Motors and Transformers Flashcards

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

Answer: 50 A

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.

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

Answer: Available fault current

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

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

Answer: Article 440

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

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

Answer: The wye-connected secondary center point

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

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:

Answer: 250% of FLC

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

Which motor nameplate data item is used to determine the correct overload protection size per NEC 430.32?

Answer: Service factor and nameplate current

NEC 430.32 bases overload sizing on the motor's nameplate full-load amperes and service factor (or temperature rise).

A 75 kVA, 480–208Y/120V transformer is considered a separately derived system. What grounding electrode conductor size is required for a 4/0 AWG secondary conductor per NEC Table 250.66?

Answer: 2 AWG copper

NEC Table 250.66 specifies a 2 AWG copper grounding electrode conductor for secondary conductors sized 2/0–3/0 AWG (or 4/0 in some brackets).