Transformers Level II Lesson 4: Understanding
Transformer Overcurrent Protection (22 Questions;
Third Year) – Qs & As
The National Electrical Code® (NEC) requirements found in Section 450.3 are
for the protection of ? . Right Ans - 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. Right Ans - 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.) Right Ans - 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) Right Ans - 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.) Right Ans - 451.59 A circuit breaker
Transformer Overcurrent Protection (22 Questions;
Third Year) – Qs & As
The National Electrical Code® (NEC) requirements found in Section 450.3 are
for the protection of ? . Right Ans - 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. Right Ans - 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.) Right Ans - 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) Right Ans - 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.) Right Ans - 451.59 A circuit breaker
