PV System Components and Design
Domain 3 covers the hardware that turns modules into a safe, code-compliant system and the sizing decisions that make it work. The exam expects you to recognize component roles, choose inverter and conductor ratings, and check string voltage against the inverter window.
Components and their jobs
Modules convert light to DC. The inverter converts DC to AC and sets grid interaction; hybrid inverters also manage batteries. Racking fixes modules at the chosen tilt and transfers wind and snow loads. DC and AC disconnects provide isolation; overcurrent devices protect conductors; ground-fault and arc-fault protection address common PV hazards. Charge controllers regulate battery charging, and rapid shutdown limits shock hazard near an array.
Design sequence
Start with the load or production goal, then the available area, orientation, tilt, and shading. Size the array, then select an inverter and check the DC/AC ratio. String sizing must stay inside the inverter's MPPT voltage window at both temperature extremes. Conductor ampacity must account for continuous current, and voltage drop should stay low enough for the application. Structural attachment, roof clearance, pathways, and labeling belong in the design, not the install punch list.
Worked example
An inverter has a 100-500 V MPPT window. A module has Voc 40 V at 25 °C and a Voc temperature coefficient of -0.30%/°C. At a record low of -10 °C, the correction is 1 + (-0.003 × (-10 - 25)) = 1.105, so Voc is 44.2 V. Maximum modules = 500 ÷ 44.2 = 11.3, so use 11. At a hot cell temperature of 70 °C with Vmp 33 V and a -0.40%/°C coefficient, Vmp falls to about 27.1 V; minimum modules = 100 ÷ 27.1 = 3.7, so use at least 4. A string of 4-11 modules fits, subject to the actual MPPT range and start voltage.
Traps
Using STC Voc for the cold maximum is a classic error. Do not size strings by nominal module voltage. Do not ignore the 1.25 continuous-current factor for PV source and output conductors where required. Do not assume any inverter can take any DC/AC ratio; check the manufacturer's limits and clipping behavior.