PC Transformers and Distribution Equipment

Question 1

A transformer has a primary voltage of 480V and a secondary voltage of 120V. What is the turns ratio (primary to secondary)?

Accepted Answer

4:1

Answer

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?

Accepted Answer

Article 450

Answer

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:

Accepted Answer

Provides voltage transformation and establishes a neutral point on the secondary

Answer

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?

Accepted Answer

To adjust secondary voltage output when primary voltage varies from nominal

Answer

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?

Accepted Answer

80%

Answer

At unity power factor, kVA equals kW, so the load percentage is 60 kW ÷ 75 kVA = 80%.

(PC) Professional Electrical Contractor Practice Test

(PC) Professional Electrical Contractor Practice Test

PC Transformers and Distribution Equipment