VFD feeder sizing

[Davebones]

Have a ( Mitsubishi FR-F740-00770-NA ) 50 HP drive . The nameplate show's " Input 115 A " . The drive's only rated for output of SLD of 77 amps . Looking at 430.122 we would then size the feed to the drive for 144 amps ? So we would need to have a 1/0 feed. Would a disconnect rated for 100 amps be sufficent or do we need to up the size because of the increase in feeder ?

 

[Smart $]

Have a ( Mitsubishi FR-F740-00770-NA ) 50 HP drive . The nameplate show's " Input 115 A " . The drive's only rated for output of SLD of 77 amps . Looking at 430.122 we would then size the feed to the drive for 144 amps ? So we would need to have a 1/0 feed. Would a disconnect rated for 100 amps be sufficent or do we need to up the size because of the increase in feeder ?

Upsize because of:

430.128 Disconnecting Means. The disconnecting means
shall be permitted to be in the incoming line to the conversion
equipment and shall have a rating not less than
115 percent of the rated input current of the conversion
unit.

 

[Davebones]

Dang , didn't see 430.128 . So we'll need a 200 amp bus switch fused down just to feed the drive that feed's a 50 hp motor that draws 65 amps ..

 

[kwired]

I realize there can be more current input than rated output because of losses in the drive, but the difference here is over 30kVA. Does that mean that unit is very inefficient? Could heat my house and then some with that kind of loss.

 

[Jraef]

I agree, something is amiss here. Typically the input current is only slightly higher than the output current, maybe 5% at most. Might be one of those "lost in translation" issues from the manual having been written in Japanese. That 115A may ALREADY factor in the 125%, i.e. Max current is 92A, x 1.25 = 115A minimum circuit size. 92A input on a 77A output drive is almost 20%, but it might factor in the overload capacity of the drive or the fact that the drive may be rated for higher current if it is used on a variable torque load, something like that.

 

[petersonra]

I used some drives once that the rated input current was almost double the rated output current.

Oddly, the drive manual showed recommended input fuses that were less than the rated drive input current, and wire sizing that seemed in line with the fuse sizes and the output current.

I think they must rate input current in Japan in a different way than we do.

 

[Jraef]

Looks like it is an issue like that. The "Input Current" rating on the nameplate is higher because it is for 380V and is based on the 110% output for 1 minute rating. Because of the way the NEC is worded, you have no choice but to abide by that rating since that is the way they listed it. By the way, MAXIUMUM Class RK-5 fuse or UL489 listed size is 175A for that drive, but you can use smaller if appropriate.