PC Transformers and Distribution Equipment 2

Question 1

Per NEC 450.3, when a transformer has primary overcurrent protection set at no more than 125% of rated current, secondary overcurrent protection:

Accepted Answer

May be omitted if primary protection does not exceed 125% of primary rated current

Answer

Must always be provided at 125% of secondary rated current

Answer

Is not required for transformers with a 2-wire secondary

Answer

Must be rated at 250% of secondary rated current

Question 2

What is the primary difference between a switchboard and a metal-clad switchgear assembly?

Accepted Answer

Metal-clad switchgear is fully enclosed with drawout components providing greater protection than open-construction switchboards

Answer

Switchboards are for outdoor use only; switchgear is for indoor use only

Answer

Switchboards have a higher interrupting capacity than switchgear

Answer

There is no practical electrical difference between the two assemblies

Question 3

A 225 kVA, 480V/240V single-phase transformer operates at full load. What is the approximate full-load secondary current?

Accepted Answer

938A

Answer

469A

Answer

270A

Answer

1,875A

Question 4

A K-rated transformer (e.g., K-13) is specifically designed to handle:

Accepted Answer

Non-linear loads that generate harmonic currents

Answer

High ambient temperatures above 50°C

Answer

Underground installation in wet or damp locations

Answer

High available short-circuit fault currents

Question 5

In a typical commercial distribution system, primary distribution feeders in urban areas operate at what voltage range?

Accepted Answer

4,160V to 15,000V

Answer

120/240V

Answer

480V

Answer

115,000V to 345,000V

Question 6

Which transformer cooling designation indicates an oil-immersed, forced-oil-cooled, forced-air-cooled unit?

Accepted Answer

OFAF

Answer

ONAN

Answer

ONAF

Answer

ODAF

Question 7

When two transformers are to be operated in parallel, which condition is NOT required for proper load sharing?

Accepted Answer

Identical kVA ratings

Answer

Same voltage ratio (turns ratio)

Answer

Same impedance percentage (%Z)

Answer

Same polarity and phase relationship