Question

Scenario 220.53 The secondary of a 300 KVA transformer delivers power to a 180- amp, 30 load. This is a 480 – 240 volt A-A transformer. Primary and secondary conductors are the minimum size required to serve the load. Refer to Scenario 220.53. The secondary OCPD can have a rating no greater than __amps. Select one: O a. 100 O b. 200 O c. 150 d. none of these e. 175

Answer 1

SOLUTION:

FOR A GENERAL CASE,

SECONDARY CURRENT = TRANSFORMER VA / $\sqrt{3}$ * SECONDARY VOLTAGE = 300k / $\sqrt{3}$ * 240

                                      = 300000/$\sqrt{3}$ * 240

                                      = 721.688 A

$\therefore$ SECONDAR CURRENT = 721.70 A

NOW, OCPD MUST BE ATLEAST RATED AS 125% OF THE RATED SECONDAR CURRENT

                                        = 721.70 * 125% = 721.70 * 1.25 = 902.125A

THEREFORE OCPD FOR SECONDARY MUST BE RATED AROUND 902.125 A

BUT HERE IT SEEMS THE TRANSFORMER IS USED TO SUPPLY THE LOAD OF 180A ONLY BECAUSE IT IS SAID THAT TRANSFORMER HAS MINIMUM SIZE OF CONDUCTOR REQUIRED TO SERVE THAT LOAD.

SO IF WE USE OCPD OF AROUND 902 A THEN IN CASE OF FAULT IN 180A LOAD THE TRANSFORMER OCPD WILL NOT RESPOND BEACAUSE THE FAULT VALUE WILL BE LESS THAN 900 A.

SO HERE WE WILL SIZE THE OCPD DEPENDING ON THE LOAD(BECAUSE TRANSFORMER IS SOLELY USED TO SERVE THAT 180 A LOAD)

SO THE REQUIRED OCPD FOR SECONDARY MUST 120% OF 180A= 180 *1.25 =225 A

ANSWER IS NONE OF THESE.