CME Electrical Systems Design & Installation 3 — Questions and Answers
Question 1: When installing a 1000 kVA, 480-208Y/120V transformer, what minimum clearance must be maintained from combustible material per NEC 450.21(A)?
- 6 inches
- 12 inches
- 300mm (approximately 12 in)
- No clearance required if ventilated (Correct answer)
Correct answer: No clearance required if ventilated
NEC 450.21(A) permits dry-type transformers rated 112.5 kVA or less to be installed with no separation from combustible material if enclosed; larger units require 12 inches or fire-resistant construction.
Question 2: Per NEC 220.87, what method allows an existing service or feeder to be evaluated for additional load without full load calculation?
- Demand factor method
- Optional calculation method
- Measured demand method using 12-month data (Correct answer)
- Connected load method
Correct answer: Measured demand method using 12-month data
NEC 220.87 permits using the maximum demand data from the previous 12-month period measured at the meter to determine available capacity for new loads.
Question 3: In a switchboard installation, what is the minimum headroom required in front of the equipment per NEC 110.26(A)(3)?
- 5.5 feet
- 6 feet
- 6.5 feet (Correct answer)
- 7 feet
Correct answer: 6.5 feet
NEC 110.26(A)(3) requires a minimum headroom of 6.5 feet (2.0 m) in front of electrical equipment 600V or less.
Question 4: A photovoltaic system is connected to a 200A residential service panel. Per NEC 705.12(B)(3), what is the maximum breaker size allowed for the PV interconnection?
- 20A
- 40A (Correct answer)
- 60A
- No limit if bus is rated
Correct answer: 40A
NEC 705.12(B)(3) limits the PV supply breaker to 120% of the bus rating minus the main breaker; for a 200A panel with 200A bus, maximum PV breaker = (200A × 120%) - 200A = 40A.
Question 5: What is the purpose of an equipment bonding jumper in an electrical system per NEC Article 250?
- Connect the neutral to ground at the panel
- Bond non-current-carrying metal parts to the grounding system (Correct answer)
- Size the grounding electrode conductor
- Provide a return path for fault current through the utility
Correct answer: Bond non-current-carrying metal parts to the grounding system
Equipment bonding jumpers connect non-current-carrying metal parts (conduit, enclosures, equipment frames) to the equipment grounding conductor to ensure fault current can flow safely.
Question 6: When designing an industrial control panel with multiple 480V motor starters, what NEC article specifically governs the internal wiring and component requirements?
- Article 409 (Correct answer)
- Article 430
- Article 440
- Article 445
Correct answer: Article 409
NEC Article 409 covers industrial control panels including short-circuit current ratings, wiring methods, and component requirements for motor control centers and industrial panels.
Question 7: A 3-phase, 4-wire, 208Y/120V panelboard serves a mixed load. Per NEC 408.36, what is the maximum number of overcurrent devices (poles) permitted in a lighting and appliance branch circuit panelboard?
- 24 poles
- 30 poles
- 42 poles (Correct answer)
- No maximum specified
Correct answer: 42 poles
NEC 408.36 limits lighting and appliance branch circuit panelboards to a maximum of 42 overcurrent devices (counting each pole of a multi-pole breaker).
CME Electrical Systems Design & Installation 3
When installing a 1000 kVA, 480-208Y/120V transformer, what minimum clearance must be maintained from combustible material per NEC 450.21(A)?