Grid-connected PV system design and installation
What the exam tests
Grid-connected PV design and installation covers interconnection to the utility, inverter selection, AC and DC sizing, busbar and feeder rules, anti-islanding, and commissioning. Expect questions on the 120% rule, voltage rise, and utility requirements.
Core rules
- Inverters must be listed to UL 1741 and meet IEEE 1547 anti-islanding and grid-support requirements; they must cease energizing a de-energized utility feeder.
- Common topologies: string inverters, module-level power electronics (microinverters or DC optimizers), and hybrid inverters with storage. Module-level electronics can provide rapid shutdown and per-module MPPT.
- Load-side interconnection commonly uses the 120% busbar rule: main breaker rating + PV breaker rating ≤ 120% of busbar rating. A supply-side connection is made ahead of the main disconnect and is not subject to that busbar rule, but it has its own disconnect and utility approval requirements.
- Size the PV breaker at not less than 125% of the inverter's continuous output current. Check conductor ampacity, voltage rise, and the utility's interconnection limit.
- AC-coupled systems need coordination between PV inverters, storage, and backup loads; never backfeed a panel in a way that exceeds its rating.
Worked example
A 200 A panelboard has a 200 A main breaker. 120% of 200 A = 240 A, so the largest PV breaker is 240 − 200 = 40 A. An inverter with 32 A continuous AC output requires 32 × 1.25 = 40 A, so a 40 A breaker just fits. A 50 A breaker would exceed the busbar allowance and require a supply-side connection or service upgrade.
Traps
- Applying the 120% rule to the utility feeder or to a supply-side connection.
- Sizing the PV breaker from array STC power rather than inverter continuous AC output.
- Ignoring continuous-load 125% factors.
- Forgetting anti-islanding, utility review, and rapid-shutdown requirements for roof-mounted arrays.