Understanding Interconnected Electric Power Production Sources
This article provides an overview of ‘point of connection’ options for electric power production sources in parallel with primary sources of electricity, such as a utility. It is essential to understand how the 2023 National Electrical Code (NEC) impacts each interconnection option. To familiarize yourself with the basics of the interconnection process and the differences between a load-side and line-side tap, see our previous article here.
NEC 705.11 Supply-Side Connections
As reviewed in the article linked above, a line-, or supply-, side connection commonly refers to systems where the inverter connects to service wires on the 'line' or utility side of the circuit breaker. Requiring approval from a licensed electrician, line-side connections are selected when the main electrical panel is unsuitable for inverter connections, usually due to sizing. A line-side connection thus bypasses the main breaker to tap into the conductors leading out of the service panel and back to the utility grid.
Generally, it is best practice to interconnect on the load side if amperage allows. However, when generation amperage is high compared to service amperage, load-side connections become trickier. Larger-sized systems have become more common as solar prices have plummeted, so installers must maintain a current understanding of supply-side connections to service conductors.
So, how does this connection take place?
We have outlined that a supply side tap is physically connected to a conductor or busbar. The 2023 NEC zeroes in on the details of supply side connection options. Section 705.11 (C) of the code offers guidance on how to install this kind of connection and where to do so within the following guidelines:
- Splices or taps
- 705.11 (C) (1) states that service-entrance spliced and tapped conductors, such as power distribution blocks and pressure connectors, can be utilized within proper code restrictions. For example, these devices must be labeled as “suitable for use on the line side of the service equipment” to legally abide by the NEC.
- Existing equipment
- As long as any modifications to existing equipment are made according to the manufacturer’s instructions, and those modifications have been thoroughly inspected, existing equipment that has historically been NEC-approved may be in continued use for supply-side interconnection.
- Utility-controlled equipment
- 705.11 (C) indicates that meter socket enclosures and other equipment ‘under the exclusive control of the electric utility’ require approval from the electric utility itself, shifting responsibility for equipment-specific requirements towards utilities.
Additionally, section 230.82 encompasses ‘Equipment Connected to the Supply Side of Service Disconnect’. Here, the NEC explicitly lists the following equipment appropriate for supply-side connections:
- Cable limiters
- Meter sockets rated below 1000V
- Meter disconnect switches capable of interrupting the load served
- Instrument transformers
- Impedance shunts
- Load management devices
The section outlines several other unique disconnect situations that would be suitable for line-side connections. This is assuming that all necessary equipment shall be ‘legibly field marked on its exterior in a manner suitable for the environment’.
What are the limitations surrounding service size and conductors?
The ‘six breaker rule’ is a widely understood standard established by the 2017 NEC stating that a main disconnect must contain a maximum of six ‘throws’ to shut off all power to a system. Assessing the service size limitations of a supply-side connection requires deliberate language. Linguistic changes from the 2020 NEC established specific guidance regarding how supply-side connections fit into the six-breaker rule. As per the 2023 NEC 705.11 (A), adding a ‘power production source service disconnecting means’ does not inherently count as one of the six service disconnects allowed in 230.71 (B). The section outlines that supply-side connections serve as an exception to this rule.
In terms of conductor sizing for overcurrent protection, section 705.11 (B) guides our understanding that conductors between the interconnection point and the first overcurrent protection device must be sized to at least 6 AWG copper / 4 AWG aluminum. Overcurrent protection guidance further explains that these devices must be readily accessible either outside the building or where power sources enter the building. In homes, overcurrent protection must fall within 10 feet of the interconnection point; in a non-dwelling (commercial building), the limit is 16.5 feet. Overcurrent devices should be calculated based on 125% of the maximum amperage.
NEC 705.12 Load-Side Source Connections
What about the majority of residential installations in which load-side taps are the more appropriate and often the only option? Load-side connections obviously must take into account the 120% rule. But how does the 2023 NEC approach this requirement?
The 2005 & 2008 NEC sections 690.64(B)(2) offered the 120% rule and no other interconnection options for load-side connections. This is the way solar professionals have traditionally understood load-side tap options. Both code editions claimed that in series-connected panelboard systems, calculations for busbars and conductors were to be based on the rating of the first overcurrent device coming from the utility-interactive inverter output. In 2011, the 120% rule was rewritten to remove the inclusion of "all panelboards connected in series". Although the 2011 NEC removed this statement from the code, many officials continued enforcing the provision in ensuing years.
