A 3-phase, 600V, 50 HP motor has a nameplate full load current (FLA) of 50A. If a time-delay fuse is used for branch circuit overcurrent protection, according to CEC Table 29, what is the maximum standard rating of the fuse permitted?

Question: A 3-phase, 600V, 50 HP motor has a nameplate full load current (FLA) of 50A. If a time-delay fuse is used for branch circuit overcurrent protection, according to CEC Table 29, what is the maximum standard rating of the fuse permitted?

Answer options:

• 60A
• 70A ✅ 80A
• 90A

Correct answer: 80A

Explanation: For a 3-phase motor, the maximum overcurrent protection using a time-delay fuse is 175% of the FLA, according to CEC Table 29. 175% of 50A is 87.5A. The next standard fuse size down is 80A, but typically you are allowed to go up to the next standard size if the calculated value is not a standard size, provided it does not exceed the maximum percentage (in this case, 250% for time-delay fuses or 300% for non-time delay fuses). However, the most conservative and often correct answer is the next standard size up from 87.5A, which is 90A, but the question asks for maximum standard rating permitted for a branch circuit, and 250% is the absolute maximum, so 50A * 1.75 = 87.5A. The closest standard size not exceeding 175% would be 80A. If 175% allows you to go to the next standard size, then 90A would be the answer. Let's re-evaluate based on common interpretation: CEC 28-206(1) and Table 29. For time-delay fuses, the maximum is 175% of FLA. 50A * 1.75 = 87.5A. If 87.5A is not a standard fuse size, the next higher standard size may be used, provided it does not exceed 250% (50A * 2.5 = 125A). The standard fuse sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200... 87.5A is not standard, so the next standard size is 90A. This is less than 125A, so 90A is permissible.