[RE-wrenches] Busbar rating with multiple main breakers

[SunHarvest]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20131219/8e032636/attachment-0003.html>