Transformers Level II Lesson 4: Understanding Transformer Overcurrent Protection (22 Questions; Third Year)

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)

45.11 A circuit breaker 50 A circuit breaker

The National Electrical Code® (NEC) requirements found in Section 450.3 are for the protection of ? .

The windings of the transformer only

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 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.

10 A

An installation consists of a 37.5-kVA, 3-phase transformer, a 480-volt primary, and a 120/208-volt secondary. Calculate the maximum standard size of the primary OCPD that can be used where the transformer protection method is both primary and secondary protection.

110 A

For the installation in the previous question, calculate the minimum size THWN copper conductors required to supply the primary when the maximum standard size overcurrent protective device is applied to the transformer primary, where the transformer protection method is both primary and secondary protection.

2 AWG THWN min. size

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.

250 A circuit breaker

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.

3 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

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).

350 A circuit breaker

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 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

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.

450 A circuit breaker

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

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

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.

125%. Table 450.3(B)

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.)

451.59 A circuit breaker

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 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

An installation consists of a 75-kVA, 3-phase, 480-volt transformer, an inverse time circuit breaker (rated at not more than 125% of the secondary current) as secondary protection, a 175-ampere circuit breaker as primary protection, and 1 AWG THWN copper installed in a rigid metal conduit protected by the 175-ampere circuit breaker as a feeder. According to the NEC, this 175-ampere circuit breaker provides acceptable protection for the feeder conductors.

False