NABCEP PVDS common mistakes
NABCEP PVDS · Understand the traps and how to avoid them.
Sizing the inverter AC output breaker to the inverter's nominal AC rating instead of applying the 125% continuous-load rule to the maximum continuous output current, and using the wrong conductor ampacity table for the chosen insulation and conditions of use.
Candidates memorize the 1.25 factor for ampacity but forget the second 1.25 multiplier that applies to the overcurrent device on continuous inverter output, and they treat the conductor and OCPD rules as interchangeable. The exam deliberately presents values where the two calculations diverge.
What to do instead
Work every inverter output problem in a fixed order: (1) find maximum continuous output current from the datasheet, (2) multiply by 1.25 for the OCPD, (3) size conductors to at least 125% of continuous current, then (4) apply correction and adjustment factors and verify the OCPD protects the conductor. Practice with two different insulation types and a rooftop temperature adder until the sequence is automatic.
Applying the ampacity correction and adjustment factors to the wrong base value, or applying them to the OCPD rating rather than to the conductor ampacity, when checking whether a derated conductor still carries the required current.
The requirement is that ampacity after correction and adjustment must still be at least 125% of continuous current, but candidates reflexively derate the final number instead of the table value, or they derate the OCPD. The exam gives a hot rooftop with conduit fill where the order of operations changes the answer.
What to do instead
Write the inequality explicitly on scratch paper: derated ampacity (table value x correction x adjustment) must be >= 1.25 x continuous current, and separately the OCPD must not exceed the conductor ampacity after adjustment for conditions of use. Check both directions every time and confirm which value the question is asking about.
Miscalculating maximum system voltage using the wrong temperature coefficient sign or using the record low ambient temperature instead of the lowest expected cell temperature.
The coefficient is negative, so voltage rises as temperature falls, and candidates either flip the sign or plug in the wrong temperature. The exam supplies a record low ambient and expects the candidate to know that cell temperature at open circuit can be higher than ambient under irradiance, or to use the specified design temperature.
What to do instead
Use the standard relation: Vmax = Voc(STC) x [1 + (coefficient) x (Tlow - 25)]. Keep the coefficient negative and subtract the negative, which increases voltage. Confirm which temperature the question specifies and use exactly that; if it gives a design low temperature, do not substitute the record low ambient. Verify the result against the charge controller or inverter maximum input voltage.
Ignoring the 690.12 rapid shutdown boundary requirement and placing the shutdown initiation device or the controlled conductors outside the allowed zone, or confusing the array boundary with the conductor boundary.
Candidates learn that rapid shutdown limits conductors to 80 V within 1 foot of the array but forget the separate requirement that the initiation device be readily accessible and located at a specified distance from the array, and they mix up which boundary applies to which conductor.
What to do instead
Draw the array boundary and the one-foot perimeter on a sketch for every rapid shutdown question, mark which conductors are inside versus outside, and label the initiation device location separately. Memorize the two limits as distinct facts (voltage and distance for controlled conductors; accessibility and location for the switch) and check each against the question stem.
Designing roof layout by counting module dimensions only, without accounting for module clamps, rail overhang, walkway and setback requirements, and the roof edge and ridge clearances that the structural and fire provisions require.
Candidates treat array layout as pure arithmetic on module width and length, forgetting that mounting hardware adds to the footprint and that code-mandated setbacks and access pathways consume usable area. The exam presents a roof with obstructions where the naive count exceeds the real capacity.
What to do instead
Build a layout checklist: module dimensions plus clamp and rail allowance, required setbacks from ridges and edges, access pathways, and obstruction clearances. Compute usable area first, then divide by the per-module footprint including hardware, and round down. Practice on a roof with a chimney and a hip so the setback subtraction becomes routine.
Skipping the structural check of roof dead load and attachment pullout, assuming that if the roof is standing it can carry the array, and not verifying the mounting system's listed wind and snow ratings against the site's design values.
Candidates focus on electrical design and treat mechanical design as a vendor problem. The exam tests whether the candidate knows that dead load, uplift, and attachment capacity must be verified against the structure and that the racking's listed ratings must meet or exceed site design loads.
What to do instead
For every mechanical question, identify three numbers: array dead load per area, site ground snow load and design wind speed, and the racking manufacturer's listed ratings. Confirm each listed rating meets or exceeds the site value, and confirm attachments are rated for the roof assembly. If a number is missing from the stem, flag it rather than assuming adequacy.
Performing a site assessment that records only roof dimensions and shading, omitting service panel rating and busbar rating, main breaker size, utility transformer and service conductor details, and the interconnection method.
Candidates treat site assessment as a roof measurement exercise because that is the visible part of the job, but the design cannot be completed without the electrical service data, and the exam tests whether the candidate knows which field data drive the design.
What to do instead
Use a standardized site survey form with mandatory fields: service voltage and phases, main breaker rating, busbar rating, panel make and model, meter location, available breaker spaces, service conductor size, utility transformer size if visible, and proposed interconnection point. Refuse to close the survey until every field is filled or explicitly marked as unavailable with a plan to obtain it.
Measuring or modeling shading only at one time of day or one season, and using a generic shading percentage instead of an hourly or interval-based analysis tied to the specific obstructions and array geometry.
Candidates take a single handheld reading or apply a rule of thumb, which misses the seasonal solar path variation and the effect of near obstructions on specific rows. The exam tests whether the candidate knows that shading loss depends on time of year and array layout.
What to do instead
Require a solar-path-based analysis: record obstruction azimuth, elevation, and distance, then evaluate shading hour by hour across the year for the actual array geometry, and apply the resulting losses per string or per row rather than a single system-wide number. Cross-check the model against a physical shade measurement at a known time.
Producing a permit package with a single-line diagram that omits the grounding and bonding conductors, the rapid shutdown equipment, the OCPD ratings, and the point of interconnection, or submitting a plan set whose calculations cannot be traced to the drawings.
Candidates treat the single-line diagram as a block diagram of major equipment and forget that the permitting authority checks the diagram against the load calculations and the code citations. Inconsistency between the drawing and the calculation sheet is a common rejection cause.
What to do instead
Adopt a standard single-line diagram template that always shows modules, string configuration, combiner, DC disconnect, inverter, AC disconnect, OCPD ratings, grounding electrode conductor and equipment grounding conductors, rapid shutdown device, and the utility interconnection point with the service panel rating. Before submission, trace every number on the drawing back to the calculation sheet and confirm the two match.
Treating arc-flash and shock hazard boundaries as generic safety talk rather than computing the incident energy and shock boundaries for the specific system, and failing to specify lockout/tagout and PPE for the DC side.
Candidates learn safety as general awareness and do not connect it to the design documents, but the exam tests whether the candidate knows that the design must document the hazard analysis, the available fault current, and the required PPE and procedures, and that DC conductors remain energized when the AC is off.
What to do instead
For each project, document the available fault current at the point of interconnection, the shock hazard boundaries, and the incident energy or arc-flash label requirement, then specify the lockout/tagout points and the PPE category for the tasks. Remember that opening the DC disconnect does not de-energize the array side, and state that explicitly in the safety notes.