Solar PV System Components and Electrical Basics
Domain 2 covers the hardware between the sun and the grid - modules, strings, combiners, inverters, disconnects, storage - plus the Article 690 and 705 calculations that size conductors and overcurrent devices.
Major components
- Module: 60/72-cell or half-cut 120/144-cell formats. Bypass diodes limit hot-spot heating when a substring is shaded.
- Inverter: string, central, micro, or hybrid. Converts DC to AC and provides MPPT, anti-islanding, and grid protection.
- Combiner box: parallels strings and holds string fuses per 690.9.
- Charge controller: MPPT tracks the array's maximum power point; PWM is simpler but efficient only when array voltage closely matches battery voltage.
- Disconnects and labeling: 690.13 PV disconnecting means, 690.15, and 690.56 signage.
Sizing rules
- Conductor ampacity per 690.8 uses 125 % of Isc (parallel strings summed), then applies conditions of use.
- String OCPD must be at least 1.25 × Isc and no greater than the module's series fuse rating.
- 690.7 sets maximum voltage from the cold-temperature Voc.
- 705.12 applies the 120 % busbar rule to the inverter's AC breaker.
Worked example
Four strings, each Isc 9.8 A, module series fuse rating 20 A, 10 AWG Cu (35 A at 75 °C). String fuse = 1.25 × 9.8 = 12.25 A → use 15 A, which is below the 20 A series rating. Minimum conductor ampacity = 9.8 × 1.25 × 1.25 = 15.3 A, so 10 AWG is more than adequate. Inverter output is 40 A continuous → 40 × 1.25 = 50 A breaker. On a 200 A bus with a 200 A main, 200 + 50 = 250 A exceeds 120 % of 200 A (240 A), so a supply-side connection or busbar upgrade is required.
Traps
- Using Imp instead of Isc for OCPD and conductor sizing.
- Exceeding the module series fuse rating when upsizing a string fuse.
- Treating the 120 % rule as 100 % and overloading the busbar.
- Mixing AC and DC disconnect requirements, or omitting rapid-shutdown labeling.