Dakota Motor Bank Overcurrent Guide for Red Seal Exam (CEC 2024)

When you have multiple motors running together from a common feeder, it’s crucial to size your main overcurrent protection device (breaker or fuse) properly — not too small that it trips constantly, and not so large that it fails to protect your system.

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The Situation:

You’re given a motor bank with 4 motors, all 230V, 3-phase, Motors 1, 2, and 3 are FVS and Motor 4 is Auto-Transformer Starting, and with the following full-load currents:

• Motor 1: 36A    
• Motor 2: 40A
• Motor 3: 50A
• Motor 4: 60A

By the way, it doesn’t matter what the individual branch circuit over-current is!

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Step 1: Focus on the Main Overcurrent Device

CEC Rule 28-204 tells us that when multiple motors are connected to a single feeder:

You size the main overcurrent device based on the highest rated motor and then add the full-load amps (FLA) of all the other motors.

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Step 2: Identify the Controlling Motor

Looking at the calculations. Since the Main Over-Current in this case is a Circuit Breaker, we use the multipliers for that.

Table 29

Motor 1: 36A * 2.5= 90A

Motor 2: 40A * 2.5= 100A

Motor 3: 50A * 2.5= 125A

Motor 4: 60A  * 2= 120A

• Motor 3 has the highest adjusted value after applying multipliers (125A)
• So this becomes the reference motor for sizing the breaker

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Step 3: Add Up the Remaining Motor Currents

Now, simply add the FLA of the other motors:

• Motor 1: 36A
• Motor 2: 40A
• Motor 4: 60A
• Total additional = 136A

Add to Motor 3's adjusted value:

125A + 36A + 40A + 60A = 261A

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Step 4: Select the Breaker (Table 13)

You now go to CEC Table 13 and select the closest breaker size that doesn’t exceed 261A.

• Nearest size at or below 261A: 250A breaker

ALWAYS GO DOWN

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Summary:

To size a breaker for a motor bank:

1. Find the motor with the highest adjusted overcurrent value.
2. Add the FLA of all remaining motors (no multipliers).
3. Select a breaker from Table 13 that does not exceed the total.

This ensures:

• Protection for startup inrush
• Safety against overload
• And compliance with CEC Rule 28-204.