Available Fault Current on Secondary Red Seal Exam Questions

A common question on System and Distribution section for Red Seal Electrical IP Exam is to determine the available fault current on secondary terminals, when there is unlimited fault current on the primary.

Practice Exam Question

If a 2,500 kVA, 3-phase, 13.8 kV/600 V transformer has an impedance of 5 percent, __________ is the transformer short circuit current at the secondary terminals with unlimited fault current on the primary terminal.

Step 1 - Identify key words in the question
Identify the question as an Available Fault Current on Secondary question by reading “% impedance” and “unlimited fault current on primary”.

Note: If they are asking about a pad mount transformer, this is categorized as high voltage question, not necessarily asking about available fault current.

Step 2: Apply Available Fault Current formula [Quick Reference pg 5]
Available Fault Current = kVA / ( √3 * Line Voltage * % impedance)
Available Fault Current = 2,500,000 kVA / ( √3 * 600 V * 0.05)
Available Fault Current = 48,170 A

Box Sizing Red Seal Exam Questions

Another common Red Seal IP exam question is box sizing. The size of the box housing conductors is based on the number and size of conductors. It’s extremely likely that one rendition of these questions will be on the test.

Practice Exam Question
If you are installing eight No. 14 AWG TW75 conductors with three wire connectors into a box, the minimum volume of the box is __________ mL.

Step 1: Find volume per conductor in Table 22
Volume of a No. 14 AWG - 24.6 mL

Step 2: Apply Rule 12-3034(4)
Number of wire connectors - 3
Number of extra conductors count - 3/2 = 1.5 (round down)
Total number of No. 14 AWG conductors = 8 + 1 = 9

Step 3
Total volume of conductors or box - 24.6 x 9 = 221.4 mL

Motor Overcurrent Protection Red Seal Exam Questions

In the motor section for the Red Seal IP exam, a topic that comes up frequently during the exam is how to determine the overcurrent protection rating for motors. Based on previous experiences from students, this comes up once to twice per exam and needs to be practiced in order to maximize your chances of passing the exam.

Practice Exam Question
A non-time-delay fuse with maximum size of __________ can protect the feeder to the splitter that connects three 3-phase, 208 V, squirrel cage induction motors that are 3 hp, 5 hp, and 10 hp.

Step 1: Identify key words in the question
The question is asking about overcurrent protection by identifying phrases like “maximum size” or “non-time-delay fuse”. However, please note that it is asking about overcurrent protection for the feeder, not for a branch circuit.

Step 2: Use Table 44 to determine FLA
3 hp FLA - 10.6
5 hp FLA - 16.7
10 hp FLA - 30.8

Step 3: Use Rule 28-204(1) to determine fuse size
Largest motor FLA x Factor (Table 29) + remaining motor FLAs
(30.8 x 300%) + 16.7 + 10.6 = 119.7 A

Step 4: Use manufacturer charts to determine maximum standard rated non-time-delay fuse
110 A

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