Breaker Size For Feeder For Combination ESS and PV

[wwhitney]

Say you have a load-side generation panel with 60A of PV inverter breakers and 60A of ESS inverter breakers (and for simplicity, assume 125% of the inverter output current equals the connected breaker size in all cases). Is there something in 690 or 705 that would require the feeder connection to this panel to be protected at 120A or greater (and the feeder sized correspondingly)?

Or would it be permitted to connect just a 100A feeder, and accept the corner case risk of tripping the 100A breaker during peak conditions, when solar generation is at a maximum, the ESS is configured to offset all site loads to export all PV, and the site loads are near the ESS inverter capacity? The smaller feeder could be helpful to qualify upstream existing panels under 705.12(D) to avoid the cost of replacing them.

Or does the ESS configuration play into the answer? If the ESS is configured for "site consumption" and say the site loads are supplied by a 100A breaker, then the ESS inverter behavior should be that the combined ESS and PV inverter outputs never exceed 100A (at least, no more than the loads on a 100A breaker are allowed to).

Thanks,
Wayne

 

[jaggedben]

Say you have a load-side generation panel with 60A of PV inverter breakers and 60A of ESS inverter breakers (and for simplicity, assume 125% of the inverter output current equals the connected breaker size in all cases). Is there something in 690 or 705 that would require the feeder connection to this panel to be protected at 120A or greater (and the feeder sized correspondingly)?

Yes. It is entailed by 705.60 and .65. (2017 NEC reference.) Note that the definition of inverter output circuit also includes the feeder all the way back to the service.

Or would it be permitted to connect just a 100A feeder, and accept the corner case risk of tripping the 100A breaker during peak conditions, when solar generation is at a maximum, the ESS is configured to offset all site loads to export all PV, and the site loads are near the ESS inverter capacity? The smaller feeder could be helpful to qualify upstream existing panels under 705.12(D) to avoid the cost of replacing them.

Or does the ESS configuration play into the answer? If the ESS is configured for "site consumption" and say the site loads are supplied by a 100A breaker, then the ESS inverter behavior should be that the combined ESS and PV inverter outputs never exceed 100A (at least, no more than the loads on a 100A breaker are allowed to).

Thanks,
Wayne

Practically speaking, you're probably all fine here. Code wise, you can't count on this being allowed unless/until the system is listed as a 'Power Control System' and you are on the 2020 NEC or equivalent, which explicitly allows it when so listed. My understanding is that PCS is new or still in development and not much if anything is listed to it yet.

 

[wwhitney]

Yes. It is entailed by 705.60 and .65. (2017 NEC reference.) Note that the definition of inverter output circuit also includes the feeder all the way back to the service.

Ah, thank you for the pointers, I was expecting to find something like that but somehow missed those sections.

Cheers, Wayne