Solar Technical Training — United States

NEC-aligned calculation drills

The sizing math NABCEP expects you to perform without a spreadsheet.

Numbers you must be able to produce without a spreadsheet.

1. Maximum system voltage at record low temperature

   Confirm a string fits inside the inverter's maximum input voltage.

   • · Module Voc at STC: 41.2 V
   • · Temperature coefficient of Voc: −0.28 %/°C
   • · Record low ambient: −12 °C
   • · Modules per string: 14
   • · Inverter maximum input: 600 V

   Find: What is the corrected maximum string voltage, and does it comply?

   Answer: ≈ 620 V — does not comply; reduce to 13 modules

   1. Temperature delta from 25 °C: 25 − (−12) = 37 °C
   2. Voc correction: 1 + (0.0028 × 37) = 1.1036
   3. Corrected Voc per module: 41.2 × 1.1036 = 45.5 V
   4. String voltage: 45.5 × 14 = 636 V → exceeds 600 V
   5. 13 modules: 45.5 × 13 = 591 V → compliant with margin

   NEC 690.7 — maximum PV system voltage

2. Continuous current and OCPD sizing

   Size the PV source circuit overcurrent device.

   • · Module Isc: 11.3 A
   • · Single string per circuit

   Find: What is the minimum OCPD rating?

   Answer: ≈ 21.2 A → 25 A standard device

   1. PV source circuit current = Isc × 1.25 = 11.3 × 1.25 = 14.13 A
   2. OCPD = circuit current × 1.25 = 14.13 × 1.25 = 17.7 A
   3. Conductor ampacity after temperature and conduit-fill correction must exceed this
   4. Select the next standard size that also protects the conductor: 20 A or 25 A depending on the corrected ampacity.

   NEC 690.8 — circuit sizing and current

3. Busbar interconnection check

   Decide whether a load-side connection is permitted.

   • · Busbar rating: 200 A
   • · Main breaker: 200 A
   • · Proposed backfed PV breaker: 40 A

   Find: Is the interconnection allowed under the 120% allowance?

   Answer: No — 240 A exceeds the 240 A limit only if equal; recheck the sum

   1. 120% of busbar = 200 × 1.2 = 240 A
   2. Sum of supply devices = 200 + 40 = 240 A
   3. 240 A is not greater than 240 A, so the connection complies at the limit with zero margin
   4. Any future addition, or a 50 A PV breaker, would fail — consider a supply-side connection or a main breaker derate.

   NEC 705.12 — load-side source connections

4. Cold-temperature Voc correction

   Correct module Voc for the record low before counting modules per string.

   • · Module Voc at STC: 45.2 V
   • · Temperature coefficient of Voc: −0.30 %/°C
   • · Record low ambient: −15 °C

   Find: What corrected Voc is used for maximum system voltage?

   Answer: 50.6 V per module

   1. ΔT from 25 °C: 25 − (−15) = 40 °C
   2. Correction factor: 1 + (40 × 0.0030) = 1.12
   3. Corrected Voc: 45.2 × 1.12 = 50.62 V
   4. Divide the inverter maximum input by this figure to get the maximum modules per string.

   NEC 690.7 — maximum PV system voltage

5. String voltage for twelve modules at −10 °C

   Check a 12-module string against a 700 V inverter input limit.

   • · Module Voc at STC: 49.8 V
   • · Temperature coefficient of Voc: −0.29 %/°C
   • · Lowest expected ambient: −10 °C
   • · Modules per string: 12

   Find: What is the maximum string voltage?

   Answer: ≈ 658 V — inside a 700 V limit, outside a 600 V limit

   1. ΔT from 25 °C: 25 − (−10) = 35 °C
   2. Correction factor: 1 + (35 × 0.0029) = 1.1015
   3. Corrected Voc: 49.8 × 1.1015 = 54.85 V
   4. String: 54.85 × 12 = 658.2 V
   5. Against a 600 V limit, reduce to 10 modules (548 V).

   NEC 690.7 — maximum PV system voltage

6. Array power from module Vmp and Imp

   Confirm nameplate array power independently of the arrangement.

   • · Module Vmp: 41.2 V
   • · Module Imp: 10.8 A
   • · 18 modules in 3 strings of 6

   Find: What is the nominal array power at STC?

   Answer: ≈ 8.01 kW

   1. Module power: 41.2 × 10.8 = 445 W
   2. Array power: 445 × 18 = 8,010 W = 8.01 kW
   3. Array Vmp: 41.2 × 6 = 247 V; array Imp: 10.8 × 3 = 32.4 A
   4. Cross-check: 247 × 32.4 = 8,003 W — same answer from the other direction.

