NABCEP Electrical & Mechanical Concepts 4 — Questions and Answers
Question 1: What is the purpose of anti-islanding protection in a grid-tied PV inverter?
- Prevent theft of electricity
- Shut down the inverter if the grid de-energizes (Correct answer)
- Boost output during peak demand
- Balance loads across phases
Correct answer: Shut down the inverter if the grid de-energizes
Anti-islanding protection disconnects the inverter from an energized output if the utility grid fails, protecting line workers.
Question 2: What is the formula for calculating voltage drop across a conductor?
- Vd = I × R (Correct answer)
- Vd = I / R
- Vd = V × R
- Vd = P / I
Correct answer: Vd = I × R
Voltage drop equals current (I) multiplied by the conductor resistance (R) per Ohm's Law.
Question 3: Which wire gauge has a larger cross-sectional area: 10 AWG or 6 AWG?
- 10 AWG
- 6 AWG (Correct answer)
- They are the same
- Depends on conductor material
Correct answer: 6 AWG
In the AWG system, lower numbers indicate larger wire diameters and greater cross-sectional area.
Question 4: What happens to PV module Voc as cell temperature increases?
- Voc increases
- Voc remains constant
- Voc decreases (Correct answer)
- Voc doubles
Correct answer: Voc decreases
Voc has a negative temperature coefficient, so it decreases as module temperature rises.
Question 5: A residential PV system uses USE-2 cable on the roof. What does USE-2 indicate?
- Utility Service Entry, 2-conductor
- Underground Service Entrance, 2000V rated
- Underground Service Entrance, dual-rated for sunlight resistance (Correct answer)
- Ultra Solar Efficiency, Class 2
Correct answer: Underground Service Entrance, dual-rated for sunlight resistance
USE-2 is a dual-rated cable suitable for underground service entrances and also rated for direct sunlight exposure, common in PV source circuits.
Question 6: What is the role of a combiner box in a large PV array?
- Convert DC to AC
- Combine multiple source circuit outputs into a single output circuit (Correct answer)
- Provide battery backup connections
- Monitor module-level performance
Correct answer: Combine multiple source circuit outputs into a single output circuit
A combiner box aggregates the output of multiple PV source circuits (strings) into one output circuit feeding the inverter.
Question 7: What is the primary reason for limiting PV system DC voltage to 600 V in residential applications per the NEC?
- Inverter efficiency peaks at 600 V
- Listed equipment and wiring methods are readily available for ≤600 V systems (Correct answer)
- Battery banks operate at 600 V
- Utility interconnection requires 600 V maximum
Correct answer: Listed equipment and wiring methods are readily available for ≤600 V systems
600 V is a common threshold in NEC Article 690 because widely available listed components, connectors, and wiring are rated for this voltage.
NABCEP Electrical & Mechanical Concepts 4
What is the purpose of anti-islanding protection in a grid-tied PV inverter?