PV system design and installation
What the exam tests
PV system design and installation tests whether you can move from a site and load assessment to a safe, code-compliant array: shading, structural mounting, string voltage, conductor sizing, and commissioning.
Core rules
- Site assessment: roof condition, age, orientation, tilt, shade, and structural capacity. Use a solar pathfinder or software to quantify shade; even partial shade can dominate a string without module-level electronics.
- Array layout must respect fire setbacks and access pathways required by the building code and local amendments, and leave room for thermal expansion and drainage.
- String sizing uses the inverter's MPPT voltage window, the module's temperature-corrected Voc at record low, and Vmp at record high cell temperature. NEC 690.7 requires temperature correction for maximum voltage.
- Conductors are sized for ampacity, voltage drop, and code factors. NEC 690.8(A) applies 125% to Isc for maximum current, and 690.8(B) then requires conductor ampacity at 125% of that value before adjustment. Keep DC and AC voltage drop within design targets, often 2–3% each.
- Mounting must be flashed and sealed; rail splices, clamps, and grounding hardware follow manufacturer torque values. Support conductors, use listed connectors, and protect exposed wiring.
Worked example
An inverter has a 100–500 V MPPT window. A module has cold-corrected Voc = 44.2 V and hot-cell Vmp = 27.0 V. Maximum modules in series = 500 ÷ 44.2 = 11.3, so use 11. Minimum = 100 ÷ 27.0 = 3.7, so use at least 4. A 10-module string gives 442 V cold and 270 V hot, inside the window.
Traps
- Using STC Voc for cold-weather string sizing, which can overvoltage the inverter.
- Forgetting the 125% current factors when selecting conductors and overcurrent devices.
- Mixing modules with different orientations or electrical characteristics on one MPPT input.
- Ignoring roof setbacks, flashing, or structural review; these are common inspection failures.