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
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