Transformers and Distribution Equipment Flashcards

A dry-type transformer with an 80°C rise rating is installed in a room with a 40°C ambient temperature. What is the maximum operating winding temperature?

Answer: 120°C

Maximum winding temperature equals ambient temperature plus temperature rise: 40°C + 80°C = 120°C, which must remain within the insulation class rating.

Per NEC 450.21(A), a dry-type transformer rated 112.5 kVA or less may be installed indoors without a transformer vault if it is:

Answer: Separated from combustible material by at least 12 inches or by a fire-resistant heat-insulating barrier

NEC 450.21(A) permits dry-type transformers 112.5 kVA or less to be installed indoors if separated from combustible materials by at least 300 mm (12 in.) or by a fire-resistant, heat-insulating barrier.

The available short-circuit current on the secondary of a transformer is primarily determined by:

Answer: The transformer's impedance (%Z) and the secondary voltage and current ratings

Secondary short-circuit current is calculated as: I_sc = I_FL ÷ (%Z / 100), so both the transformer's full-load current (from kVA and voltage) and its impedance directly determine available fault current.

A transfer switch in an emergency power system serves what primary function?

Answer: To automatically or manually switch electrical loads from the normal source to the emergency power source

A transfer switch disconnects loads from the normal utility source and connects them to the emergency source (such as a generator), ensuring loads are never connected to both sources simultaneously.

Per NEC 450.9, indoor transformer ventilation must be adequate to:

Answer: Limit the temperature rise of the transformer to its nameplate rating under full-load conditions

NEC 450.9 requires that ventilation be provided so that transformer temperature does not exceed its nameplate temperature rise under full-load operation.

What is the purpose of a zig-zag (interconnected-wye) transformer used in a distribution system?

Answer: To derive a neutral point and suppress third-harmonic currents in ungrounded or delta systems

A zig-zag transformer provides a low-impedance path for zero-sequence (ground fault) currents to establish a neutral reference in delta or ungrounded systems and helps mitigate third-harmonic voltages.

When calculating the available fault current at a panelboard fed from a transformer secondary, which transformer parameter must be obtained from the nameplate?

Answer: Impedance voltage percentage (%Z)

The impedance percentage (%Z) is used directly in the short-circuit current formula (I_sc = I_FL ÷ %Z) and is the critical nameplate value needed for fault current calculations.