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.
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!
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.
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
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
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
Summary:
To size a breaker for a motor bank:
- Find the motor with the highest adjusted overcurrent value.
- Add the FLA of all remaining motors (no multipliers).
- 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.