Sounds like I may be misinterpreting something but I don't think so...
NFPA 70, 2014 NEC 705.12(D2)(3)(b) states, "The sum of 125% of the inverter(s)
output circuit current and the rating of the overcurrent device protecting the
busbar shall not exceed 120% of the ampacity of the busbar."
Based on this wording, here's an example of the way I calculate whether or not
I'm compliant with the 120% rule:
1. Circuit#1 inverter output current rating = 13.5A X 1.25 = 16.88 (OCPD for
this circuit = 20A)...(I run 10AWG for these circuits)
2. Circuit#2 inverter output current rating = 9.0A X 1.25 = 11.25 (OCPD for
this circuit = 20A)
3. 16.88A + 11.25A = 28.13A
4. 28.13A + 100A (rating of main OCPD in subpanel) = 128.13A
5. Ampacity of busbar in 125A rated subpanel = 125A
6. 125A X 1.2 = 150A
7. 128.13A < 150A, therefore, this installation is compliant with 2014 NEC
705.12(D1-D6).
If I used the 100A rating of the OCPD instead of the 125A rating of the busbar,
I would be limited to 20A of PV instead of 50A.
According to the wording in the NEC I ignore load breakers and only concern
myself with OCPD for PV inverter output circuits and OCPD for the busbar.
Eric Stikes
SunHarvest
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