4A adjusted output current is installed in a new or existing 200A subpanel using a 40A breaker, at a subpanel busbar location in compliance with Scenario a or Scenario b, above. The subpanel itself is fed by a feeder breaker installed in the Main Panel, in a breaker space created by moving one or more existing loads to the subpanel.
According to Scenario A, above, if the subpanel feeder breaker is not located at the opposite end of the Main Panel breaker from the utility feed, the Main Panel busbar must be rated no lower than the sum of (39.4A plus the rating of the utility OCPD.)
According to Scenario B, above, if the subpanel feeder breaker is located at the opposite end of the Main Panel breaker from the utility feed, the Main Panel busbar must be rated no less than 83.3% (the inverse of 120%) of the sum of (39.4A plus the rating of the utility OCPD.)
Interconnection at a Feeder Tap
An existing residential electrical wiring system may include zero, one, or more feeders. A PV system NEC-compliant interconnection can be made to any of the feeders on a premises.
An electrical wiring system using an “all-in-one meter” enclosure containing the utility meter, the Main Panel, and the Main Disconnect (if present), includes zero feeders unless there is also a load subpanel on the premises.
An electrical wiring system with a meter/Main Disconnect enclosure and a physically separate Main Panel includes at least one feeder; the one between the Main Disconnect and the Main Panel. There may be other feeders if other load subpanels exist.
An electrical wiring system with one or more load subpanels fed from the Main Panel includes a feeder for each load subpanel, in addition to the main feeder, if any.
If the feeder interconnection is made at the opposite end of the feeder from the utility OCPD, the feeder ampacity must be not less than 125% of the inverters’ output circuit rated current. (This rule holds irrespective of the presence or not of an OCPD at the end of the feeder opposite the utility OCPD. The rules governing busbar rating on the load side of the feeder interconnection still apply to the busbar). An example of this interconnection point is labeled “A” below.
If the feeder interconnection is not made at the opposite end of the feeder from the utility OCPD, there are two ways to ensure an NEC-compliant feeder interconnection, as outlined in NEC Article 705.12(D)(2)(1).
Method A: The portion of the feeder on the load side of the inverter interconnection must have an ampacity not less than the sum of (125% of the inverters’ output circuit rated current plus the rating of the utility source OCPD). See Diagram 5a.
Method B: An OCPD on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder. See Diagram 5b.
Method A applies to feeders that originate from a utility source OCPD and do not terminate on an OCPD. In this case the two supply currents – utility OCPD-limited supply current and adjusted inverter supply current – must sum to a current not greater than the feeder ampacity.
Method B, if not strictly interpreted, applies to feeders (including taps) that terminate on an OCPD. In this case (governed by tap rules found in NEC Article 240.21) the current in the portion of the feeder on the load side of the inverter connection is limited by the OCPD where the feeder terminates.
Note: there are two possible interpretations of Method B. The strict interpretation requires the OCPD device protecting the inverter load-side portion of the feeder to be located at the inverter connection point. The tap-based interpretation of Method B allows the inverter load-side portion of the feeder to terminate on an OCPD under the rules found in NEC Article 240.21. To avoid rejection of a Method B installation by the AHJ at inspection time, the location of the Method B OCPD should be approved by the AHJ prior to construction.
Note: if the Method B inverter load-side feeder portion OCPD is not located at the inverter connection, no tap can pre-exist on the portion of the feeder on the load side of the inverter connection, and no tap can be installed on this portion of the feeder in future, according to NEC Article 240.21(B) (paraphrasing – it is prohibited to tap a tap.). This means that the existing feeder, before installation of the PV system, can directly feed only one panel, and the panel being fed must have a main breaker.
Note: under all circumstances of a supply-side interconnection, the equipment being connected to is live until de-energized by the utility. DO NOT ATTEMPT TO INTERCONNECT TO A LIVE CONDUCTOR OR BUSBAR.
A supply-side interconnection (also called a line-side interconnection) is a PV system interconnection made between the premises Main Disconnect (or disconnecting means) and the utility meter, almost always through a fused disconnect rated for 125% of the PV system rated output current. In a supply-side connection the premises Main Disconnect OCPD limits the sum of utility current and PV current onto the existing load-side feeder or busbar to the rating of the OCPD.
When a supply-side interconnection is made through a fused PV system disconnect switch, the PV system disconnect switch constitutes a Service Disconnect, and must comply with Section VI of NEC Article 230, as well as with the provisions of NEC Article 250.24(C).
Supply-side interconnections are made complicated by two predominant features of residential electrical system equipment.
- Existing residential electrical Services seldom include accessible portions of feeder cable between the utility meter and the line lugs of the Main Disconnect.
- Physical space for a tapping or splicing device is usually found only in the utility’s portion of the Meter/Main enclosure, which is not legally accessible to the customer.
