PC Transformers and Distribution Equipment 3

Question 1

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?

Accepted Answer

120°C

Answer

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

Question 2

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:

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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

(PC) Professional Electrical Contractor Practice Test

(PC) Professional Electrical Contractor Practice Test

PC Transformers and Distribution Equipment 3