Back-feeding from main lug sub-panels

[craigmw]

I'm in the process of planning a photovoltaic system that will include initially two arrays on west- and east-facing roofs of our pool house (total of 3.65 kW), and later, I'd also like to include a separate south-facing roof from over the BBQ island (2.2 kW). The pool-house is wired to the main breaker panel (rated 200A, main breaker is 150A) on the house via a 50A breaker and 8 gauge copper (2 hots, neutral and ground), about 100 ft of underground wiring in conduit. This feeder wire connects to a main lug sub-panel (rated 100A) on the back of the pool house that feeds the pool house and pool equipment sub-panel. This pool house sub-panel also feeds a second sub-panel (rated 100A) via underground conduit (10 gauge copper, 2 hots, neutral and ground) via a 30A breaker in the sub-panel. The idea is to take the output from the two arrays (which will use micro-inverters) on the pool-house roof and connect them to the pool house sub-panel via a 20A/220 backfeed breaker. When I add the additional array on top of the BBQ island roof, I plan to wire that array into a 15A back-feed breaker in the second sub-panel on the BBQ island. Based on my reading of the NEC code regarding this (690.64 Point of Connection), this should be permissible since the total wattage back-fed to the main panel will be less than 25A. The permissible back-feed on a 200A panel with a 150A main breaker would be 90A.

I was originally thinking that I would have to install new conduit to do home-runs from the three arrays back to the main panel, but after reading the NEC, it seems I can just back-feed the existing wiring, assuming I stay within the 120% rule and respect the wire gauge ampacity ratings. Note that all sub-panels have separate ground buses, and all are tied to the ground bus on the main panel. I may need to add an additional grounding rod at the pool house panel... I'm not quite sure about that (likely depends on what our building inspector requires). Any thoughts?

[scrambler]

I think you should create a wiring diagram showing the various panels their breakers interconnections, as well as the panel rating, all the way from the meter to all the sub panels, and where the microinverter breakers will be

It will make it a lot easier to validate the 120% rule

[Mike90250]

The other thing - your AC line resistance and voltage rise. With long runs, your inverters have some resistance from inverter to Power Company Transformer. You may need to upsize your wire gauge to prevent your inverters from tripping offline from local voltage rise.
You should consider going 3 or 4 sizes larger wire, and using aluminum power cable. Aluminum pulls easier and costs less per watt handling.

[solarix]

Your jurisdiction may be different, but around here there needs to be just one disconnecting means for all the solar. All the solar circuits need to go through one switch. (The evil solar power needs to be tightly controlled you know - despite what grief and dollars it may cost you)

[inetdog]

As far as applying the 120% rule to the situation of a main lug only subpanel, the feed wire from the upstream breaker and the panel bus in the subpanel form one "bus" for computation purposes. The feeder breaker in the main panel is equivalent to a main breaker in the subpanel So the PV amps plus that breaker rating must be less than 120% of both the subpanel bus ampacity and the feeder wiring ampacity. And the PV backfeed must be at the bottom of the subpanel.

[PVAndy]

Your jurisdiction may be different, but around here there needs to be just one disconnecting means for all the solar. All the solar circuits need to go through one switch. (The evil solar power needs to be tightly controlled you know - despite what grief and dollars it may cost you)

Curious what jurisdiction that is. NEC allows a maximum of 6 disconnects

[SunEagle]

Curious what jurisdiction that is. NEC allows a maximum of 6 disconnects

I believe the 6 allowed disconnects are for an electrical distribution service without requiring a single main breaker. But I do not think it applies to solar generation systems.

[PVAndy]

I believe the 6 allowed disconnects are for an electrical distribution service without requiring a single main breaker. But I do not think it applies to solar generation systems.

I didn’t look at the code just now. We routinely have more than one disconnect on larger systems in Mass, Conn & New York

Andy

[SunEagle]

I didn’t look at the code just now. We routinely have more than one disconnect on larger systems in Mass, Conn & New York

Andy

I understand. The larger Utility solar generating systems can have a number of disconnects for each "bank". But usually they terminate into a single CB per inverter and there could be a large number of inverters depending on the MWatt of the panel system. I worked on a 1MW system in Nevada that included 2 500kw 3 phase inverters each having a separate CB for it as well as one for each string. There were 6 strings per each inverter.

If the array is much larger than 1MW I would expect many more disconnects for the system.

For a home owner things go a little different because the pv array is much smaller and usually does not require a large amount of disconnects for the system to be to code.