NEC 2017 705.12 (B)(2)(3)(a)

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thamessolar

Member
Location
Denver, Co
Can you give me your opinion of which of the following methods you agree with? Our state inspector uses you as a reference for requiring method two.
Question: Is the 100 amp busbar in the garage panel adequate or does the code require it to be increased to 200 amps?


Method one – My understanding using NEC 2017 705.12(B)(2)(3)(a) – 125% of the inverter output current rating plus the utility fed main breaker does not exceed the busbar rating.
20.96A x 1.25 = 26.20A +50A main = 76.2A < 100 busbar.
Therefore, the existing 100a garage panel is adequate.

Method two – Colorado State Electrical inspectors understanding of the NEC. The code requires that the size of the breaker being backfed by a PV inverter onto a panel busbar does not exceed 20% of the busbar rating
200A busbar x 20% = 40A backfed breaker
Therefore, the panel busbar must be increased to 200 amps.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Inspector is wrong.

First of all, your reading of 705.12(B)(2)(3)(a) is correct.

Second, the '120% rule' which is nowadays 705.12(B)(2)(3)(b), has never stated the maximum as 20 percent of the busbar. It has always been the sum of the backfeed plus the main feed cannot exceed 120 percent of the busbar. Thus a 20 percent limit only applies if the main breaker and busbar ratings are equal. It seems like your inspector is not familiar with scenarios that differ from that, or what the complete rules actually allow.
 

pv_n00b

Senior Member
Location
CA, USA
Up through the 2011 NEC the PV circuit breaker rating was used when calculating the max back feed allowance. That changed to 125% of the PV inverter max output current in the 2014 NEC. Buy them a new NEC.
 
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