Topic:
Photovoltaic System Interconnection on Feeders With Meter/Main Combination Devices
Question:
How is NEC 705.12(D)(2)(3)(c) applied when an inverter output connection is made on feeder conductors fed from a primary source via feed-through lugs in a meter/main combination service entrance device? Does the inverter output overcurrent protection device (OCPD) count toward the sum of the OCPD devices "on" the primarily supply panelboard as described in this section and allow up to the busbar rating for the sum of the load and supply overcurrent devices, regardless of additional loads at the end of the feeder? This question applies equally to NEC 2014 currently in effect and 2017 which is imminently to be in effect.
Comment:
It is my interpretation that the attached drawing is code compliant with this section. The sum of the inverter output connection plus all load OCPD devices "on" the panelboard as described in this section does not exceed the rating of the meter/main combination service entrance device busbar. The main breaker in the subpanel on the load side of the inverter connection adequately protects the subpanel busbar and complies with NEC 705.12(D)(2)(1)(b). There is no segment of the main panel busbar nor the feeder where the ampacity can be exceeded under any scenario without the primary supply OCPD tripping. The subpanel load is not relevant to the calculation in 705.12(D)(2)(3)(c) because that portion of the circuit is protected via the provisions of 705.12(D)(2)(1)(b). Interestingly, NEC 2020 addresses feeder conductors fed via feed-through lugs in 705.12(B)(3)(6) and appears to support this interpretation.
Existing Interpretations and/or Declaratory Statements Related to this request
Input Received:
No. The inverter output protection device (OCPD) does not count towards the sum of the OCPD on the primary supply panel board because that is not what 705.12(D)(2)(3)(a) says. The article is looking at 125% of the inverter output current; not the protection devices. The issue is the potential current applied to the buss of the main panel.
The main panel buss potential is defined as the rating of the service OCPD plus 125% of the inverters output current. That must not exceed 120% of the ampacity of the buss bar. Whatever OCPD are in the panel is irrelevant. If the inverter output plus the main OCPD is over the buss capacity, the way this has been defined allows a reduction in the main OCPD to allow a larger inverter current capacity. The issue then becomes can the new service OCPD support the calculated load for the unit. When the inverter is no longer supplying power, can the main OCPD supply the load?
I agree
Answer:
No. The inverter output protection device (OCPD) does not count towards the sum of the OCPD on the primary supply panel board because that is not what 705.12(D)(2)(3)(a) says. The article is looking at 125% of the inverter output current; not the protection devices. The issue is the potential current applied to the buss of the main panel.
The main panel buss potential is defined as the rating of the service OCPD plus 125% of the inverters output current. That must not exceed 120% of the ampacity of the buss bar. Whatever OCPD are in the panel is irrelevant. If the inverter output plus the main OCPD is over the buss capacity, the way this has been defined allows a reduction in the main OCPD to allow a larger inverter current capacity. The issue then becomes can the new service OCPD support the calculated load for the unit. When the inverter is no longer supplying power, can the main OCPD supply the load?
