2011 NEC Inverter Output Conncetion

[inspector23]

2011NEC 705.12(D)(7) "..In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s)shall be used in the calculations for all busbars and conductors. ..."

I have an inverter output connection with a 30A OCPD interconnecting to a 100A MLO subpanel in a garage.The feeder for the subpanel comes from the MSP, protected by a 100A OCPD located on the MSP busbar next to the middle-fed main breaker (200A MB, 200A rated bus service panel).My interpretation of this code section, this configuration will not be allowed since I have to use the 30A OCPD for calculations for ALL busbars and conductors, not just the subpanel, but the MSP as well. Since the MSP has a 200A rated bus and a 200A MB and it is a middle fed configuration, it is not eligible for the 120% rule. I understand there may be a possibility of downsizing the MB after existing load calculations justify, but as presented, it is not compliant.

AT least that is my interpretation of this code section. I am questioning if my interpretation of "ALL busbars and conductors" is valid. Any comments/insights/ viewpoints welcome.

Thanks -

 

[GoldDigger]

2011NEC 705.12(D)(7) "..In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s)shall be used in the calculations for all busbars and conductors. ..."

I have an inverter output connection with a 30A OCPD interconnecting to a 100A MLO subpanel in a garage.The feeder for the subpanel comes from the MSP, protected by a 100A OCPD located on the MSP busbar next to the middle-fed main breaker (200A MB, 200A rated bus service panel).My interpretation of this code section, this configuration will not be allowed since I have to use the 30A OCPD for calculations for ALL busbars and conductors, not just the subpanel, but the MSP as well. Since the MSP has a 200A rated bus and a 200A MB and it is a middle fed configuration, it is not eligible for the 120% rule. I understand there may be a possibility of downsizing the MB after existing load calculations justify, but as presented, it is not compliant.

AT least that is my interpretation of this code section. I am questioning if my interpretation of "ALL busbars and conductors" is valid. Any comments/insights/ viewpoints welcome.

Thanks -

Completely correct as far as you go.
But if the 100A panel has a 100A bus and also houses circuits other than the PV you will be violating the 120% rule in the subpanel unless you downsize the feeder or the main breaker to 90A or less.
If the subpanel has a 125A or higher bus, you would be OK.
Same thing for the main. It is possible that the bus in the main panel is actually rated higher than 200A. But with the center feed location even 225A would not be enough to meet the 100% rule.

 

[inspector23]

Completely correct as far as you go.
But if the 100A panel has a 100A bus and also houses circuits other than the PV you will be violating the 120% rule in the subpanel unless you downsize the feeder or the main breaker to 90A or less.
If the subpanel has a 125A or higher bus, you would be OK.
Same thing for the main. It is possible that the bus in the main panel is actually rated higher than 200A. But with the center feed location even 225A would not be enough to meet the 100% rule.

Thank you, I appreciate the insight and the added input.

 

[jaggedben]

As Goldigger said, both panels have issues.

The subpanel issue could be solved by downsizing the 100A breaker feeding the sub to a 90A. You could ask the installer to provide load calcs for the sub if you're concerned whether that's legit.

Likewise, the main breaker in the service panel could be downsized to 150A. Again, load calculations could be called for. In most cases the calculated load is nowhere close to the equivalent of 200A.

For what it's worth, the 2017 NEC will allow center-fed panels in dwellings to use the 120% rule as long as the inverter breaker is placed at one end or the other (not both). Some AHJs I work with allow the 120% rule on centerfed panels and, as the 2017 revision shows, the code making panel doesn't think there's a safety issue with this.