Conductor sizing based on 430.22 and 430.122

[aNerdyEngineer]

Hi, I'm a little confused on conductor sizing based on NEC 430.22 and NEC 430.122. NEC 430.122 is pretty straight forward that the branch LINE side conductors with a drive in place should be 125% of the drive input rating. First, is this the rated input or the 'seen input'? If my drive is over-sized, can I go with the efficiency of the drive with relation to the motor to get the 'seen input' current and use that?

Additionally, for sizing the LOAD side conductors, are these to be based on 430.122 as well or 430.22? If 430.22, how is it that a 60min motor should allow conductor sizes of 150% when continuous motor is 125%. This seems backwards. Thoughts?

 

[Jraef]

430.122 is the RATED input amps of the VFD, as stated on the unit or literature. If you over size the VFD, you must use the VFD rating, which is why people need to pay attention to that. I keep seeing project specs calling for a VFD to be sized for 110% or 120% of the motor FLA, which then means go9ng up one size in most cases and the conductors end up much larger than the same engineer specified. It can be an expensive trap.

Nothing special on sizing the output conductors, same as per usual.

The subtlety that you are missing in the non-continuous duty issue is that continuous duty motors are sized at 125% of the TABLE values for the HP rating, i.e. Tables 430.247, 248, 249 or 250, whereas for the non-continuous duty motors, you use those Table 430.22(E) percentages of the MOTOR NAMEPLATE current rating.