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Photovoltaic - 2017 NEC 705.12(B)(2)(3)(a) and 705.12(B)(2)(3)(b)

Question asked by chiefns830 on Oct 17, 2019
Latest reply on Oct 22, 2019 by john.hal

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.

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