Motor Circuit

[Bob Clukey]

I have a question on tapping from a busbar to a single motor that
hopefully you can shed some light on.

I have a 32hp, 460v, 3ph motor (lathe) which has an internal overload
~40amp fuse and 75amp CB in its control panel and I was going to feed
this with a 100amp plug-in circuit breaker (mounted 30feet away) with
#8 AWG THHN. My concern is the wire size - I originally thought #8 is
adequate with 40A FLC x 1.25 = 50 since this is for a motor.

However, now considering the tap rule and that the machine is located
more than (10 or the 25feet away) - Does this change my conductor and
plug-in CB? How do you size for this?

Thanks in advance for your help!

 

[meternerd]

Size it for the 100A breaker.

 

[raider1]

I have a question on tapping from a busbar to a single motor that
hopefully you can shed some light on.

I have a 32hp, 460v, 3ph motor (lathe) which has an internal overload
~40amp fuse and 75amp CB in its control panel and I was going to feed
this with a 100amp plug-in circuit breaker (mounted 30feet away) with
#8 AWG THHN. My concern is the wire size - I originally thought #8 is
adequate with 40A FLC x 1.25 = 50 since this is for a motor.

However, now considering the tap rule and that the machine is located
more than (10 or the 25feet away) - Does this change my conductor and
plug-in CB? How do you size for this?

Thanks in advance for your help!

The conductors are not tap conductors they are motor feeder conductors since there is a branch circuit overcurrent device in the control panel. The feeder conductors are sized at 125% of the motor FLC and the feeder OCPD is sized not larger than the largest rating of the branch circuit overcurrent device based on the Table 430.52.

So assuming that the FLC is 40 amps the conductors must have an ampacity of at least 50 amps so #8 Copper THHN/THWN-2 would have an ampacity of 50 amps. Now because this is a motor load we can size the branch circuit breaker in accordance with Table 430.52 which for a 3 Phase motor using a inverse time circuit breaker would allow a breaker to be sized at 250% of the FLC of the motor so at 40 amps the breaker could be a 100 amp breaker.

Again this is not a feeder tap but a motor circuit.

Chris