Transformers Level II Lesson 4:
Understanding Transformer Overcurrent Protection (22 Questions; Third Year)
Question 1: The National Electrical Code® (NEC)
requirements found in Section 450.3 are for the protection of ?
CORRECT ANSWER : The windings of the transformer only
Question 2: Where primary-only protection for transformers
is provided with rated currents of nine amperes or more, the basic rule requires that each transformer rated 1,000 volts, nominal, or less be protected by an individual overcurrent protective device on the primary side, rated or set at not more than ? of the rated transformer primary current.
CORRECT ANSWER : 125%. Table 450.3(B)
Question 3: For an installation with a 25-kVA, 3-phase
transformer, a 440-volt primary, and a 120-volt secondary, calculate the maximum overcurrent protection value (not the standard fuse or circuit breaker rating) using the protection
method for primary-only protection. (Round the final answer to two decimal places.)
CORRECT ANSWER : 41.05 A
Question 4: An installation consists of a 30-kVA, 3-phase
transformer, a 480-volt primary, and a 240-volt secondary.Calculate the largest standard size circuit breaker permitted for primary-only protection to be used without applying Note
- of Table 450.3(B)
CORRECT ANSWER : 45.11 A circuit breaker
Question 5: An installation consists of a 30-kVA, 3-phase
transformer, a 480-volt primary, and a 240-volt secondary.Calculate the maximum standard size circuit breaker permitted for primary-only protection using Table 450.3(B)
CORRECT ANSWER : 50 A circuit breaker
Question 6: An installation consists of a 300-kVA, 3-phase
transformer, a 480-volt primary, and a 240-volt secondary.Calculate the maximum overcurrent protection value (not the fuse or circuit breaker rating) using the protection method for
primary-only protection. (Round the final answer to two decimal places.)
CORRECT ANSWER: 451.59 A circuit breaker
Question 7: For the installation in the previous question,
calculate the maximum standard size circuit breaker permitted for primary-only protection to be used without application of Table 450.3(B), Note 1.
CORRECT ANSWER : 450 A circuit breaker
Question 8: For an installation with a 150-kVA, 3-phase
transformer, a 480-volt primary, and a 240-volt secondary, calculate the maximum standard size circuit breaker permitted for primary-only protection using Table 450.3(B), Note 1.
CORRECT ANSWER : 250 A circuit breaker
Question 9: For an installation with a 225-kVA, 3-phase
transformer and a 480-volt primary, determine the maximum standard size circuit breaker permitted where the method of primary-only protection is used according to Table 450.3(B).
CORRECT ANSWER : 350 A circuit breaker
Question 10: For an installation with a 6-kVA, 3-phase
transformer and a 480-volt primary, determine the maximum standard size fuse permitted using Table 450.3(B), for primary-only protection, where transformer primary currents are less than nine amperes.
CORRECT ANSWER : 10 A
Question 11: For an installation with a primary current of 1.75
amperes, determine the maximum standard size fuse permitted using Table 450.3(B), for primary-only protection, where the primary currents are less than two amperes.
CORRECT ANSWER : 3 A
Question 12: A 75-kVA, 3-phase transformer, 480-volt
primary is supplied by 3 AWG THW copper feeder conductors and protected by a 100-ampere (inverse time) circuit breaker. The feeder conductors supplying the transformer are sized according to the Code. (All conductors and terminations are marked 75°C.)