Load Side Connection at Sub Panel

[SunsUp]

We are planning to install a small PV system on this house and tie in through one of the two sub panels. The main breaker is located at the meter with 2 sub panels downstream. The main breaker is 125a in GE equipment (no label). The 1st sub panel has a 125a rated bus fed by the 125a main breaker. Based on the 120 rule there should be no problem installing a 20a PV breaker here. Room would be available in this sub panel at the bottom of the busing if the existing breaker(s) are move up into an open space currently existing. There is a 2nd sub panel fed by a 125a breaker installed in the 1st sub panel. Will this work and be NEC compliant. 1st Sub Panel Bus - resized.jpgMain area pic no label resized.jpg2nd Sub panel area pic.JPG1st Sub Panel Area Pic.JPG1st Sub Panel Label.JPGMain Breaker at Meter.JPG

[thastinger]

AFAIK, the Square D panels are the only ones NEC lists as acceptable for DC breakers as well

I just used a fused lever style DC rated disconnect between my panels and CC.

[inetdog]

We are planning to install a small PV system on this house and tie in through one of the two sub panels. The main breaker is located at the meter with 2 sub panels downstream. The main breaker is 125a in GE equipment (no label). The 1st sub panel has a 125a rated bus fed by the 125a main breaker. Based on the 120 rule there should be no problem installing a 20a PV breaker here. Room would be available in this sub panel at the bottom of the busing if the existing breaker(s) are move up into an open space currently existing. There is a 2nd sub panel fed by a 125a breaker installed in the 1st sub panel. Will this work and be NEC compliant.

It sounds like it could comply based on what you have said. But you have to look at all three panels.
The PV backfeed breaker must be at the far end of the bus from the main to take advantage of the 120% rule, and with a 125A main feeding at least a 125A bus you have up to 25A of PV backfeed allowed. (Second subpanel)
The same 25A allowance exists at the first subpanel , as long as the breaker feeding the 2nd subpanel is at the opposite end of that bus from that panel's main.
You figure the backfeed at the first subpanel and at the main panel by the size of the first OCPD (20A) in line from the GTI.
Moving the first subpanel's feeder breaker in the main to the far end of the main bus may not be simple, but it is also technically required by the NEC.

In your case, the main disconnect at the meter is not a distribution panel, since the only thing fed from it is what you call the first subpanel. It is a disconnect with overcurrent protection. The backfeed to the "main" is OK since the only two things on the bus are the incoming service and the (potentially) backfed breaker. More accurately, there is not really a bus in this panel, just the breaker itself with input and output wiring.
At the first subpanel, you do not have a main breaker, so the feed is limited only by the 125A at the meter. The feed breaker to the second panel should be at the bottom of the bus since the feed lugs are at the top.
At the second subpanel, whether there is a main breaker or not, the PV backfeed breaker must be located at the bottom end of the bus.

[SunsUp]

It sounds like it could comply based on what you have said. But you have to look at all three panels.
The PV backfeed breaker must be at the far end of the bus from the main to take advantage of the 120% rule, and with a 125A main feeding at least a 125A bus you have up to 25A of PV backfeed allowed. (Second subpanel)
The same 25A allowance exists at the first subpanel , as long as the breaker feeding the 2nd subpanel is at the opposite end of that bus from that panel's main.
You figure the backfeed at the first subpanel and at the main panel by the size of the first OCPD (20A) in line from the GTI.
Moving the first subpanel's feeder breaker in the main to the far end of the main bus may not be simple, but it is also technically required by the NEC.

In your case, the main disconnect at the meter is not a distribution panel, since the only thing fed from it is what you call the first subpanel. It is a disconnect with overcurrent protection. The backfeed to the "main" is OK since the only two things on the bus are the incoming service and the (potentially) backfed breaker. More accurately, there is not really a bus in this panel, just the breaker itself with input and output wiring.
At the first subpanel, you do not have a main breaker, so the feed is limited only by the 125A at the meter. The feed breaker to the second panel should be at the bottom of the bus since the feed lugs are at the top.
At the second subpanel, whether there is a main breaker or not, the PV backfeed breaker must be located at the bottom end of the bus.

Thanks for the clearly written and easy to follow explanation. You bring up an interesting issue when you say, “The same 25A allowance exists at the first subpanel, as long as the breaker feeding the 2nd subpanel is at the opposite end of that bus from that panel's main”, as code wants the inverter output connection (pv backfeed breaker) at opposite end from the input feeder as well (in 1st Sub panel). Which one would be at the end of the bus in the 1st sub panel, the pv backfeed breaker or the breaker feeding the 2nd sub panel?

690.64(B)(7). Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220.

[inetdog]

Which one would be at the end of the bus in the 1st sub panel, the pv backfeed breaker or the breaker feeding the 2nd sub panel?

I am not sure I understand the question. The first panel is between the main disconnect at the meter and the second subpanel. The feed from that main into the first panel comes in at the top, either directly via lugs or through a main breaker. In either case, the backfed breaker, the one which is connected to the second subpanel, must be at the opposite (bottom) end. And the 20A backfed breaker is at the bottom of the second subpanel's bus, with the feeder from the first subpanel coming in at the top.

[SunsUp]

I am not sure I understand the question. The first panel is between the main disconnect at the meter and the second subpanel. The feed from that main into the first panel comes in at the top, either directly via lugs or through a main breaker. In either case, the backfed breaker, the one which is connected to the second subpanel, must be at the opposite (bottom) end. And the 20A backfed breaker is at the bottom of the second subpanel's bus, with the feeder from the first subpanel coming in at the top.

I believe I understand what you are saying, but the second sub panel’s bus is full, so what I am trying to ask is it possible to install the PV backfed breaker in the 1st sub panel as there is space available on its bus.

[inetdog]

I believe I understand what you are saying, but the second sub panel’s bus is full, so what I am trying to ask is it possible to install the PV backfed breaker in the 1st sub panel as there is space available on its bus.

Ok, I get it now. No problem with putting the 20a breaker at the bottom of the bus in the first panel and running the wiring directly to it instead of to/through the second panel.
If you want to pull the wire from the second panel through existing raceway into the first panel to connect that will be Ok too, as long as there is room in the raceway and the wires are not at their ampacity limit for the original number of conductors.