Size of the Motor Breakers
Size of the Motor Breakers
(OP)
For the motors 100HP and 300HP, 48o Volts size of the breakers has been determined. Now we have a new information including FLC currents ( lower than conventional)
100HP, 48o Volts with FLC=110A
300HP, 48o Volts with FLC=335A
Can we use 200A and 600A breakers ?
100HP, 48o Volts with FLC=110A
300HP, 48o Volts with FLC=335A
Can we use 200A and 600A breakers ?






RE: Size of the Motor Breakers
It depends on the breaker you are using, i.e. magnetic Only or Thermal Magnetic?
It depends on the circuit, i.e. branch feeder with other loads downstream of the breaker or dedicated just to that one motor?
It depends on where you are (although one can assume you are in North America because you said 480V), because different codes have different rules.
It depends on the wire used, i.e. if you oversize a Thermal Mag breaker to accomodate motor inrush current, you may need to increase the wire size because the breaker may be the protective device for the wire (which goes back to the 1st issue). Or if the wire is already there and you don't want to replace it, you may need to find a suitable breaker that allows the correct range of adjustment.
Post some more details.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Size of the Motor Breakers
Jason Buda, PE
Greenville, SC
jbuda54@yahoo.com
RE: Size of the Motor Breakers
The only people who can use MCPs (other than replacement applications) are starter manufacturers who can afford to add them to their listing and coordinated testing procedures (and of course, people who don't care about UL listing and fire insurance).
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Size of the Motor Breakers
No extra load, CB's are to protect motor and wire.
The problem is un-standardized FLC's, little lower than FLC's specified by NEC.
Can I use a smaller breakers - or can I use 200A and 600A breakers in my case ?
Thanks.
RE: Size of the Motor Breakers
RE: Size of the Motor Breakers
RE: Size of the Motor Breakers
RE: Size of the Motor Breakers
So applied to your application pristava, your breakers for the 100HP motors must have a magnetic trip setting of no more than 1430A, and the ones for th 300HP cannot be above 4355A. Anything under that is OK.
Now look at the instantaneous trip setting or range of adjustment of your breakers and see how they fare. If the mag trips are fixed at values higher than those, you cannot use them. If they are lower or adjustable to be lower, they are OK, lthough too much lower may result in nuisance tripping. Many thermal-mag breakers with fixed trips are set at 7x the breaker rating. So your 200A breaker would be 1400A, which is <1430A, and your 600A breaker would be 4200A, also <4355A, so both should be fine. Just make sure the breaker instantaneous trip settings are correct, this is a generalization. Also, most newer breakers with electronic trip elements are adjustable over a wide range, so they may too be just fine for you.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Size of the Motor Breakers
Jason Buda, PE
Greenville, SC
jbuda54@yahoo.com