A supply side connection can be made by splicing into existing utility feeder conductors, tapping into bus bars on the utility side of the existing service disconnect, or double-lugging terminals (connecting a second set of conductors) either on the customer (load) side of the utility meter or the main AC disconnect. However, some of these methods can be difficult to accomplish in a code-compliant and practical manner—many residential and commercial load centers do not have means for connecting on the supply side without violating their listing requirements For example, bus bars often are not allowed to have additional holes tapped into them, and frequently connection points are not rated for more than one conductor. It is recommended to contact the manufacturer of the existing equipment before proceeding, as additional equipment may be available or required, such as connectors that allow two conductors to be landed on each pole of a meter base.
Insulated piercing connectors allow a new conductor to be “spliced” to an existing conductor without having to cut or disconnect the existing conductor. While the temptation is to install them without shutting off utility power at the site, NFPA 70E is specific that working on energized conductors is allowed only in very limited circumstances.
More common in commercial applications is cutting into conduit or raceways between the meter and the main AC service disconnect to install an additional junction box inline. The box provides an access point where the service conductors can be cut to use Polaris-style insulated connectors for making connections. Piercing or Polaris-style connectors can also be used in residential and commercial applications if the conductors are accessible in the main AC service panel. Be sure to verify that this will not violate the listing of the equipment (if the conductors are insulated bus bars then this likely will not be allowed) as they can’t be tapped and Polaris connectors cannot be used.
The requirements of NEC Article 230, “Services,” must be followed. A service-rated fused disconnect—the minimum size is 60 amps per Section 230.79(D)—is typically installed for supply side connections (using breakers can result in the panel needing to be oversized, for reasons that will be discussed in part 2 on load-side connections). The load side of the disconnect is connected to the PV system, and the line side to the utility. If the supply side connection is occurring between the main disconnect and the meter, then the new disconnect will count towards the maximum of six switches or breakers allowed per Section 230.71. However, Section 230.2(A) allows for additional services for parallel power production systems—the new meter and disconnecting means for a buy-all sell-all type interconnection should count as a separate service, but this will depend on the interpretation of the AHJ.
Section 230.91 dictates overcurrent protection device (OCPD) requirements, stating that the OCPD must either be integrated with or immediately adjacent to the disconnecting means. The current interrupting capability (AIC, not the “size”) of the fuses in the new disconnect must be sufficient to comply with Sections 110.9 and 110.10, typically meaning it must at least match the AIC rating of the existing AC service disconnect.
Sections 230.23(B) and 230.31(B) require a minimum conductor size of 8 AWG copper or 6 AWG aluminum between the point of connection and the fused disconnect. Remember that these conductors are exposed to the full current potential of the grid—keeping them as short as possible, protecting them from physical damage, and oversizing them is recommended. However, be aware of the limitations of the terminals in the new, fused disconnect—a maximum size of 2 AWG is common for 60-amp rated switches.
Especially on residential systems, the overcurrent protection device size for the inverter output circuit required by Section 690.8 will be less than 60 amps, The rating of the fused disconnect must still be 60 amps, but fuse holders that allow smaller fuses to be inserted can be used so that, for example, a smaller 40 A fuse can be installed.
Some utilities may require a second, lockable disconnect near the point of interconnection. If required, it can be installed between the new, fused disconnect and the inverter, and thus overcurrent protection is not required in this additional “utility” disconnect. It can simply be a switch, ideally located immediately adjacent to the fused disconnect and labeled per utility requirements.
Depending on the AHJ, for buy-all sell-all connections an equipment-grounding conductor (EGC) may or may not be installed between the new meter base and the service disconnecting means, depending on where the AC system ground (the connection between neutral and ground) is formed.
Previously, the size limit for a supply side connection was not specified in the NEC. As of 2011, the Code takes a common sense approach, stating in Section 705.12(A) that the sum of the ratings of all overcurrent protection devices connected to the power production sources cannot be more than the rating of the service. For example, if the required inverter output circuit OCPD—which would typically be a fused disconnect sized per Sections 705.60 and 690.8—for a PV system is 200 amps, the existing AC service would have to be at least 200 amps in order to connect the PV system on the supply side. If this isn’t the case, the existing service, service entrance conductors, and possibly the utility transformer, may have to be upgraded.
Brian Mehalic is a NABCEP-certified PV installer and ISPQ-certified PV instructor. He has experience designing, installing, and servicing both PV and solar thermal systems, and is a curriculum developer and instructor for Solar Energy International.