Motor protection

[jake21]

I have a 75HP, 480V motor with 84.1A FLA. The service factor is 1.25 with 105A. I am planning to feed it thru a direct online starter. Power will be fed from a bus feeder with J type fuse.

I have few questions:

1. What should be my fuse size at bus feeder (i think it should be 105 x 1.75 = 183A ~180A, so my bus feeder has to be 200A power source)
2. What should be the MPCB size, O/L range and setting on MPCB ( i think MPCB shall be 105 x 1.25 = 131.5A ~150A with O/L range from 90A to 130A and set @ 105 x 1.05 = 110A).
3. What should be the Contactor size? ( I think it should be 150A)

Am I going the right path or missing something here? Please reply.

 

[augie47]

Art 430.6 states the NEC tables will be sued for all general motor applications for everthing except motor overload protection so for those calculations you need to reference Table 430.250 or 96 amps for a 75 hp motor.
Per 430.52, your maximum SC/GF Protection would be 175% of that number for a fuse and 250% for a Inverse Time Circuit Breaker.
Your contactor would normally be part of a motor starter with overload protection,. The contactor must be HP rated and you would normally select your overload protection based on the nameplate rating and the manufacturers OL instructions

 

[Jraef]

Do you need to use the Service Factor? Sometimes OEMs play that game because they would rather use a 75HP motor and run it into the SF rather than buying a 100HP motor, but it's the end user who pays the price in the form of sacrificing motor life. The OEM only cares that it outlasts their warranty. If you are the end user that's not a great plan, unless maybe the motor is not used very often.

If you do need to run it into the SF, then there are exceptions to the OL selection that you can factor on, but do so with the full knowledge of the instructions provided by whatever MPCB you select. You have to understand that most of them ALREADY factor in a pickup point that is based on the NEC values, ie 115% for 1.0SF or 125% for a 1.15 SF, so using the 1.25 SF requires adjusting the trip point higher based on where THEIR DESIGN starts. You can't generalize.

 

[petersonra]

I have a 75HP, 480V motor with 84.1A FLA. The service factor is 1.25 with 105A. I am planning to feed it thru a direct online starter. Power will be fed from a bus feeder with J type fuse.

I have few questions:

1. What should be my fuse size at bus feeder (i think it should be 105 x 1.75 = 183A ~180A, so my bus feeder has to be 200A power source)

It can be whatever you want it to be. personally I would make it 200A if you have a 200A fuse block.

2. What should be the MPCB size, O/L range and setting on MPCB ( i think MPCB shall be 105 x 1.25 = 131.5A ~150A with O/L range from 90A to 130A and set @ 105 x 1.05 = 110A).
I would follow the manufacturer's recommendations.

3. What should be the Contactor size? ( I think it should be 150A)
I would follow the manufacturer's recommendations.
Am I going the right path or missing something here? Please reply.

I think you are headed in the right way.