A 100 kVA, 600V delta primary to 208Y/120V secondary transformer is installed in a commercial building. The electrician notes that the primary feeders are protected by 125 A circuit breakers. What is the approximate full load current on the primary side of this transformer, and is the protection within typical CEC guidelines for this kVA rating?

Question: A 100 kVA, 600V delta primary to 208Y/120V secondary transformer is installed in a commercial building. The electrician notes that the primary feeders are protected by 125 A circuit breakers. What is the approximate full load current on the primary side of this transformer, and is the protection within typical CEC guidelines for this kVA rating?

Answer options:

• Primary FLC ≈ 96 A; Protection is too high. ✅ Primary FLC ≈ 96 A; Protection is within guidelines.
• Primary FLC ≈ 167 A; Protection is too low.
• Primary FLC ≈ 167 A; Protection is within guidelines.

Correct answer: Primary FLC ≈ 96 A; Protection is within guidelines.

Explanation: The full load current (FLC) for a three-phase transformer primary is calculated as FLC = kVA * 1000 / (V_primary * √3). So, 100 kVA * 1000 / (600 V * √3) ≈ 100000 / (600 * 1.732) ≈ 100000 / 1039.2 ≈ 96.2 A. According to CEC Rule 26-254(1)(a)(i), for a transformer with a rated primary current of 96.2 A, the overcurrent device shall not exceed 125% of the rated primary current without exceeding 250% where 125% is not a standard size. 96.2 A * 1.25 = 120.25 A. A 125 A breaker is the next standard size above 120.25 A and does not exceed 250% of the transformer's rated primary current (96.2 A * 2.5 = 240.5 A), so 125 A is within guidelines.