NEC decision drills — ampacity, rapid shutdown and interconnection

Four numbers an inspector or a PVIP examiner can ask for cold. Show the correction factors; an unsupported answer scores nothing.

Numbers you must be able to produce without a spreadsheet.

NEC Articles 690 and 705 as applied in PVIP-style judgment questions.

1. Conductor ampacity with temperature and conduit-fill correction

   PV source circuit conductors run in conduit on the roof, in the sun, with other current-carrying conductors.

   • · Module Isc = 11.0 A
   • · Six current-carrying conductors in the raceway
   • · Ambient design temperature 45 °C, conduit within 20 mm of the roof surface
   • · 90 °C rated conductor, base ampacity 40 A for the size under consideration
   • · Temperature correction factor at the corrected ambient = 0.71
   • · Adjustment factor for 6 conductors = 0.80

   Find: What continuous-current requirement must the conductor meet, and does the candidate conductor pass?

   Answer: Required 20.6 A minimum ampacity after 125% × 125%; corrected conductor ampacity 22.7 A — it passes, with little margin.

   1. Maximum circuit current = 1.25 × Isc = 1.25 × 11.0 = 13.75 A (690.8(A)).
   2. Continuous-duty conductor sizing = 1.25 × 13.75 = 17.19 A before corrections (690.8(B)).
   3. Corrected conductor ampacity = 40 A × 0.71 × 0.80 = 22.7 A.
   4. 22.7 A exceeds both the 17.19 A continuous requirement and the 13.75 A maximum circuit current, so the conductor is acceptable.
   5. Margin is thin: raise the raceway off the roof or upsize if the design ambient is revised upward.

   NEC 690.8(A), 690.8(B), 310.15

2. OCPD selection for the same source circuit

   Select the overcurrent device protecting the PV source circuit above.

   • · Module Isc = 11.0 A
   • · Module series fuse rating = 20 A
   • · Standard device ratings available: 15, 20, 25, 30 A

   Find: What OCPD rating do you select?

   Answer: 20 A.

   1. Minimum OCPD = 1.25 × 1.25 × Isc = 1.5625 × 11.0 = 17.19 A.
   2. Round up to the next standard rating: 20 A.
   3. Check the module series fuse rating: 20 A is not exceeded, so 20 A is acceptable.
   4. A 25 A device would exceed the module series fuse rating and is not permitted.

   NEC 690.9, 240.6

3. Busbar loading on a load-side interconnection

   A PV backfeed breaker is added to an existing service panel at the opposite end from the main.

   • · Panel busbar rating 200 A
   • · Main breaker 200 A
   • · Inverter continuous output 32 A

   Find: What is the largest backfeed breaker permitted, and does this installation comply?

   Answer: The 120% rule allows 40 A of backfeed. The required 40 A breaker for a 32 A inverter fits exactly, so it complies at the opposite end of the bus.

   1. Allowed sum of supply devices = 1.2 × 200 = 240 A (705.12(B)(3)(2)).
   2. Subtract the main: 240 − 200 = 40 A available for backfeed.
   3. Required breaker for the inverter = 1.25 × 32 = 40 A.
   4. 40 A permitted equals 40 A required, so it complies — and only if the breaker sits at the opposite end of the busbar from the main supply.
   5. A 40 A busbar-warning label is required at the panel.

   NEC 705.12(B)

4. Rapid shutdown verification — what the reading has to prove

   You initiate rapid shutdown and take readings inside the array boundary and at the conductors leaving it.

   • · Array boundary defined as 1 ft from the array
   • · Reading taken 30 s after initiation outside the boundary: 22 V
   • · Reading taken 30 s after initiation inside the boundary: 78 V

   Find: Does the system pass, and what does each reading tell you?

   Answer: It fails. Outside the boundary must be ≤ 30 V within 30 s — 22 V passes. Inside the boundary must be ≤ 80 V, and 78 V passes, but only if that is the controlled value at every module; verify the function per module rather than at one point.

   1. 690.12(B)(1): conductors outside the array boundary must be limited to ≤ 30 V within 30 s of initiation. 22 V satisfies it.
   2. 690.12(B)(2): conductors inside the boundary must be limited to ≤ 80 V within 30 s. 78 V satisfies it at the point measured.
   3. Both readings pass at the points measured, so the correct finding is 'not yet verified', not 'compliant'.
   4. Verify the shutdown function itself at each device — an absent reading at one point can also mean an open connection, which is a fault, not compliance.

   NEC 690.12(B)

NABCEP practice questions

Code-aligned questions across safety, design verification, commissioning and troubleshooting.

Exam-style questions with the reasoning behind every answer.

