eurekah
Member
- Location
- BC, Canada
Hello, I have a question regarding sizing a motor protective circuit breaker, the thermal-magnetic type that can provide me with overload and short circuit protection.
I've notice while looking at some Motor SCPD model parameters if I find one that has ideal thermal overload settings for a motor the short circuit parameters will not match up to what is require. I will give an example below. Quick note, I am using the Canadian Electrical Code guidelines but I've noticed it is very similar to what is in the NEC, at least when it comes to motors.
Example.
Motor in question is a 460V, 3PH, squirrel cage motor with a FLA of 5.9A and S.F. of 1.10. The max thermal setting should be 5.9A*115%=6.79A, and for an instantaneous magnetic circuit breaker the release current max should be 5.9A*1300%=76.7A.
Using these parameters I find the motor SCPD. One I found was the Eaton PKZM0-10 which has a overload range of 6.3-10A (great), and a short-circuit release current max of 155A( 2600% of the motors FLA).
Here is where my problem comes in. I often find a thermal characteristic that fits the requirements, but the short-circuit current is always significantly higher that the max 1300%*FLC that is required.
Even the next model size down which has a thermal range of 4-6.3A has a short-circuit setting of 97.7A.
Am I missing something when sizing these protection devices? If I find a device that has the desired short circuit setting the thermal overload settings are always below the FLC of the motor. A bit of background of the project. These motor protection devices will be in a control enclosure which has a main fused disconnect of ~100A, which will then feed into about 5 other motors, each with their own motor protection devices and contactors.
Thanks for the help! I typically only do the design from the power pole on the road and end at the power going into the control enclosures. This is the first time I've had to actually work with control enclosure internals.
I've notice while looking at some Motor SCPD model parameters if I find one that has ideal thermal overload settings for a motor the short circuit parameters will not match up to what is require. I will give an example below. Quick note, I am using the Canadian Electrical Code guidelines but I've noticed it is very similar to what is in the NEC, at least when it comes to motors.
Example.
Motor in question is a 460V, 3PH, squirrel cage motor with a FLA of 5.9A and S.F. of 1.10. The max thermal setting should be 5.9A*115%=6.79A, and for an instantaneous magnetic circuit breaker the release current max should be 5.9A*1300%=76.7A.
Using these parameters I find the motor SCPD. One I found was the Eaton PKZM0-10 which has a overload range of 6.3-10A (great), and a short-circuit release current max of 155A( 2600% of the motors FLA).
Here is where my problem comes in. I often find a thermal characteristic that fits the requirements, but the short-circuit current is always significantly higher that the max 1300%*FLC that is required.
Even the next model size down which has a thermal range of 4-6.3A has a short-circuit setting of 97.7A.
Am I missing something when sizing these protection devices? If I find a device that has the desired short circuit setting the thermal overload settings are always below the FLC of the motor. A bit of background of the project. These motor protection devices will be in a control enclosure which has a main fused disconnect of ~100A, which will then feed into about 5 other motors, each with their own motor protection devices and contactors.
Thanks for the help! I typically only do the design from the power pole on the road and end at the power going into the control enclosures. This is the first time I've had to actually work with control enclosure internals.