PC Transformers and Distribution Equipment 1 — Questions and Answers
Question 1: A transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio (primary to secondary)?
- 2:1
- 4:1 (Correct answer)
- 8:1
- 6:1
Correct answer: 4:1
The turns ratio equals primary voltage divided by secondary voltage: 480V ÷ 120V = 4:1.
Question 2: Which NEC article governs the installation and protection of transformers?
- Article 230
- Article 450 (Correct answer)
- Article 480
- Article 410
Correct answer: Article 450
NEC Article 450 covers transformers, including overcurrent protection, installation requirements, and marking.
Question 3: A delta-wye transformer configuration is commonly used in electrical distribution primarily because it:
- Eliminates the need for overcurrent protection
- Provides voltage transformation and establishes a neutral point on the secondary (Correct answer)
- Reduces transformer losses to zero
- Requires no grounding on either winding
Correct answer: Provides voltage transformation and establishes a neutral point on the secondary
A delta-wye transformer steps voltage up or down and provides a neutral conductor on the wye secondary, essential for 208Y/120V or 480Y/277V distribution systems.
Question 4: What is the primary purpose of taps on a transformer primary winding?
- To increase the transformer's kVA rating
- To adjust secondary voltage output when primary voltage varies from nominal (Correct answer)
- To provide multiple secondary voltages simultaneously
- To reduce the transformer's impedance percentage
Correct answer: To adjust secondary voltage output when primary voltage varies from nominal
Taps allow the primary turns to be adjusted so the secondary voltage remains correct even when utility supply voltage is slightly above or below nominal.
Question 5: A 75 kVA single-phase transformer supplies a 60 kW resistive load at unity power factor. What is the approximate load percentage?
- 60%
- 75%
- 80% (Correct answer)
- 85%
Correct answer: 80%
At unity power factor, kVA equals kW, so the load percentage is 60 kW ÷ 75 kVA = 80%.
Question 6: Per NEC 450.8, which marking must appear on a transformer nameplate?
- Maximum ambient temperature only
- Rated kVA, frequency, primary and secondary voltage, and impedance (Correct answer)
- Only the primary voltage and kVA rating
- Manufacturer's serial number and country of origin only
Correct answer: Rated kVA, frequency, primary and secondary voltage, and impedance
NEC 450.8 requires transformer nameplates to include rated kVA, frequency, primary and secondary voltage, impedance (on transformers 25 kVA and larger), and other information.
Question 7: What does a transformer's impedance percentage (%Z) represent?
- The ratio of primary to secondary turns in the winding
- The percentage of rated primary voltage required to circulate full-load current with the secondary shorted (Correct answer)
- The transformer efficiency at rated full load
- The power factor of the transformer under normal load conditions
Correct answer: The percentage of rated primary voltage required to circulate full-load current with the secondary shorted
%Z is determined by measuring what percentage of rated primary voltage is needed to drive rated current through a bolted secondary short circuit, indicating voltage regulation and fault current contribution.
PC Transformers and Distribution Equipment 1
A transformer has a primary voltage of 480V and a secondary voltage of 120V.
What is the turns ratio (primary to secondary)?