althomas49
New member
- Location
- Milton, PA, USA
I want to make sure I understand about how to properly size motor protection. As an example, let's say I am building a control panel that will house a 10HP 460V 3 phase IEC contactor and overload block, 5A DC power supply, and associated control circuitry. The panel will also have a fusible disconnect. FLA of the motor is 13.3A so the overload block will be set no higher than 13.3 X 125%. FLC from table 430.250 is 14A. Using time delay fuses, I would use 25A (14A X 175% = 24.5) fuses for the short circuit and ground fault protection. The draw of the other loads in the panel is only 3A. Am I ok to put the 25A fuses in the panel disconnect or do I need to put them in the disconnect and another set before the IEC contactor if I am pulling the 3A of control power after the disconnect?
This also plays into another example I had a question about. Same sort of setup but I would have a 60HP and 1.5HP 460V 3 phase IEC contactor with overloads in the panel. I do the math and set the overloads to no more than 125% of nameplate FLA. FLC of the 60HP is 77A, 1.5HP is 6A. 60HP would need 150A time delay fuses (77A X 175% = 134A) and the 1.5HP would need 10A fuses for short circuit and ground fault protection. Now for the fusible panel disconnect, I would consider it the feeder for the 2 and go by the rule of no larger than the biggest branch circuit short circuit protection plus the FLC of the other motors. This would be 150A + 6A = 156A, so would I need a set 150A fuses for the disconnect and another set for the motor branch, just before the IEC contactor? Or can I use the 150A fuses as feeder and branch protection?
Thanks for any advice and assistance.
This also plays into another example I had a question about. Same sort of setup but I would have a 60HP and 1.5HP 460V 3 phase IEC contactor with overloads in the panel. I do the math and set the overloads to no more than 125% of nameplate FLA. FLC of the 60HP is 77A, 1.5HP is 6A. 60HP would need 150A time delay fuses (77A X 175% = 134A) and the 1.5HP would need 10A fuses for short circuit and ground fault protection. Now for the fusible panel disconnect, I would consider it the feeder for the 2 and go by the rule of no larger than the biggest branch circuit short circuit protection plus the FLC of the other motors. This would be 150A + 6A = 156A, so would I need a set 150A fuses for the disconnect and another set for the motor branch, just before the IEC contactor? Or can I use the 150A fuses as feeder and branch protection?
Thanks for any advice and assistance.