NABCEP Electrical Component Inspection 5 — Questions and Answers
Question 1: An inspector reviews commissioning records and notices that the measured Isc of a PV string is 18% higher than the nameplate Isc value. What is the most likely explanation?
- The inverter is incorrectly measuring current
- Irradiance at the time of measurement exceeded 1000 W/m² STC conditions (Correct answer)
- The modules have degraded, increasing current output
- The overcurrent protection devices are undersized
Correct answer: Irradiance at the time of measurement exceeded 1000 W/m² STC conditions
Isc increases proportionally with irradiance; measurements taken under conditions exceeding STC irradiance (e.g., on high-altitude clear days with reflected light) can produce Isc values higher than nameplate.
Question 2: During an inspection, a technician finds that the AC output conductors from the inverter to the AC disconnect are 8 AWG THWN-2 copper rated at 50A, and the AC disconnect has a 60A fuse. The inverter's output circuit breaker is rated at 30A. Is there an issue?
- No, the upstream 60A fuse protects the conductor
- Yes, the 60A fuse exceeds the conductor ampacity and must be replaced with a 50A fuse (Correct answer)
- Yes, the wire must be upsized to match the 60A fuse
- No, the inverter's 30A breaker protects the conductor from the inverter side
Correct answer: Yes, the 60A fuse exceeds the conductor ampacity and must be replaced with a 50A fuse
The overcurrent device protecting a conductor must not exceed the conductor's ampacity; 8 AWG THWN-2 copper is rated for 50A, so the 60A fuse in the AC disconnect violates this requirement.
Question 3: During inspection of a rooftop installation, you find that PV conductors run across a flat EPDM roof membrane without any protection or support. What is the primary concern?
- UV degradation of the roof membrane from conductor shading
- Abrasion damage to conductor insulation from roof traffic and thermal movement (Correct answer)
- The conductors will cause increased electrical resistance from heat absorption
- Unprotected conductors will interfere with roof drainage patterns
Correct answer: Abrasion damage to conductor insulation from roof traffic and thermal movement
Conductors on roof membranes are subject to insulation abrasion from foot traffic, thermal expansion, and debris, which can cause insulation damage and eventual ground faults.
Question 4: An inspector finds that a PV system uses a transformer-less inverter and the modules are frameless thin-film. Which grounding approach is required?
- Standard equipment grounding with a grounding electrode system only
- Functional ground connection per manufacturer requirements, not traditional EGC bonding (Correct answer)
- A resistance-grounded system with a 1 kΩ grounding resistor
- An isolated ground system with no connection to the building grounding electrode
Correct answer: Functional ground connection per manufacturer requirements, not traditional EGC bonding
Certain thin-film module technologies require a specific functional grounding polarity per manufacturer instructions to prevent potential-induced degradation (PID), overriding standard EGC approaches.
Question 5: During a system inspection, you find that a microinverter AC branch circuit has 15 microinverters, each with a maximum output of 230VA. The branch circuit breaker is rated at 20A at 240V single-phase. Does this comply with NEC 705.12?
- Yes, because 15 × 230VA ÷ 240V = 14.4A, which is within the 20A breaker rating
- No, the output current of 14.4A exceeds 80% of the 20A breaker (16A) and must be on a 20A breaker after applying the 125% continuous load rule (Correct answer)
- Yes, microinverter circuits are exempt from the 80% continuous load rule
- No, microinverter branch circuits are limited to 10 units per circuit regardless of current
Correct answer: No, the output current of 14.4A exceeds 80% of the 20A breaker (16A) and must be on a 20A breaker after applying the 125% continuous load rule
Inverter output circuits are considered continuous loads, so the calculated load (14.4A) must not exceed 80% of the overcurrent device rating (16A for a 20A breaker); 14.4A is within the 16A limit, making this installation compliant.
Question 6: A PV system inspection reveals that junction boxes on the roof have not been sealed and show evidence of water intrusion with oxidized connections inside. Which NEC article most directly governs enclosure weatherproofing requirements for this outdoor installation?
- NEC 110.28 requiring enclosures suitable for the environment (Correct answer)
- NEC 690.35 covering ungrounded systems
- NEC 250.66 covering grounding electrode conductor sizing
- NEC 230.70 covering service disconnecting means
Correct answer: NEC 110.28 requiring enclosures suitable for the environment
NEC 110.28 requires that enclosures be suitable for the environmental conditions where they are installed; outdoor PV junction boxes must have an appropriate NEMA/IP weatherproof rating.
Question 7: During a performance inspection of a 50 kW commercial PV system, you calculate a performance ratio (PR) of 0.62 when the typical expected value is 0.75–0.80. Which of the following is the LEAST likely cause of the low PR?
- Significant soiling on module surfaces
- Inverter clipping due to undersized inverter
- Modules operating at correct STC temperature (Correct answer)
- Shading from nearby equipment or structures
Correct answer: Modules operating at correct STC temperature
Modules operating at STC (25°C) would produce optimal output and a higher PR; elevated real-world operating temperatures reduce output and PR, not the other way around.
NABCEP Electrical Component Inspection 5
An inspector reviews commissioning records and notices that the measured Isc of a PV string is 18% higher than the nameplate Isc value.
What is the most likely explanation?