Transformers Level II Lesson 4: Understanding
Transformer Overcurrent Protection (22 Questions;
Third Year) questions and answers
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.)✔✔True
The 100-ampere circuit breaker in the previous question protects the transformer according to
Code.✔✔True
- Determine if the 350-ampere circuit breaker protects the transformer
- Determine if the feeder is sized properly for the transformer load✔✔- Yes
- Yes
For an installation with a 125-kVA, 3-phase transformer, a 240-volt primary, and THWN copper
conductors, determine the maximum size OCPD permitted that will protect the conductors and
the primary of the transformer where the protection method is primary-only protection. There is
more than one option here, so be sure to select only the maximum circuit breaker and wire
combination that is NEC compliant.✔✔400A circuit breaker with 500 kcmil THWN copper
, For an installation with a 25-kVA, 3-phase transformer, a 208-volt primary, and THWN copper
conductors, determine the maximum size OCPD permitted that will protect the 4 AWG THWN
copper conductors and the primary of the transformer where the method is primary-only
protection. This solution works without the need for additional options. Note: All conductor
terminations are rated 75°C.✔✔90 A
For an installation using primary and secondary protection for a 35-kVA, 3-phase transformer
and a 240-volt, 3-phase, 3-wire secondary, calculate the maximum rating of the secondary OCPD
using Table 450.3(B), Note 1.✔✔110 A
Using non-time-delay fuses, calculate the maximum standard rating for the secondary
overcurrent protective device of a transformer with a secondary current of five amperes. The
transformer is rated 480/120 volts, single-phase, and primary and secondary protection is
provided.✔✔6 A
For an installation with a 45-kVA, 3-phase transformer, a 460-volt primary, a 220-volt secondary,
and secondary protection where the transformer secondary overcurrent protection does not
exceed 125% of the secondary current, determine the maximum standard rating of the primary
feeder OCPD, where primary and secondary overcurrent protection are provided for the
transformer.✔✔125 A
For an installation with a 5-kVA, single-phase transformer, a 480-volt primary, a 220-volt
secondary, 25-ampere fuses as secondary protection, and a 30-ampere circuit breaker as primary
winding and feeder protection, the 30-ampere circuit breaker provides the Code-required primary
protection at not more than 250% of the rated primary current.✔✔False
Transformer Overcurrent Protection (22 Questions;
Third Year) questions and answers
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.)✔✔True
The 100-ampere circuit breaker in the previous question protects the transformer according to
Code.✔✔True
- Determine if the 350-ampere circuit breaker protects the transformer
- Determine if the feeder is sized properly for the transformer load✔✔- Yes
- Yes
For an installation with a 125-kVA, 3-phase transformer, a 240-volt primary, and THWN copper
conductors, determine the maximum size OCPD permitted that will protect the conductors and
the primary of the transformer where the protection method is primary-only protection. There is
more than one option here, so be sure to select only the maximum circuit breaker and wire
combination that is NEC compliant.✔✔400A circuit breaker with 500 kcmil THWN copper
, For an installation with a 25-kVA, 3-phase transformer, a 208-volt primary, and THWN copper
conductors, determine the maximum size OCPD permitted that will protect the 4 AWG THWN
copper conductors and the primary of the transformer where the method is primary-only
protection. This solution works without the need for additional options. Note: All conductor
terminations are rated 75°C.✔✔90 A
For an installation using primary and secondary protection for a 35-kVA, 3-phase transformer
and a 240-volt, 3-phase, 3-wire secondary, calculate the maximum rating of the secondary OCPD
using Table 450.3(B), Note 1.✔✔110 A
Using non-time-delay fuses, calculate the maximum standard rating for the secondary
overcurrent protective device of a transformer with a secondary current of five amperes. The
transformer is rated 480/120 volts, single-phase, and primary and secondary protection is
provided.✔✔6 A
For an installation with a 45-kVA, 3-phase transformer, a 460-volt primary, a 220-volt secondary,
and secondary protection where the transformer secondary overcurrent protection does not
exceed 125% of the secondary current, determine the maximum standard rating of the primary
feeder OCPD, where primary and secondary overcurrent protection are provided for the
transformer.✔✔125 A
For an installation with a 5-kVA, single-phase transformer, a 480-volt primary, a 220-volt
secondary, 25-ampere fuses as secondary protection, and a 30-ampere circuit breaker as primary
winding and feeder protection, the 30-ampere circuit breaker provides the Code-required primary
protection at not more than 250% of the rated primary current.✔✔False
