How to properly size a motor circuit breaker?
This article will tackle about the sizing of Inverse Time Circuit Breaker with regards to National Electrical Code (NEC). I have learned this not just in school, but also in review center for our national board exam. It’s one of the essential topics you should know for future use especially in the design field.
What is Circuit Breaker?
Circuit Breaker is an over current protective device that can manually switch on/off and automatically trip when a short circuit fault occurs.
Why do we need to size a circuit breaker?
Circuit Breaker rating is sized according to its load/s. Thus, undersized circuit can always give false alarm to operation. While oversized can lead to equipment breakdown or affect other circuits. That’s why it’s good to follow standards safety practice as mentioned in NEC. It’s tested by calculation and actual demonstration.
What is Inverse Time Tripping?
It’s a tripping characteristic of circuit breakers in which the breaker trips longer time with lower over-current. Thus, short time with higher over-current. According to National Electrical Code Article 100: “Inverse Time (as applied to circuit breakers). A qualifying term indicating that there is purposely introduced a delay in the tripping action of the circuit breaker, which delay decreases as the magnitude of the current increases.”
How to size Circuit Breaker for Three Phase Motor?
Note: This article will only cover Inverse Time Circuit Breakers as defined above.
It’s always required to have the motor data especially the power rating in HP and supply 3~ Voltage. Type of motor (three phase induction motor)
If it’s kW, try to convert to HP with this conversion.
Once you able to get the HP, you can now find the corresponding Full Load Current using NEC Handbook. See Table 1 below:
Table above shown the HP rating of motor and corresponding Full Load Current per Supply Voltage.
After you determine the FLC, you have to multiply it to 250% since you will be using an Inverse Time Circuit Breaker. Means, you will be able to get the cb ampacity.
CB Ampacity = FLC × 250%
Once you able to determine the Ampacity, you can now select a standard Inverse Time Circuit Breaker Size which has to be equal or next standard size higher. See Table 2 below:
- Motor Nameplate: 2HP, 230V
- Based on Table 1 above, FLC for 2HP, 230V is 6.8A
- CB Ampacity = FLC x 250%
- CB Ampacity = 6.8A x 250%
- CB Ampacity = 17A
- Based on Table 2 above, we now can select standard Circuit Breaker Size. Or CB Ampacity is 17A which means we can select 20A as our Circuit Breaker Size.
What if you have multiple motor loads? How to calculate the main circuit breaker?
National Electrical Code stated a standard calculation of Main Circuit Breaker with multiple motor load.
There are instances that one panel may have multiple motor starter installed and supplying different sizes of motors.
Again, please keep in mind that for multiple motors, HP rating is required for all as this value will be used to select the equivalent full load current with regards to National Electrical Code.
Then, you have to know which motor has the highest HP Rating. If all are same rating, you can use that value for calculation.
The motor with the highest HP rating will be used for calculation to after finding out the equivalent full load current with regards to National Electrical Code. That full load current will be multiplied to 250% or 2.5. Means, we will still use Inverse-Time Circuit Breaker for this example as it the most commonly used circuit breaker for motor application.
Once you got the highest FLC value multiplied to 250%. You have to add the remaining FLC of each motors that will be supplied by our main circuit breaker. I repeat, only the Motor with highest FLC will be multiplied to 250%, then add the remaining FLC.
CB Ampacity = (Highest FLC x 250%) + Remaining FLC
After adding all the value, you can now select the standard circuit breaker size referring to same standard size given on first example.
If you’re not able to get the exact value, the National Electrical Code does not permit going to the next standard size higher. Therefore, permit to drop down one size lower than the computed value. The National Electrical Code permits next standard size higher on branch over-current devices only just like the first example.
- Motor Nameplate:
- 1st motor = 2HP, 230V
- 2nd motor = 5HP, 230V
- 3rd motor = 3HP, 230V
- Based on Table 1 above, FLC are as follows
- 1st motor = 2HP, 230V is 6.8A
- 2nd motor = 5HP, 230V is 15.2A
- 3rd motor = 3HP, 230V is 9.6A
- This means 2nd motor has the highest FLC which is 15.2A to be multiplied to 250%
- CB Ampacity = (Highest FLC x 250%) + 6.8A + 9.6A
- CB Ampacity = (15.2A x 250%) + 6.8A + 9.6A
- CB Ampacity = 38A + 6.8A + 9.6A
- CB Ampacity = 54.4A
- Based on Table 2 above, we now can select standard Circuit Breaker Size. Our CB Ampacity is 54.4A which means we can select 50A as our Circuit Breaker Size as we need to select one standard size lower.