Motors and Industrial Control Panels

[althomas49]

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.

 

[petersonra]

There is no requirement that the fuses for a motor not be installed in the main disconnect. As long as they are suitable. It is in fact pretty common.

Not a lot different than a combination starter that has a built in xfmr.

 

[augie47]

Welcome to the Forum.
A few comments (remember, we all just provide you with opinions )

The OL settings for motors is normally set at nameplate current without any "adjustment". Your 13.3 amp (namplate) motor OL would be set at 13.3.

I agree with petersonra, where your "feeder" protection is adequate for your branch circuit there is no need to add additional protection. Your 150 amp fuses can also protect the 60HP motor.

Keep in mind that many jurisdictions would require your control panel have a NRTL certification in which case the NEC would not necessarily apply.

 

[Jraef]

Welcome to the Forum.
A few comments (remember, we all just provide you with opinions )

The OL settings for motors is normally set at nameplate current without any "adjustment". Your 13.3 amp (namplate) motor OL would be set at 13.3.

I agree with petersonra, where your "feeder" protection is adequate for your branch circuit there is no need to add additional protection. Your 150 amp fuses can also protect the 60HP motor.

Keep in mind that many jurisdictions would require your control panel have a NRTL certification in which case the NEC would not necessarily apply.

Agree on all points.

This issue of the "125% of the motor FLC" setting, undoubtedly taken from 430.32(A)(1), causes more confusion than it is worth, I wish the NFPA would expound on that better. 90% of overload relays on the market ALREADY have that 115 or 125% pick-up point factored into the design of the OLR and they SPECIFICALLY tell you in the instruction manuals NOT to add it in again when setting the dial or selecting the replaceable heater elements. Yet a lot of people misinterpret that and set the OLR at 125%, which then means it doesn't pick up an overload until it is ALREADY at 156% of FLC, and then the trip curve is biased too high in addition. That alone probably causes a lot of fried motors...

Maybe the motor rewinder's lobby keeps the wording unclear.

:ashamed:

 

[MSU]

Best to pull down the OLR curve, with the anticipated heater, and overlay the breaker or fuse curve. By looking at both curves on a single graph, you can clearly see the coordination of the protection system. OLR basically provides motor overload protection (<2-3X Rated load). Fuse or breaker serves as S/C protection. Most (but perhaps not all) contactors are rated to interrupt 6X current (they commonly interrupt inrush, which is 5-8X) , so fuse or breaker needs to operate before the contactor below that current level (or whatever the interrupt rating of your contactor may be).

Agreed on set OLR for FLA. Service Factor of the motor rarely exceeds 1.15. Some smaller motors are 1.25. These SF ratings are based on a specified ambient temp., voltage, unimpeded cooling air, etc. For instance, operation above rated voltage increases core loss, and therefore degrades SF. Best use of SF in my opinion is to allow headroom in the design for differing ambient temps, load cycles, etc. Motors are usually very happy at 80-90% rated load.