Supply-Side PV Connection

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Leo1

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Location
Los Alamos, NM
Article 705.12(A) states the sum of the rating of all overcurrent protection device connected to the "power production sources" cannot be more than the rating of the service.

The NEC states the local utility is not one of the "power production sources" referred to in 705.12. Therefore if the existing service was a 200 AMP it seems I could install 200 AMPs of PV system. However, I have read other material that states Article 705.12(A) is implying all overcurrent protection devices - utility and power production sources. Therefore if a 200 AMP service is protected by a 200 CB or fuse then to add any PV to the supply-side means the entire service must be upgraded? Is that correct?
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
Article 705.12(A) states the sum of the rating of all overcurrent protection device connected to the "power production sources" cannot be more than the rating of the service.

The NEC states the local utility is not one of the "power production sources" referred to in 705.12. Therefore if the existing service was a 200 AMP it seems I could install 200 AMPs of PV system. However, I have read other material that states Article 705.12(A) is implying all overcurrent protection devices - utility and power production sources. Therefore if a 200 AMP service is protected by a 200 CB or fuse then to add any PV to the supply-side means the entire service must be upgraded? Is that correct?
Possibly the authors of the other material are confusing the line-side tap configuration with the backed-panel-bus configuration. In the second case, the 120% rule is intended to protect the busbars. Since the individual load breakers can exceed the main breaker rating, all sources of power are added together. In the case of a line-side tap, there is no way that any existing OCPD protected load could draw the sum of the utility power and the PV power. The two worst case situations in terms of the service will be:
1. the loads drawing the full rated current of the service with no PV contribution, and
2. No loads and the PV sending the full rated service current back to POCO.
 

SolarPro

Senior Member
Location
Austin, TX
Leo,

705.12(A) applies to supply side connections, connections made on the utility side of the service disconnecting means. The size of the PV system is limited by the size of the service. If the service is rated for 200 A and you make a supply side interconnection, then you could have 200 A fuses in the disconnecting means for the PV system, provided the PV system is the only electric power production source.

Connections made inside of a panel are subject to 705.12(D), which covers connections on the load side of the service disconnecting means. 705.12(D)(2) is the source of the so-called 120% rule, which says the sum of the breakers feeding a panel may not exceed 120% of the bus rating. Note that the 120% allowance only applies if the PV breaker is at the opposite end of the bus as the main breaker; if the PV breaker is installed anywhere else on the bus, the sum of the breakers cannot exceed the bus rating.

There are any number of threads here discussing this in great detail. If you're having a hard time understanding your design options, pick up a copy of Mike Holt's "Understanding NEC Requirements for Solar Photovoltaic Systems." It has good examples of real world scenarios.
 
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Leo1

Member
Location
Los Alamos, NM
SolarPro,

Thank you.

I have Mike Holt's Solar Photovoltaic Systems, based on the 2011 NEC. On page 303, top right of page, in Fig, 705-8 it states "The ampere ratings of the PV and service overcurrent device, is not permitted to exceed the ampere rating of the service conductor."

To me this means if I wanted to add a 100 amps of PV power onto an existing 200 amp service I would have to upgrade the service to 300 amps. The confusion here for me is two part: a) the existing utility service should never see more than a 200 amp load whether the utility is providing the entire 200 amps or only 100 amps when the PV is providing 100 amps. b) Article 705.12(A) states the sum of the rating of all overcurrent protection device connected to the "power production sources" cannot be more than the rating of the service, and 705.12 states the local utility is not one of the "power production sources". In my example the 100 amps of PV power is less than the rating of the 200 amp service/conductors. So must I upgrade the 200 amp service/conductors to 300 amps?
 

shortcircuit2

Senior Member
Location
South of Bawstin
SolarPro,

Thank you.

I have Mike Holt's Solar Photovoltaic Systems, based on the 2011 NEC. On page 303, top right of page, in Fig, 705-8 it states "The ampere ratings of the PV and service overcurrent device, is not permitted to exceed the ampere rating of the service conductor."

To me this means if I wanted to add a 100 amps of PV power onto an existing 200 amp service I would have to upgrade the service to 300 amps. The confusion here for me is two part: a) the existing utility service should never see more than a 200 amp load whether the utility is providing the entire 200 amps or only 100 amps when the PV is providing 100 amps. b) Article 705.12(A) states the sum of the rating of all overcurrent protection device connected to the "power production sources" cannot be more than the rating of the service, and 705.12 states the local utility is not one of the "power production sources". In my example the 100 amps of PV power is less than the rating of the 200 amp service/conductors. So must I upgrade the 200 amp service/conductors to 300 amps?

Leo1...IMO MH's book has some errors and conflicting statements.

For example on the same page 303...figure 705-6 states that the PV Disconnect is not a service disconnect.

But just below in figure 705-7 they tell us to bond the PV Disconnect as service equipment...

IMO...don't live by MH book, go by the NEC...

705.12(A) says to add the sum of the breakers from power production sources and it shall not exceed the rating of the service.

This is pretty simple...your 100 amps of solar doesn't exceed your 200amp service.
 

Smart $

Esteemed Member
Location
Ohio
SolarPro,

Thank you.

I have Mike Holt's Solar Photovoltaic Systems, based on the 2011 NEC. On page 303, top right of page, in Fig, 705-8 it states "The ampere ratings of the PV and service overcurrent device, is not permitted to exceed the ampere rating of the service conductor."

To me this means if I wanted to add a 100 amps of PV power onto an existing 200 amp service I would have to upgrade the service to 300 amps. The confusion here for me is two part: a) the existing utility service should never see more than a 200 amp load whether the utility is providing the entire 200 amps or only 100 amps when the PV is providing 100 amps. b) Article 705.12(A) states the sum of the rating of all overcurrent protection device connected to the "power production sources" cannot be more than the rating of the service, and 705.12 states the local utility is not one of the "power production sources". In my example the 100 amps of PV power is less than the rating of the 200 amp service/conductors. So must I upgrade the 200 amp service/conductors to 300 amps?
The 200A service ocpd is not connected to a power production source. It is load only... so it does not count in the summing of breaker ratings. The sum of breaker ratings connected to power production sources in your case is 100A, and less than the rating of the service.
 

Leo1

Member
Location
Los Alamos, NM
Leo1...IMO MH's book has some errors and conflicting statements.

For example on the same page 303...figure 705-6 states that the PV Disconnect is not a service disconnect.

But just below in figure 705-7 they tell us to bond the PV Disconnect as service equipment...

IMO...don't live by MH book, go by the NEC...

705.12(A) says to add the sum of the breakers from power production sources and it shall not exceed the rating of the service.

This is pretty simple...your 100 amps of solar doesn't exceed your 200amp service.


Agree. Thank you.
 

Leo1

Member
Location
Los Alamos, NM
The 200A service ocpd is not connected to a power production source. It is load only... so it does not count in the summing of breaker ratings. The sum of breaker ratings connected to power production sources in your case is 100A, and less than the rating of the service.

Agree, all the feedback is consistent and making more sense now. Thanks.
 
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