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TRANSFORMERS
LEVEL
II
LESSON
4:
UNDERSTANDING
TRANSFORMER
OVERCURRENT PROTECTION
(22
QUESTIONS;
THIRD
YEAR)
1.The National Electrical Code® (NEC) requirements found in Section 450.3 are for the
protection of ? .: The windings of the transformer only
2.Where primary-only protection for transformers is provided with rated cur- rents 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.:
125%. Table 450.3(B)
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.)-
: 41.05 A
