The way I’m interpreting this section, the Photovoltaic installation must comply with either 705.12(D)(2)(3)(a) OR 705.12(D)(2)(3)(b):

**(a)** says that 125% of the inverter output circuit current (can be obtained from the label on the inverter) gets added to the rating of the main overcurrent device protecting the busbar on the panel (or subpanel). So, for example if I have a 100-amp subpanel (with 100-amp bus), and it has a PV circuit with an output of 35-amps (I merely read the label on the inverter) and it is being backfed into the 100-amp panel using a 35-amp circuit breaker, **THEN**

35 x 1.25 = 43.75-amps

43.75 + 100 = 143.75-amps. So this is exceeding the ampacity of the busbar, and this method cannot be used. Moving on to (b):

**(b)** says basically the same thing as (a), except it allows for a few more amps because of the language, *“shall not exceed 120 percent of the ampacity of the busbar,”* SINCE THE BREAKER IS INSTALLED AT THE OPPOSITE END OF THE BUS, so as not to overheat the bus. **SO**

35 x 1.25 = 43.75-amps

43.75 + 100 = 143.75-amps

100 x 120% = 120-amps

so once again, this is over the limit and the installation does not comply. It seems that they would require at least a 225-amp panelboard to make this PV installation compliant, with the Photovoltaic circuit installed at the opposite end of the bus (method b) and the warning sign so as not to relocate the overcurrent device:

35 x 1.25 = 43.75-amps

43.75 + 225 = 268.75-amps

225 x 120% = 270-amps = INSTALLATION IS COMPLIANT, WHEN PV CIRCUIT IS INSTALLED ON A 225-AMP PANELBOARD

Am I interpreting this section correctly?

PS – My question only concerns (a) and (b), please do not comment about (c), (d), or (e). I would like to keep this question on track. Thank you in advance.

While there is no "Transfer Switch" such as applications which have been around longer - when the Primary Power is lost, the Transfer Switch kicks over to the Generator. This has morphed - creating an 'intuitive' thought process about something we can't see.

This application is an OR circuit to a fixed (predetermined) Load. Whether pulling Power from either or both - the Load can only pull "X" amount of Amps. For Residential, there could be (10) 20-Amp Breakers set up for distribution off the 100-Amp Main Breaker even though by rights, if they all started to get close to the 20A, the 100A Main Breaker is going to trip.

From the other Power production source (other than Utility), it will only put out as many Amps that the Load is requesting. But what also happens is, the Power production source can back feed thru the 100-Amp Main Breaker reducing the Load current to zero from the 100-Amp source. This is 'counter-intuitive' from what many people have developed as to how electricity works.

To bring this full circle, if the 100-Amp Utility Service is actually consuming (30) Amps at the Load where the Power production source can provide (35) Amps [when requested], it will, as actually being requested from the Utility Source, send 5-Amps back into the Utility Source system for the neighbor to consume.