Size of the Motor Breakers

[pristava]

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 ?

 

[jraef]

As in all things, it depends.

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

 

[JBUDA54]

The circuit breaker, per se, is put into the circuit for Short Circuit protection and the motor overload is to protect motor and cable. The NEC addresses the maximum Rating of the Short circuit protective device in Table 430.52. Using a thermal magnetic breaker on a motor that has overload relays is impractical. Most applications will use what is called Motor Circuit Protectors (MCPs) which is a breaker without thermal capability.

Jason Buda, PE
Greenville, SC
jbuda54@yahoo.com

 

[jraef]

Yes, but if UL listing is important, you are not allowed to install MCPs in your own built-up starters because they are only UL recognized, not UL listed as TM breakers are. Most people don't realize that and do it anyway, but it isn't safe to assume they will be willing to violate UL listing as many insurance investigators will look for that if a fire happens. So if you are building your own starter, you must use Thermal Magnetic breakers to avoid violating UL rules.

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

 

[pristava]

I need two motor breakers (thermal magnetic) for 100HP, 480V and 300HP, 480V motors.
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.

 

[davidbeach]

No, if the NEC is the governing code, you have to just the FLA numbers in the code for breaker and conductor sizing; actual motor FLA numbers are only used to set the overload.

 

[wareagle]

See 430.6(A)(1). It requires you to use tables listed to determine ampacity of conductors and overcurrent protection and not the FLA of the motor.

 

[dpc]

Motor feeders are handled differently than normal feeders in the NEC. As mentioned, the feeder is sized for 125% of the motor FLA in the tables in the NEC. The breakers are sized based on the motor FLA as well. There is no **direct** relationship between the breaker size and the feeder size for motor feeders.

 

[jraef]

Right. The correct answer generally is to size the conductors for the 125% motor FLA and voltage drop over distance, then size the circuit breaker to the conductors. But there are exceptions in a motor branch circuit that allow oversizing of the breaker if the magnetic trip settings will not work to allow starting, becasue the Overload Relay provides the continuous overcurrent protection for them. So what that means is that in a motor branch circuit with a starter, as long as you don't exceed the maximum allowable setting of the instantaneous trip (1300% motor FLA if I remeber correctly) you can use any circuit breaker..

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

 

[JBUDA54]

The assumption I made was that he was not going to build his own starter and that it was fed from a motor control center...

Jason Buda, PE
Greenville, SC
jbuda54@yahoo.com