E2 Motors and Transformers 3

Question 1

A 3-phase, 460V motor draws 28 A at full load. What is the minimum ampacity of the disconnecting means per NEC 430.110?

Accepted Answer

35 A

Answer

NEC 430.110 requires the motor disconnecting means to have an ampere rating of at least 115% of the motor's full-load current.

Question 2

What is the purpose of a motor's locked-rotor code letter on the nameplate?

Accepted Answer

It identifies the kilovolt-ampere input per horsepower at locked rotor

Answer

NEC Table 430.7(B) lists locked-rotor code letters that represent kVA/HP ranges used to size motor branch-circuit protection.

Question 3

Per NEC 450.9, the minimum ventilation clearance around a dry-type transformer rated over 112.5 kVA installed indoors must allow for adequate:

Accepted Answer

Cooling air circulation

Answer

NEC 450.9 requires transformers to be installed so that cooling is not blocked, ensuring thermal dissipation from the windings.

Question 4

A 100 HP, 460V, 3-phase motor has a FLC of 124 A per Table 430.250. Using dual-element time-delay fuses, what is the maximum fuse rating per NEC Table 430.52?

Accepted Answer

225 A

Answer

NEC Table 430.52 allows dual-element time-delay fuses up to 175% of FLC, and 175% × 124 A = 217 A, rounded up to the next standard size of 225 A.

Question 5

What transformer connection produces a 30-degree phase shift between primary and secondary voltages?

Accepted Answer

Delta–wye

Answer

A delta–wye (or wye–delta) transformer connection inherently produces a 30-degree phase displacement between primary and secondary line voltages.

(E2) E2 Commercial Electrical Inspector Practice Test

(E2) E2 Commercial Electrical Inspector Practice Test

E2 Motors and Transformers 3