E2 Motors and Transformers 2

Question 1

According to the NEC, what is the minimum branch-circuit conductor ampacity for a single motor with a full-load current of 40 A?

Accepted Answer

50 A

Answer

NEC 430.22 requires motor branch-circuit conductors to have an ampacity of not less than 125% of the motor's full-load current rating.

Question 2

A transformer nameplate lists an impedance of 5.75%. This value is primarily used to calculate:

Accepted Answer

Available fault current

Answer

Transformer impedance (%Z) is the key parameter for calculating available short-circuit current on the secondary side.

Question 3

What NEC article governs the installation of air-conditioning and refrigeration equipment motors?

Accepted Answer

Article 440

Answer

Article 440 covers motor-driven air-conditioning and refrigerating equipment, supplementing Article 430.

Question 4

A 480V delta–wye transformer secondary supplies a 208Y/120V panelboard. The neutral conductor on the secondary is derived from:

Accepted Answer

The wye-connected secondary center point

Answer

In a wye-configured secondary, the neutral is the center (star) point of the three wye windings connected to ground.

Question 5

Per NEC 430.52, the maximum rating of an inverse-time circuit breaker used as motor branch-circuit protection for a Design B squirrel-cage motor is:

Accepted Answer

250% of FLC

Answer

NEC Table 430.52 allows inverse-time breakers to be rated up to 250% of the motor FLC for Design B squirrel-cage motors.

(E2) E2 Commercial Electrical Inspector Practice Test