Inspectors will see several different electrical connections to the utility grid in their daily activities of inspecting photovoltaic (PV) systems. This article will deal with the more common single-phase utility-interactive systems only and will not cover standalone systems or battery-backed-up systems. Those systems will appear in subsequent articles. The National Electrical Code (NEC) covers two distinct methods of connecting these systems to the grid. One is a connection before the main utility service disconnect for the facility, known as a supply-side connection, and the other is a connection after the main utility service disconnect, known as a load-side connection. In addition to the NEC requirements, there are other considerations that a typical installer must address in selecting which of these two connections to make. References below will be made to the main content in paragraphs in the 2023 NEC. The text of the NEC should be referenced to see all the Exceptions and Informational Notes.
These considerations include items like the accessibility of the existing electrical wiring system, the cost of making a connection to the existing system, the requirement for replacement of existing hardware, the relative size of the utility service versus the rated output of the PV system, and in some cases utility restrictions on where the connection can be made.
In some commercial and residential installations, the electrical wiring is not readily accessible where a connection might be desired. Removing structural elements and interior finished surfaces may not be acceptable. In many cases, particularly residential systems, the size of the desired PV system exceeds the rating of the existing service and main load center, which may dictate the replacement of service equipment and/or service drops to the utility transformer. This size restriction could apply to either supply-side or load-side connections.
Supply-Side Connections
Source Connections to a Service. Section 705.11 has been significantly revised and expanded in content in the 2020 and 2023 editions of the NEC. Section 705.11(A) allows three methods to connect an electric power production source to a service. They include adding a new service in accordance with Section 230.2(A), connecting to the supply side of the service disconnecting means in accordance with Section 230.82(6), and adding an additional set of service entrance conductors per 230.40, Exception No. 5. These new service connections must meet a considerable number of requirements that are found in 705.11(B) through (F).
Conductors. Section 705.11(B)(1) requires that the service conductors connected to the power production source service disconnecting means shall not be rated less than the sum of the power production source maximum circuit current established by Section 705.28(A). The conductors connected to the power production source output disconnecting means shall be sized in accordance with 705.28 and not be smaller than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum [705.11(B)(2)]. The ampacity of any other service conductors to which the power production sources are connected shall not be less than the conductor sizes required for the power source output conductors stated above.
Service Disconnecting Means [705.11(D)]. Disconnecting means in accordance with Parts VI through VII of Article 230 shall be provided to disconnect all underground conductors of a power production source from the conductors of other systems.
Bonding and Grounding [705.11(E)]. All metal enclosures, metal wiring methods, and metal parts associated with the service connected to a power production source should be bonded (and subsequently grounded) in accordance with Parts II through V and VIII of Article 250.
Overcurrent Protection [705.11(F)]. Part VII of Article 230 shall be used to determine the overcurrent protection for the power production source service conductors. Section 230.95 shall be used to determine the requirement for ground fault protection of equipment based on the rating of the overcurrent protection device in the power production source service disconnecting means.
A Special Note. New to the 2020 and 2023 editions of the NEC is the requirement in Section 230.85 for an Emergency Disconnect on one- and two-family dwellings. This disconnect may be installed in one of three outdoor locations. It can be marked as either a service disconnect or not a service disconnect but will have to be marked as an emergency disconnect in all locations. Hopefully, this emergency disconnect will be located on the load side of the service meter, be marked as the service disconnect, and therefore allow for a normal supply-side connection of the power production source. This would be a normal utility service electrical connection where the main service disconnect is located outside the building in a readily accessible location.
Unfortunately, the NEC may allow, without specifically stating so, that this emergency disconnect, marked as the service disconnect, be located on the supply side of the meter and, therefore, make it impossible to correctly place a power production source (the PV system output) supply-side connection to the service that will allow the service meter to measure the included output of the power production source. Coordination between the PV installer, the dwelling electrician, and the AHJ may be necessary to rectify this situation.
It should also be noted that if any other power source emergency disconnect or isolation disconnect required by this Code is not located with the dwelling emergency disconnect, signage shall be installed at the dwelling emergency disconnect location, showing the location of all other power source disconnects.
Author’s Note: Code Making Panel 4 has steadfastly refused over the years and editions of the Code to allow the output conductors of the PV interactive inverter, or production power source, to be called a “service.” It is interesting to note how often in Section 705.11(B) Conductors, the word “service” is associated with these power source output conductors and their disconnects. In my mind, the use of the word “service” in this context confuses the issue.
Load-Side Connections
Low-Cost Option. The lowest cost connection is generally considered to be a load-side connection using a single circuit breaker in the main load center for a residence or commercial building where there is no modification of any equipment required. However, Section 705.12, Load-Side Source Connections, imposes several requirements on such a connection which may make this simple option not possible.
For example, the sum of 125% of the power source output circuit current and the rating of the overcurrent device protecting the busbar in the load center shall not exceed the busbar and ampere rating [705.12(B)(1)]. However, if two sources, one a primary power source and the other power source (PV), are located opposite ends of a busbar that contains loads, the sum of 125% of the power source output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120% with the busbar ampere rating [705.12(B)(2)]. With a 100-amp load center, the maximum back-fed PV current that could be connected would be 16 A (16×1.25 = 20 where the 20A is 120% of 100A minus the 100A main breaker). For a 200-amp load center, the maximum back-fed PV current is 32 A (32×1.25 = 40 where the 40A is 120% of 200A minus the 200 A main breaker).
