PC Transformers and Distribution Equipment 2 — Questions and Answers
Question 1: Per NEC 450.3, when a transformer has primary overcurrent protection set at no more than 125% of rated current, secondary overcurrent protection:
- Must always be provided at 125% of secondary rated current
- Is not required for transformers with a 2-wire secondary
- May be omitted if primary protection does not exceed 125% of primary rated current (Correct answer)
- Must be rated at 250% of secondary rated current
Correct answer: May be omitted if primary protection does not exceed 125% of primary rated current
NEC 450.3 Table 450.3(B) permits omission of secondary overcurrent protection for transformers 600V and under when primary protection does not exceed 125% of rated primary current.
Question 2: What is the primary difference between a switchboard and a metal-clad switchgear assembly?
- Switchboards are for outdoor use only; switchgear is for indoor use only
- Metal-clad switchgear is fully enclosed with drawout components providing greater protection than open-construction switchboards (Correct answer)
- Switchboards have a higher interrupting capacity than switchgear
- There is no practical electrical difference between the two assemblies
Correct answer: Metal-clad switchgear is fully enclosed with drawout components providing greater protection than open-construction switchboards
Metal-clad switchgear uses enclosed drawout circuit breakers with interlocks and bus barriers, providing higher protection levels than switchboards which are typically accessible from the front only.
Question 3: A 225 kVA, 480V/240V single-phase transformer operates at full load. What is the approximate full-load secondary current?
- 469A
- 938A (Correct answer)
- 270A
- 1,875A
Correct answer: 938A
Full-load secondary current = kVA × 1,000 ÷ secondary voltage = 225,000 ÷ 240 = 937.5A ≈ 938A.
Question 4: A K-rated transformer (e.g., K-13) is specifically designed to handle:
- High ambient temperatures above 50°C
- Non-linear loads that generate harmonic currents (Correct answer)
- Underground installation in wet or damp locations
- High available short-circuit fault currents
Correct answer: Non-linear loads that generate harmonic currents
K-rated transformers use heavier neutral conductors and upgraded windings to tolerate the additional eddy-current heating caused by harmonic currents from non-linear loads such as variable frequency drives and computers.
Question 5: In a typical commercial distribution system, primary distribution feeders in urban areas operate at what voltage range?
- 120/240V
- 480V
- 4,160V to 15,000V (Correct answer)
- 115,000V to 345,000V
Correct answer: 4,160V to 15,000V
Urban primary distribution feeders typically operate at medium voltages between 4,160V and 15,000V (commonly 4.16 kV, 12.47 kV, or 13.2 kV) before being stepped down to utilization voltage at the service point.
Question 6: Which transformer cooling designation indicates an oil-immersed, forced-oil-cooled, forced-air-cooled unit?
- ONAN
- ONAF
- OFAF (Correct answer)
- ODAF
Correct answer: OFAF
OFAF stands for Oil Forced (pumped oil circulation) / Air Forced (fans on radiators), distinguishing it from ONAN (natural oil, natural air convection cooling).
Question 7: When two transformers are to be operated in parallel, which condition is NOT required for proper load sharing?
- Same voltage ratio (turns ratio)
- Identical kVA ratings (Correct answer)
- Same impedance percentage (%Z)
- Same polarity and phase relationship
Correct answer: Identical kVA ratings
Transformers operating in parallel do not need identical kVA ratings; however, mismatched kVA ratings cause load sharing to be proportional to each unit's rating based on their impedances.
PC Transformers and Distribution Equipment 2
Per NEC 450.3, when a transformer has primary overcurrent protection set at no more than 125% of rated current, secondary overcurrent protection: