IEC Transformers and Distribution Systems

Question 1

What is the turns ratio of a single-phase transformer with a 480V primary winding and a 120V secondary winding?

Accepted Answer

4:1

Answer

Turns ratio equals primary voltage divided by secondary voltage: 480V ÷ 120V = 4:1.

Question 2

A 25 kVA, single-phase, 480V/240V transformer is operating at full load. What is the approximate rated secondary current?

Accepted Answer

104A

Answer

Secondary current = kVA × 1,000 ÷ secondary voltage = 25,000 ÷ 240 ≈ 104A.

Question 3

Which three-phase transformer connection provides a neutral conductor on the secondary side?

Accepted Answer

Delta-Wye

Answer

A Delta-Wye transformer provides a neutral from the center point of the wye-connected secondary windings.

Question 4

According to NEC 450.3(B), a 480V transformer with a rated primary current of 9A requires primary overcurrent protection not exceeding what value?

Accepted Answer

15A

Answer

NEC allows 125% of 9A = 11.25A; since no standard device matches that value, the next standard size of 15A is permitted.

Question 5

What is the primary purpose of a K-rated transformer?

Accepted Answer

To handle harmonic currents from non-linear loads

Answer

K-rated transformers are built to withstand the additional heat generated by harmonic currents produced by non-linear loads such as computers and variable frequency drives.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC Transformers and Distribution Systems