Section 705.12(D)(3) requires that interconnection occur on the line side of all ground-fault protection equipment. The Exception allows load-side connections, but the ground-fault protection device (GFPD) has to be listed as suitable for backfeeding and protection must be in place for equipment from all ground-fault sources. For residential and small commercial systems, ground-fault breakers typically cannot be used for interconnection. Larger systems may require ground-fault protection as part of the main AC breaker per Section 230.95 (480Y/277 VAC services rated at 1,000 A or more). If this is the case, documentation may need to be obtained from the breaker manufacturer stating that the breaker can be back-fed. If it cannot, then a supply-side connection is the only option.
When multiple sources can supply power to a piece of equipment, Section 705.12(D)(4) requires that the equipment be marked to indicate the presence of all power sources. Usually, the marking for the main breaker in a panel will be stamped into the metal cover. Additional labeling needs to be included at any panel where there is one or more breakers being back-fed by an inverter. This includes subpanels where the interconnection is, as well as the main service panel. Section 690.54 expands upon this, requiring that all points of interconnection be marked at their disconnect as a power source and with the rated AC output current and nominal AC operating voltage of the interconnected PV systems.
Sections 705.12(D)(5) and (6) detail requirements for circuit breakers back-fed by an inverter. They must be suitable for the application, and an Informational Note about the sections states that if the breakers are marked with a line and a load side, they cannot be used. Breakers connected to grid-tied inverters do not need additional fastening—clip-on breakers can be used because the inverter will immediately turn off if the breaker is pulled from the busbar per the UL1741 listing requirements, meaning no part of the breaker will be energized. However, in a grid-tied system with battery backup, an inverter output breaker supplying power to a backed-up loads subpanel needs to be fastened per Section 408.36(D). The breaker will remain energized as long as it is wired to the inverter, even when it is physically removed from the panel.
Finally, Section 705.12(D)(7) requires that when the sum of the breakers supplying power to a busbar exceeds the busbar’s rating (as allowed per the 120% rule), that the breakers be located at opposite ends of the panel’s busbar. For example, if the main breaker is at the top of the busbar and the sum of it and the inverter output-circuit OCPD exceeds the busbar’s rating, the inverter OCPD would have to be located at the bottom of the busbar. A permanent label must be applied so that the inverter breaker is not moved in the future (see label sample, below).
INVERTER OUTPUT CONNECTION.
DO NOT RELOCATE THIS
This rule applies equally to main service panels and subpanels, whether or not they have a main disconnect. Locating the breakers at opposite ends ensures that no point on the busbar will be subject to the full sum of the supply breakers—if they were adjacent to each other, then the section of the busbar below them could have to carry up to 120% of its rating.
There is no clear answer on how to accomplish this on service panels where the main breaker lies in the center, which is common in residences. In this case, consultation with the local AHJ will be necessary to determine if a parallel power production system can be interconnected on the load side. If not, a supply-side connection or service equipment upgrade will be required.
Section 705.12(D)(7) also clarifies that when the point of interconnection occurs in a subpanel, the first overcurrent device to which the inverter is connected should be used for calculations in all panels. This means that if there is a 20 A inverter output-circuit OCPD in a subpanel that is fed by a 100 A breaker in the main panel, the 20 A rating should be used for calculating the load on the busbar in the main panel.
Brian Mehalic is a NABCEP-certified PV installer and ISPQ-certified PV instructor. He has experience designing, installing, servicing, and inspecting PV and solar thermal systems, and is a curriculum developer and instructor for Solar Energy International.