Grid Interconnection & Commissioning
Backfeed, 120% rule, line-side taps, anti-islanding, utility disconnect, production metering, PTO, and testing.
- Apply the 120% rule for PV backfeed in dwelling services
- Describe line-side taps as an interconnection alternative
- Explain anti-islanding protection and utility disconnect requirements
- Describe commissioning, testing, and insulation resistance testing
- Define Permission to Operate (PTO) and its significance
Lección 1
Backfeed, the 120% Rule & Busbar Loading
The 120% Rule
The busbar loading rule for PV backfeed in most dwelling services is the 120% rule (NEC 705.12). The sum of the main breaker rating plus all backfed source breakers (PV, battery, etc.) cannot exceed 120% of the bus rating.
For a 200A panel with a 200A bus:
200A x 1.20 = 240A maximum
240A - 200A (main) = 40A maximum PV breaker
The backfeed breaker opposite end placement is intended to achieve even current distribution on the bus. The PV breaker must be placed at the opposite end of the bus from the main breaker. This ensures the section of bus between main and PV breaker carries the highest current, and that section is rated for it.
The 120% rule limits PV backfeed breaker plus main breaker to 120% of bus rating. For a 200A panel, maximum PV breaker is 40A. Place the PV breaker at the opposite end of the bus from the main breaker.