NABCEP PVIS common mistakes
NABCEP PVIS · Understand the traps and how to avoid them.
Treating the PVIP exam as a memory test on module nameplate numbers and reciting STC values instead of computing site-specific I-V behavior (temperature-corrected Voc, Vmp at high cell temperature) before selecting string size and conductor ampacity.
Candidates study datasheets as static facts and never practice the calculation chain: record-low ambient → cell temperature via the temperature coefficient and the 25 °C offset, then Voc at that temperature against the inverter's maximum input voltage, then Vmp at record-high cell temperature against the MPPT window. The exam tests the chain, not the datasheet.
What to do instead
Build a one-page worksheet that walks ambient low/high → cell temperature → temperature-corrected Voc and Vmp → string count window → conductor and OCPD sizing. Practice it on five different modules and three inverter models until the sequence is automatic, and always carry the module's Voc and Vmp temperature coefficients in %/°C or V/°C as given.
Applying NEC 690.12 rapid-shutdown language from memory and answering with the old '10 feet / 5 feet within array boundary' rule rather than the current controlled-conductor limits measured from the array boundary.
Code text has been revised multiple times, and study material floating around online mixes editions. Candidates memorize a number without knowing which edition it belongs to or what 'array boundary' means for a roof-mounted vs ground-mounted system.
What to do instead
Read the actual NEC 690.12 text in the edition the exam references, sketch the array boundary for a roof and a ground mount, and mark where the controlled conductors must be limited. Note the exception for systems with no exposed conductors and the listing requirement for the shutdown device.
Sizing the inverter AC output OCPD and conductor from the module array nameplate rather than from the inverter's continuous output current, and forgetting the 125% continuous-load factor and the 690.8/705.12 interconnection rules.
Candidates conflate DC array size with AC current and skip the inverter datasheet's rated output current. They also miss that the inverter output is treated as a continuous source, so the OCPD must be at least 125% of rated output current.
What to do instead
For every practice problem, write the inverter's maximum continuous output current first, multiply by 1.25, then select the next standard OCPD and size conductors to the OCPD. Separately practice the 705.12 busbar and feeder tap calculations with the 125% factor applied to the inverter output.
Ignoring the difference between module-level and string-level maximum current when sizing source-circuit conductors and the combiner OCPD, and using Isc directly without the 1.25 factors in 690.8(A) and (B).
The two-step 125% rule (125% of Isc for the source circuit, then 125% of the summed currents for the output circuit) is easy to collapse into one factor. Candidates also forget that parallel strings require string fuses or combiners rated for the fault current from the other strings.
What to do instead
Practice the calculation as two distinct steps and label each factor's code basis. For a three-string combiner, compute the maximum current each string can see from the other two strings and confirm the string fuse rating and module series fuse rating cover it.
Answering grounding and bonding questions with generic electrical assumptions instead of distinguishing system grounding (DC negative or functional ground) from equipment grounding and bonding of module frames, rails, and mounting hardware per the manufacturer's listed method.
Candidates treat grounding as a single concept and miss that listed bonding hardware, star washers, and lay-in lugs are manufacturer-specific and that the equipment grounding conductor sizing follows 250.122 based on the OCPD, not the array current.
What to do instead
Make a table with three columns: system grounding, equipment grounding conductor sizing, and module-to-rail bonding method. Fill it from the inverter manual and module installation manual, and practice identifying when an ungrounded (transformerless) inverter changes the ground-fault protection requirements.
Underestimating the site-assessment and shade-analysis questions, answering with a rule-of-thumb azimuth/tilt or a generic 'southern exposure' instead of reading the described obstructions, tilt, azimuth, and horizon and computing the specific production impact.
Candidates from a hands-on background rely on field intuition and skip the quantitative shade and irradiance reasoning. The exam presents described geometry and expects the candidate to reason about row spacing, inter-row shading, and the effect of tilt and azimuth on annual yield.
What to do instead
Practice with a sun-path chart and a described array: compute row spacing for a given tilt and latitude using the winter solstice shadow angle, and estimate the production penalty of a stated azimuth deviation. Learn the difference between near-shading (module-level, mismatch and bypass-diode effects) and far-shading (uniform irradiance reduction).
Designing string layouts and conductor routing that ignore bypass-diode behavior and mismatch, then answering questions about partial shading as if the whole string loses output proportional to the shaded area.
Candidates memorize 'shading is bad' without understanding that a shaded cell string within a module is bypassed by its diode, that half-cut and multi-diode modules behave differently, and that mismatch losses depend on how many cells are bypassed versus how the MPPT settles.
What to do instead
Draw the cell-string and bypass-diode topology for a 60-cell, 72-cell, and half-cut module, then trace what happens when one cell string is fully shaded. Practice distinguishing a partial-string bypass (one diode conducts) from a fully shaded module (whole module bypassed) and predict the voltage and current at the MPPT.
Skipping the mechanical and structural checks on mounting and attachment: answering roof attachment questions without verifying rafter spacing, span, uplift, and the difference between a flashed penetration and a comp-shingle sealant-only attachment.
Candidates focus on electrical calculations and treat racking as a commodity. The exam tests whether the installer knows that attachment spacing depends on the structural member, that flashing must be integrated into the roof's water-shedding layer, and that wind and snow loads are site-specific.
What to do instead
Study a manufacturer's racking installation manual and tabulate allowable spans for the common rafter and truss spacings. Practice describing a flashed attachment detail in words: penetration, flashing, sealant placement, and how water is directed. Review the load-path concept from module to rail to attachment to structure.
Treating commissioning and testing as a formality: answering with 'check the voltage' instead of specifying open-circuit voltage, short-circuit current, insulation resistance, polarity, ground-fault detection, and the inverter's required startup sequence in order.
Candidates memorize the steps as a list and forget the purpose of each test and the safe order. They also miss that measured Isc and Voc should be compared to expected values corrected for irradiance and temperature, not to STC nameplate.
What to do instead
Write a commissioning checklist with the test, the instrument, the expected value, and the pass criterion for each item. Practice correcting a measured Voc and Isc to the ambient and irradiance at the time of test, and rehearse the sequence from array isolation through inverter energization.
Answering troubleshooting scenarios by replacing components rather than isolating the fault: not using the open-circuit voltage and short-circuit current readings to distinguish a failed module, a blown string fuse, an open connector, and a ground fault before touching hardware.
Field experience is often pattern-matching, and candidates jump to the most familiar failure. The exam presents measured values and expects a diagnostic sequence that localizes the fault to a string, a module, or a conductor before any replacement.
What to do instead
Build a decision tree: measure string Voc and Isc, compare across strings, then measure module-level Voc and Isc within the suspect string, then insulation resistance to ground. Practice with invented readings for a 10-string array and identify which single fault explains all the numbers.