Similarly, this 120% limitation (known as the 120% rule) applies to any busbar located throughout the system that is subjected to currents from the utility and a second source. The system must be examined from any remotely located load center (busbar) throughout the system through any other load center subjected to PV source currents and currents from the utility.
One change, several editions ago in the NEC requirements, is that in these intermediate busbars and feeder calculations, only 125% of the rated PV output current must be used. Previously, the rating of each circuit breaker handling that current had to be used in those calculations.
Busbar Calculations. Additional requirements are established by 705.12(B)(3), where the sum of all overcurrent devices on panel boards for both loads and sources, excluding the rating of the main overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the main overcurrent device protecting the busbar may not exceed the rating of the busbar. Special signage is required when this option is used. Section 705.12(B)(4) establishes requirements for center-fed panelboards. There are several requirements that are related to the configuration of the center for the panelboard and the location of the source connections, and these requirements should be studied carefully to ensure that the requirements are met.
The connection may be made at either end of the center-fed panel board on dwellings. The 120% rule shall be applied where the sum of 125% of the power production output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120% of the busbar rating.
Where busbars are supplied by feedthrough lugs or connected to feedthrough conductors, the feedthrough conductor shall be sized in accordance with 705.12(A). Where an overcurrent device is installed at either end of the feedthrough conductors, panel board busbars on either side of the feedthrough conductor shall be permitted to be sized per 705.12(B)(1) through (B)(3).
Feeder calculations. Where a PV power source connection is anywhere in the middle of a feeder (other than at the ends) rather than the opposite end from the primary power source (utility), the requirements of section 705.12(A) apply. This requirement says that the feeder ampacity must be increased to the sum of the rating of the primary source overcurrent device protecting the feeder plus 125% of the power source output circuit current or an overcurrent device rated at the feeder ampacity be installed on the feeder on the load side at the power source point of connection to the feeder. It should be noted that the NEC does not directly address the situation when the power source connection is at the opposite end of the feeder from the main source input. In this situation, there is no place on the feeder where the currents can add together since they oppose each other throughout the feeder, and therefore, no ampacity adjustments of the feeder or insertion of an overcurrent device are required. Section 705.12(A)(3) establishes additional requirements for feeders with existing load taps.
Other Considerations
Section 705.20, Source Disconnecting Means, establishes the requirements for the disconnecting means used to disconnect the ungrounded source output conductors from all other conductors in the system. These are clearly written and need no interpretation but are detailed and should be reviewed.
Section 705.25, Wiring Methods, establishes the requirements for the various wiring types for the source circuit output conductors. Flexible cords and cables can be used to connect moving parts of power production equipment or where the equipment is required for ready removal for maintenance and repair, and they should be listed as DG cables or other cables suitable for extra hard use and shall be water-resistant. If exposed to sunlight, they shall also be marked sunlight resistant. Flexible, fine-stranded cable shall be terminated only with terminals, lugs devices, or connections per 110.14(A).
Section 705.28, Circuit Sizing and Current, states multiple requirements for these items. The maximum output current for power production equipment shall be the continuous output current rating. If an energy management system (EMS) is involved, the set point of that energy management system shall be considered in the calculation [705.28(A)]. Conductor ampacity is the larger of the three calculations [705.28(B)].
- The maximum current in 705.28(A) multiplied by 125% without adjustment or correction factors. There are three exceptions to this particular requirement, and they should be read.
- The maximum current in 705.28(A) after the application of adjustment and correction factors in accordance with 310.14.
- If the conductor is smaller than a feeder that it is connected to, the ampacity is calculated based on the overcurrent device protecting the feeder.
Section 705.30, Overcurrent Protection, deals with overcurrent protective devices used to protect circuits in the output of a power source. Power source output conductors and equipment shall be protected with overcurrent protection from all electrical sources [705.30(A)]. Overcurrent device ratings shall be sized to carry not less than 125% of the maximum current as calculated in 705.28(A). See 705.30 (B). Equipment containing overcurrent devices supplied from interconnected power sources shall be marked to indicate the presence of all sources [705.30(C)]. Fuse disconnects, circuit breakers marked not marked line and load, and circuit breakers listed for backfeeding shall be deemed suitable for the backfeed application [705.30(D)].
Section 705.40, Loss Primary Source, requires that the output of an interactive electrical power production source shall automatically disconnect from all underground conductors of the primary source when one or more of the phases of the primary source to which it is connected opens. The production source shall not reconnect to the primary source until all phases of the primary source are present and stable. Interactive power production equipment shall be permitted to operate in island mode to supply loads that have been disconnected from the electric utility or other electric power production or distribution network.
Summary
Connecting a utility-interactive PV power system (power production source) can be accomplished on either the supply side or the load side of the facility’s main service disconnect. Several considerations drive the decision on which of these two connections to use. The NEC provides significant details for both types of connections, and there are numerous possibilities for combinations of the various methods that are not specifically stated in the Code for making these connections. It behooves all dealing with these systems, both AHJs and installers, to thoroughly review the requirements in 705.11 and 705.12 to see all the requirements, the exceptions, and any informational notes.