In addition, because the equipment being interconnected to can only be de-energized by the utility (which is mandatory for a safe work environment during a supply-side interconnection), the de-energization, re-energization, and intervening service inspection by the AHJ complicate the work flow and timeline associated with a supply side interconnection.
One special situation that can exist at a residential Service is the “six disconnect” definition of a Main Disconnect. The NEC allows a Service with no more than six load disconnects in the Service to operate without a Main Disconnect device, as governed by NEC Article 230.71. In this case, the load-side of the service disconnecting means is defined as the load side of any or all of the six or fewer load disconnects. Therefore, any point on the supply side of the six or fewer load disconnects meets the definition of the supply side of the service disconnecting means for the purposes of compliance with NEC Aritcle 705.12(A). In a residential Service Panel having six or fewer breakers as the service disconnecting means, interconnection to the panel busbar would constitute a supply-side connection. If there is breaker space on the busbar, the interconnection can be made through a breaker sized at 125% of the PV system rated output current, under the following restrictions:
- An AHJ will probably require a separate fused disconnect on the PV side of this interconnection breaker in order to specifically delineate the Service Disconnect for what is now an additional service allowed by NEC Article 230.2(A)(5).
- The sum of [125% of the PV system rated output current plus the ratings of the six or fewer existing utility service disconnecting means] must be no greater than the rating of the Service Panel busbar being connected to, unless the PV interconnection is made at the end of the busbar opposite the utility supply. In this case, the sum of [ ] must be no greater than 120% of the rating of the Service Panel busbar being connected to.
If breaker space does not exist on the Service Panel busbar, a tapped connection through newly installed drill holes on the busbar may be possible under the following conditions:
3. The AHJ and/or utility may require certification from the panel manufacturer that this modification of the busbar does not violate the UL listing of the service equipment.
4. The AHJ and/or utility may require the work to be performed by an entity certified for field modifications by the panel manufacturer.
One other pre-existing situation may benefit the extremely fortunate customer/installer who encounters an existing Main Disconnect device that has open, parallel lugs on the supply side of the utility Main Disconnect. After de-energization of the service by the utility, the PV system output conductors can be terminated in the empty Main Disconnect supply lugs, after passing through a closely located fused PV system disconnect switch.
Finally, one additional type of supply-side interconnection option may exist for the customer/installer. Cooper Eaton offers a PV Meter Adapter (PVMA) for supply-side taps. Information and a product data sheet are available here:
The PVMA is a modified residential meter socket that replaces the customer’s meter socket. The PVMA comprises a meter socket ring that includes screw lugs in parallel with the load side of the meter socket. Once the PVMA is installed, a PV system can be connected to the parallel lugs through a fused PV system disconnect switch.
A similar product containing an integral PV system interconnect breaker can be found here: connectder.com
Note: not all utilities accept modifications to their meter equipment. Contact your utility before purchasing or installing these products.
For San Diego Gas & Electric service area customers, San Diego Gas & Electric offers on their website a similar device called a “renewable meter adaptor” (RMA).
Visible Lockable Disconnect (or PV System Disconnect)
Some utilities require customers with on-site customer generation to install a generation facility disconnect switch for the utility’s use. For residential PV systems, this is commonly called a VLD, for visible blade, lockable disconnect switch. Some utilities do not require a VLD, depending on system size and existing meter configuration. These utilities will disconnect the customer generator by removing the customer revenue meter.
Supply side interconnections and load side interconnections to feeders nearly always require a fused disconnect to comply with the NEC requirements for tap conductors, which the PV system output conductors are in the vast majority (if not all) of these interconnections. In these cases, the fused disconnect can act as the VLD, if located in compliance with utility requirements (Utilities may have a maximum allowed distance between the meter and VLD, and will always require the VLD to be accessible to utility personnel.)
Load side interconnections to busbars do not usually require a disconnect switch for NEC compliance. If utility requirements dictate the addition of a VLD, the VLD does not have to be fused, unless existing premises available utility fault current exceeds the specification of an available unfused disconnect. (The value of available fault current can be obtained by contacting the customer’s utility service account representative, or the distribution engineering department of the utility).
For load side interconnections, the VLD can be installed in the PV system output circuit between the PV system and the busbar being interconnected to.
If the utility’s required location for the VLD, relative to the location of the PV system, forces an impractically long PV system output circuit, the VLD can alternately disconnect the subpanel being interconnected to, or the premises main panel. While these latter two options seem impractical and intrusive, it should be noted that the VLD is seldom, if ever, operated by the utility. It is, however, always operated at the time of system inspection/clearance by the utility.
For rules governing VLD requirements, including whether a VLD is required, and where it can be located, contact the interconnection or customer generation department of your utility. This information may also be contained on the Interconnect Application required by every utility, or in a technical requirements document covering customer generation (to the distribution system) possibly found on the utility’s website.