Regardless, the 2011 NEC offered no alternate point of connection rules. In 2014, point of connection requirements were expanded to section 705.12(D)(2)(3), which stated for the first time that "one of the following methods" shall be used. The 120% rule was listed as one of four load-side connection options, which grew to five in 2017, and six in 2020. The full list of load-side connection methods as of 2023 in section 705.12(B) is below:
- (1) Inverter (power source) x 125% + busbar does not exceed the rating of the busbar. This is not a commonly used method.
- (2) 120% Rule - Explanation & Math.
- (3) Sum of all Breakers Rule - All breakers on the busbar cannot exceed the rating of the busbar; the main breaker does not count in this calculation.
- (4) 120% Rule can be used with “center fed” panels if the connection is made at either end, but not both.
- (5) As prescribed or designed under a licensed engineer's supervision. Requires fault-current & load calculations.
- (6) Using feed through lugs. Sizing conductors as in 705.12 (B)(1) Load Side Tap & 705.12 (B)(3) 1, 2, & 3 will be used.
In 2020, 705.12 became labeled as ‘Load-Side Source Connections’. Although there was significant verbiage added to address PCS (Power Control Systems), the basic structure of the code was still intact.
Regardless, confusion still exists around the question of applying point of connection rules at two separate places. In some scenarios, a solar project may pursue a costly main panel upgrade to meet two separate load-side connection rules, despite the updated code indicating that only one rule need be applied. Specifically, some projects may attempt to connect via breaker in an existing sub panel which would pass either the ‘120%’ or ‘sum of all breakers’ rule. In this type of scenario, the protected busbar is at the point of connection, or the place where the PV system connects to the existing electrical system. Additional application of the code at the home’s main panel many times would require an MPU.
So, if the code writers had intended the series application, why would such a clear statement have been removed in 2011?
In another example scenario, if specific loads were relocated to a new sub-panel at which the inverter breaker was landed, those loads 'when in operation' would mitigate the current from the inverter. This would prevent full amperage from going back to the main panel’s bus, and an AC breaker and several 20A branch circuits would be relocated. When sub-panel loads are not in use, the current flow from the inverter back through the bus of the sub-panel would not be at full load either as loads will have been removed from the main panel and relocated to the sub. This can be verified by a load calculation. Essentially, unless the main panel is already overloaded, there would be no issue.
We’ve recently seen improvements in the clarity of these rules as compared to previous NEC revisions. The code introduced Power Control Systems (PCS) as options for overload protection. PCS limits the amount of current flowing through the busbar meaning that the current will not surpass the conductor or busbar rating. Using PCS, installers can design systems that do not have to consider the 120% rule. As the NEC continues to adapt to PCS and energy management updates, restrictions around load-side source connections will further develop.
The current code also implements dedicated overcurrent and disconnect requirements that specify how to determine feeder or bus ampacity with these devices. This rating is based on the inverter output circuit, not the OCPD in the inverter.
Alternative Code Compliance Methods
Some alternative methods that meet code compliance requirements are slightly less common. The ‘Hawaiian tie-in method’ is considered a sound alternative for supply-side connections in which one makes a connection to the load-side feeder from the service disconnect. Note that this method requires inspection and labeling to fully meet appropriate standards.
Another alternate method for interconnection consists of a sub-panel addition or replacement. Installers have commented on our PV Interconnection: Load Side VS Line Side article that they have successfully added a sub-panel and used it as the interconnection point of the solar breaker. In all scenarios, installers must check this strategy with local codes and ensure that existing feeders and lugs have the appropriate capacity for the current.
Ensuring your solar installation’s interconnection is safe, legal, and efficient can be nuanced, but it is crucial to align with your local AHJ to ensure the interconnection process goes as smoothly as possible. You can refer to our checklist of questions to ask while navigating the permitting and interconnection process of your projects. Our Greentech Renewables design services can also support you at any stage of the interconnection process, whether you are an entry-level installer or a seasoned professional. Connect with our design team today to get the support you need from our trusted engineers and designers.