Supports the NABCEP Associate → PV Installer / PV Installation Professional (PVIP) progression.

Practise with Maya

Domain-weighted PVIP practice — judgment and decision order

Questions tagged to the exam domains: site assessment, design and installation, commissioning, and safety. Each one asks which decision comes first, not merely which fact is true.

Exam-style questions with the reasoning behind every answer.

Weighted towards the PVIP job task analysis domains.

Practise with Maya

NABCEP-aligned service method

The method behind the exam objectives: safety, verification against design, and a commissioning record an inspector can follow.

What an assessor is actually watching while you work.

1. 1. Select PPE and fall protection appropriate to the task and the roofSafety-fail

   Arc-flash and fall exposure are assessed alongside electrical method.

2. 2. Verify the rapid shutdown function operates, not just that voltage is absentSafety-fail

   Absence of voltage can have another cause. The function itself is the requirement.

3. 3. Lockout/tagout every disconnect and prove dead with a proved meterSafety-fail

   The universal automatic fail; the required order does not change with jurisdiction.

4. 4. Verify the as-built system against the permitted plan set

   String counts, conductor sizes and OCPD ratings that differ from the plans are the inspector's first question.

5. 5. Confirm required placards and labels are present, correct and durable

   Labeling is an inspected deliverable and the most common re-inspection item.

6. 6. Verify equipment grounding and bonding continuity end to endSafety-fail

   A single missed bonding jumper undermines the protective scheme.

7. 7. Record commissioning measurements with the conditions they were taken in

   It is the baseline every future O&M comparison depends on.

NABCEP PVIP timed mock

Ninety minutes across code, commissioning and troubleshooting.

Sit it against the clock and read the score report honestly.

Aligned to the PV Installation Professional (PVIP) job task analysis.

U.S. commissioning and inspection pack

The records an AHJ inspection and a later warranty claim both rely on.

The paperwork, filled in the way it will be audited.

• Commissioning report

  Baseline performance and code-compliance evidence at handover.

  • · Plan set revision and as-built deviations
  • · String configuration, Voc and operating current per string
  • · Rapid shutdown functional verification
  • · Grounding and bonding continuity results
  • · Insulation resistance results
  • · Labeling and placard verification
  • · Conditions: irradiance, ambient and module temperature, time
  Open the printable document →

• Ground-fault investigation report

  Documents a fault that repeats, so the next visit starts where this one ended.

  • · Trip history and weather correlation
  • · Isolation performed and verified
  • · Insulation resistance by string, wet and dry
  • · Located fault, cause and remediation
  • · Post-repair verification
  Open the printable document →

Shared diagnostic core — practice questions

Method and safety questions phrased the way each region's examiners phrase them.

Exam-style questions with the reasoning behind every answer.

Practise with Maya

Proving isolation on a hybrid system

The isolation method every region assesses, in the order it is assessed. Switching something off is not isolation; isolation is a state you prove and keep proving.

What an assessor is actually watching while you work.

1. 1. Identify every energy source before touching anythingSafety-fail

   PV, battery, grid and any generator are separate sources. Missing one is how people get hurt on a system that 'was off'.

2. 2. Shut down in the manufacturer's stated orderSafety-fail

   Most hybrids specify inverter to standby, then AC, then PV, then battery. The order decides where energy is left trapped.

3. 3. Prove the instrument on a known live source before and after testingSafety-fail

   Testers fail silently. A dead reading from an unproven instrument is not evidence.

4. 4. Test every combination at the point of workSafety-fail

   Positive to earth, negative to earth and positive to negative on DC; line to neutral and line to earth on AC. One reading is not a proof.

5. 5. Lock and tag every isolator you operated, and keep the key

   A note on the panel does not stop a homeowner or a colleague restoring power while you are inside the equipment.

6. 6. Respect the stated DC-link discharge time, then verify it

   Capacitors hold charge after isolation. The waiting time is a minimum, not a guarantee.

7. 7. Re-prove dead after any interruption or absence from the work area

   Isolation you did not personally maintain is isolation you no longer own.

8. 8. Photograph the isolation state and the applied lock

   It is a safety record and diagnostic evidence at the same time, and it is what a defensible job file rests on.

The systematic diagnostic sequence

One order of work, used on every call, in every region. It is the order that keeps a hard fault from costing a whole day.

What an assessor is actually watching while you work.

1. 1. Write the symptom as reported, then separate it from any assumed cause

   'The inverter is broken' is a conclusion, not a symptom. Assessors mark whether you can tell them apart.

2. 2. Prove isolation before any intrusive workSafety-fail

   The single most common automatic fail across EPRA, NABCEP and QCTO practice.

