Transformers Level Ii Lesson 4:
Understanding Transformer Overcurrent
Protection (22 Questions; Third Year)
The National Electrical Code® (NEC) requirements found in Section 450.3 are for the
protection of ? . - correct answers✔✔The windings of the transformer only
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 answers✔✔125%. Table 450.3(B)
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 answers✔✔41.05 A
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 1 of Table 450.3(B)
Calculate the maximum standard size circuit breaker permitted for primary-only
protection using Table 450.3(B) - correct answers✔✔45.11 A circuit breaker
50 A circuit breaker
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 answers✔✔451.59 A circuit breaker
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 answers✔✔450 A circuit breaker
, 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 answers✔✔250 A circuit
breaker
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 answers✔✔350 A
circuit breaker
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
answers✔✔10 A
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 answers✔✔3 A
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.) - correct answers✔✔True
The 100-ampere circuit breaker in the previous question protects the transformer
according to Code. - correct answers✔✔True
- Determine if the 350-ampere circuit breaker protects the transformer
- Determine if the feeder is sized properly for the transformer load - correct answers✔✔-
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. - correct
answers✔✔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. - correct answers✔✔90 A
Understanding Transformer Overcurrent
Protection (22 Questions; Third Year)
The National Electrical Code® (NEC) requirements found in Section 450.3 are for the
protection of ? . - correct answers✔✔The windings of the transformer only
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 answers✔✔125%. Table 450.3(B)
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 answers✔✔41.05 A
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 1 of Table 450.3(B)
Calculate the maximum standard size circuit breaker permitted for primary-only
protection using Table 450.3(B) - correct answers✔✔45.11 A circuit breaker
50 A circuit breaker
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 answers✔✔451.59 A circuit breaker
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 answers✔✔450 A circuit breaker
, 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 answers✔✔250 A circuit
breaker
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 answers✔✔350 A
circuit breaker
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
answers✔✔10 A
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 answers✔✔3 A
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.) - correct answers✔✔True
The 100-ampere circuit breaker in the previous question protects the transformer
according to Code. - correct answers✔✔True
- Determine if the 350-ampere circuit breaker protects the transformer
- Determine if the feeder is sized properly for the transformer load - correct answers✔✔-
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. - correct
answers✔✔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. - correct answers✔✔90 A
