Protection of VFD feeder using tap rules

[petersonra]

More on motor feeder taps. Someone suggested the feeder tap had to comply with another section.

According to the contents chart of article 430, motor feeders are covered in part II.

430.28 covers feeder taps. it requires only that the feeder tap be not less than the ampacity required in part II with the 10% and 33% rules.

430.28 Feeder Taps. Feeder tap conductors shall have an
ampacity not less than that required by Part II, shall terminate
in a branch-circuit protective device, and, in addition,
shall meet one of the following requirements:
(1) Be enclosed either by an enclosed controller or by a raceway,
be not more than 3.0 m (10 ft) in length, and, for
field installation, be protected by an overcurrent device on
the line side of the tap conductor, the rating or setting of
which shall not exceed 1000 percent of the tap conductor
ampacity
(2) Have an ampacity of at least one-third that of the feeder
conductors, be suitably protected from physical damage
or enclosed in a raceway, and be not more than 7.5 m
(25 ft) in length
(3) Have an ampacity not less than the feeder conductors

Note that there is no specific requirement for the size of the OCPD the tap has to terminate at.

 

[kwired]

I just picked that as an example because I can remember the required EGC sizes for those amperages.

Lets pick another case.

20 HP 480V 54FLA

#4 wire according to my chart

90 Amp TM breaker

But, I am allowed to go up to 250% which would 135A, the next size breaker is 150A.

Normally with a 150A CB one would need a #6 EGC, but with a 90A only a #8 EGC is required according to T250.122.

So how is a motor circuit with a 150A CB adequately protected against SC with #8 EGC, when any other circuit with a 150A CB requires a #6 EGC to be adequately protected?

Your data is all for 20 Hp @ 240 volts not 480.

T250.122 determines size of EGC based on setting of overcurrent device. Doesn't matter that your 20 hp motor could have a range of 90 to 150 for the overcurrent device, what matters is which overcurrent device is used. If you assume 90 amp device, size EGC to that, and later find out it needs to be at least 110 amps then you will change the breaker as well as the EGC if you want to comply with NEC.

The 4 AWG minimum size for the motor circuit does not change, even if higher overcurrent device setting is needed to allow motor to start. 4 AWG will still be protected from short circuits and ground faults by the branch circuit overcurrent device, and will still be protected from overload by the motor overload protection device.

 

[petersonra]

Your data is all for 20 Hp @ 240 volts not 480.

T250.122 determines size of EGC based on setting of overcurrent device. Doesn't matter that your 20 hp motor could have a range of 90 to 150 for the overcurrent device, what matters is which overcurrent device is used. If you assume 90 amp device, size EGC to that, and later find out it needs to be at least 110 amps then you will change the breaker as well as the EGC if you want to comply with NEC.

The 4 AWG minimum size for the motor circuit does not change, even if higher overcurrent device setting is needed to allow motor to start. 4 AWG will still be protected from short circuits and ground faults by the branch circuit overcurrent device, and will still be protected from overload by the motor overload protection device.

sorry, looked on the wrong line of my chart.

I agree that the EGC has to be sized by the CB feeding it. But it is common practice not to do so with motor circuits. 430 does not say anywhere that we can make the EGC smaller then 250 says it has to be, although it does say the CB can be increased in size for the same size conductor beyond what previous articles would otherwise allow.

 

[kwired]

sorry, looked on the wrong line of my chart.

I agree that the EGC has to be sized by the CB feeding it. But it is common practice not to do so with motor circuits. 430 does not say anywhere that we can make the EGC smaller then 250 says it has to be, although it does say the CB can be increased in size for the same size conductor beyond what previous articles would otherwise allow.

And if you have to increase the breaker - you may have to increase EGC, your motor circuit conductors are still adequately protected.

Kind of no different than if you find out you need to pull additional conductors through raceway because you find out the 20 hp driven load also requires an oil pump or cooling fan powered by separate motor. If adding those conductors should cause deration for number of conductors in raceway or would happen to overfill raceway then you may have to replace original conductors or raceway (or both) to have a compliant install when done.