3. 3. Observe a full start-up cycle before intervening

   Where the sequence stops names the gating condition that failed — and eliminates everything downstream of it.

4. 4. Take measurements that answer a specific question

   Random voltage checks score poorly everywhere. Each measurement should eliminate a branch.

5. 5. Compare output against a condition-corrected expectation, never nameplate

   Without irradiance and cell temperature you cannot yet say whether there is a fault at all.

6. 6. Eliminate the simple high-probability faults before the expensive part

   Connections, settings, soiling and shading account for most calls. Methodical elimination beats confident guessing.

7. 7. Verify the repair under the conditions that produced the fault

   A fault that only appears after rain or at midday is not fixed because it is quiet at 9am.

8. 8. Record what you actually measured, including inconvenient numbers

   Inflated commissioning sheets are recognised instantly and destroy credibility.

Core method mock — 45 minutes

A timed check on the shared core before you sit a region mock. Method and safety only; no regional codes.

Sit it against the clock and read the score report honestly.

Faults that waste a whole day

The handful of misdiagnoses that account for most wasted visits — the correct call, and the reasoning that usually wins instead.

Correct answer, why it is correct, and the mistakes people make.

• Inverter showing a generic fault code

  Verify DC voltage, polarity, earthing and battery voltage against the inverter's operating window before condemning the unit.

  A fault code names the protection that operated, not the cause. Most are downstream of a DC-side or storage condition.

  • × Replacing the inverter first and inheriting the same code
  • × Reading the display instead of measuring at the terminals
  • × Ignoring the fault-history timestamps that would have shown the correlation

• Battery not charging, or charging very slowly

  Measure voltage at the battery terminals under charge and compare it with the voltage at the charge source. The difference tells the story.

  Charge settings, high-resistance connections, temperature sensing and one weak cell in a bank all present the same way.

  • × Replacing the bank without checking the configured charge profile
  • × Missing a single weak battery dragging a string down
  • × Treating a BMS temperature limit as a hardware fault

• Worked yesterday, dead today

  Visual inspection plus systematic voltage checks from source to load, looking for an intermittent connection, a loose connector, a failed surge device or a breaker that was never properly reset.

  Sudden total loss is almost always a connection or a protective device, not a degraded component.

  • × Starting with the most accessible component instead of the source-to-load path
  • × Resetting a breaker without asking what tripped it

• 'Low power' on an otherwise working system

  String-by-string current comparison under similar irradiance isolates it in under thirty minutes.

  Shading, soiling, mismatch or one underperforming string all read as generic underproduction at the meter.

  • × Jumping to inverter replacement
  • × Comparing against nameplate rather than a condition-corrected expectation

• Nuisance tripping of residual-current or earth-leakage protection

  Insulation-resistance testing plus visual inspection of terminations and cable routes, correlated with weather.

  Moisture ingress, damaged insulation and incorrect earthing arrangements trip protection that is working correctly.

  • × Replacing the protective device
  • × Testing at midday when the fault only appears after overnight rain

Safety-fail scenario labs

Graded labs from the curriculum where an unsafe selection ends the attempt. Work these until the order is automatic.

Graded scenario labs — a safety-critical error still fails outright.

• Isolating a storage system in the right order

  Storage holds energy independently of sun and grid. The order is the assessment.

• Isolation fault — finding the string without becoming the path

  Weather-correlated isolation faults reward patience and punish shortcuts.

• Capstone stage 1 — arrival and safety

  A safety failure resets the stage rather than letting you carry on.

Commissioning, service and as-built records

The three documents every region expects. Printable, and written to be read by someone who was not there.

The paperwork, filled in the way it will be audited.

• Commissioning sheet

  Proves the system was tested to spec, under stated conditions, on a stated date.

  • · Site and system identification
  • · Array and string configuration as built
  • · Isolation and prove-dead record
  • · Measured values with irradiance and temperature
  • · Protection settings and grid parameters applied
  • · Defects, deviations and outstanding items
  • · Technician declaration and signature
  Open the printable document →

• Service report

  Records the reported symptom, what was measured, what was decided and what was verified.

  • · Reported symptom in the customer's words
  • · Conditions on arrival
  • · Checks performed and results
  • · Diagnosis and the evidence supporting it
  • · Work carried out and parts used
  • · Verification after repair
  • · Advice given and follow-up required
  Open the printable document →

• As-built notes

  Captures what is actually installed once the design met the roof, so the next technician is not re-surveying.

  • · Array layout with string routing and module counts
  • · Isolator and disconnect locations
  • · Cable types, routes and protection
  • · Earthing and bonding arrangement
  • · Equipment serials and firmware versions
  • · Deviations from the approved design, with reasons