simple motor problem

[jefftino]

Hi there,

I am studying on my own and I am confused by motors.

See if this is right. Questions at bottom

5 hp. motor 240v. single phase integral thermal protection

table 430.248 28A FLC
430.22(A) 28x1.25 = 35A
table 310.16 75C column #10 THHN = 35A

short circuit protection (circuit breaker) 430.52(C)(1)
table 430.52 single phase

FLC 28x2.50 = 70A inverse time breaker

430.32(B) overload not required integral thermal protection otherwise use nameplate rating

430.102(B) disconnect required if motor in different location to circuit breaker

430.110 rating of disconnect 28x1.15 = 32A

430.109(F) horsepower rated cord and plug 5hp

SO...

1. Difference between a standard breaker and inverse breaker?
2.Which breaker protects ground fault or do you use fuses?
3.When do you use fuses?

Thanks

Jeff

 

[augie47]

motor

motor

calculations appear correct for a standard motor (normal speed & torque)

To me, an Inverse breaker, is a "standard breaker". The other breaker allowed is Instaneous trip, which must be part of a listed motor controller.

Both Inverse and Instaneous provide ground fault protection, as do fuses.

Fuses can be used as overload or short circuit protection. The "current limiting" value of fuses can be beneficial in some short circuit situations.

Note also that the cord and plug you mentioned might serve as the disconnecting means.

 

[jefftino]

430.109(F) states:

For a cord and plug connected motor, a hp-rated attachment plug and recep having ratings no less than the motor ratings shall be permitted to serve as the disconnecting means...

Does this mean the NEC table or nameplate rating?

Thanks,
Jeff