MPCB selection for 3 phase motor

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jake21

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I have a 3-phase, 480V, 25hp motor. Its full load current is 34 Amperes. I want to provide a MPCB for branch protection of this motor. Since the service factor of the motor is 1.15, I am going with a MPCB having overload protection range from 35 to 45 Amps. But when I see NEC code (430.52), it says Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit should be 175% for time delay fuses and 800% for instantaneous trip breaker. This got me confused that if I go with 35-45A MPCB then what happens to short circuit protection. Also, whether this is good enough to sustain the starting inrush current of the motor.

Please guide me if I am missing a point here. Thanks.
 
With an MPCB, you select it like you would an overload relay by sizing for the motor nameplate FLA. The SCPD portion of it is already covered by the mfr when they UL listed it.

As to sizing for the SF, it depends on whether or not you intend to use the SF or not. Using the SF affects the life of the motor. If you are not going to use it, then don't use that value for sizing the MPCB, just size if for the FLA of the motor.
 
Jraef said:
With an MPCB, you select it like you would an overload relay by sizing for the motor nameplate FLA. The SCPD portion of it is already covered by the mfr when they UL listed it.

As to sizing for the SF, it depends on whether or not you intend to use the SF or not. Using the SF affects the life of the motor. If you are not going to use it, then don't use that value for sizing the MPCB, just size if for the FLA of the motor.
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I got your point. Thanks. Just one more thing, How to make sure the inrush current will not trip my MPCB? Should I be considering looking at overload relay's curve & Motor's starting current curve to make sure it will not trip during the inrush time frame.
 
jake21 said:
I got your point. Thanks. Just one more thing, How to make sure the inrush current will not trip my MPCB? Should I be considering looking at overload relay's curve & Motor's starting current curve to make sure it will not trip during the inrush time frame.
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It's not usually a problem, unless your machine is something unusual that takes a long time to accelerate, such as a centrifuge or something else with a high inertia load. Most MPCBs will be Class 10 and a basic Class 10 overload relay trip curve means it will take 600% current for 10 seconds to trip. If your motor takes longer than that to get to about 80% speed, you need to think about it, otherwise, don't worry.
 
jake21 said:
I have a 3-phase, 480V, 25hp motor. Its full load current is 34 Amperes. I want to provide a MPCB for branch protection of this motor. Since the service factor of the motor is 1.15, I am going with a MPCB having overload protection range from 35 to 45 Amps. But when I see NEC code (430.52), it says Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit should be 175% for time delay fuses and 800% for instantaneous trip breaker. This got me confused that if I go with 35-45A MPCB then what happens to short circuit protection. Also, whether this is good enough to sustain the starting inrush current of the motor.

Please guide me if I am missing a point here. Thanks.
Click to expand...
Have you considered a NEMA 2 combination motor stasrer in a NEMA 1 encosure? It will be supplied with a correctly MCP disconnect, starter, and OLR.
The MVP would be adjusted per NEC art 430-52 (C)(3) and the OLD configured per the motor FLA. If the OLR requires heaters to be installed there should be a chart somewhere mounted in the enclosure which illustrates the correct heaters to be installed.
Then you also must consider how the motor is to be controlled. Commonly you would have a contactor with a 120vac coil. You could supply external 120vac source or include a CPT in the enclosure itself. Then are you using 3wire control with a start stop pushbuttons on the cover or a remotely located push button station.
I bring the control up because you sem to be overlooking the relation of a common motor starter package on putting together you own. Are you making this more complicated than it needs to be?
You also mentioned a branch circuit. The combination starter would be protecting the circuit from that location to the motor. The, you have the cable feeding the combination starter for which you must provide a OCPD to protect the branch circuit.
 
Jraef said:
It's not usually a problem, unless your machine is something unusual that takes a long time to accelerate, such as a centrifuge or something else with a high inertia load. Most MPCBs will be Class 10 and a basic Class 10 overload relay trip curve means it will take 600% current for 10 seconds to trip. If your motor takes longer than that to get to about 80% speed, you need to think about it, otherwise, don't worry.
Click to expand...

Thanks, I got it now. :)
 
templdl said:
Have you considered a NEMA 2 combination motor stasrer in a NEMA 1 encosure? It will be supplied with a correctly MCP disconnect, starter, and OLR.
The MVP would be adjusted per NEC art 430-52 (C)(3) and the OLD configured per the motor FLA. If the OLR requires heaters to be installed there should be a chart somewhere mounted in the enclosure which illustrates the correct heaters to be installed.
Then you also must consider how the motor is to be controlled. Commonly you would have a contactor with a 120vac coil. You could supply external 120vac source or include a CPT in the enclosure itself. Then are you using 3wire control with a start stop pushbuttons on the cover or a remotely located push button station.
I bring the control up because you sem to be overlooking the relation of a common motor starter package on putting together you own. Are you making this more complicated than it needs to be?
You also mentioned a branch circuit. The combination starter would be protecting the circuit from that location to the motor. The, you have the cable feeding the combination starter for which you must provide a OCPD to protect the branch circuit.
Click to expand...

I guess you are right, I was making more complicated than it needs to be. Since I am new in field, just wanted to make sure I am not over looking anything. Thanks for the answer.
 
jake21 said:
I guess you are right, I was making more complicated than it needs to be. Since I am new in field, just wanted to make sure I am not over looking anything. Thanks for the answer.
Click to expand...
You did good as it is important to at least have an understanding of motor control. Purchased assemble enclosed control makes it simple plus the assembly does have a UL listing complete with an interrupting rating because it has been tested as an assembly.
 
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