Breaker size for primary of VFD

[philly]

I am trying to size a breaker and cables from an MCC feeder breaker to feed an VFD.

When sizing a breaker for a VFD I believe the code says that the cables must be rated at 125% of VFD input current. I would assume then that the feeder breaker would have to be at least 125% as well? Or would you want to size the breaker up to the capacity of the cables?

Does it matter if the cables and breakers are rated above 125%? Is there a limit to how high these ratings can be over drive input current?

I know the drive protects itself from overload, but will it protect itself from short circuits as well?

If there is already a feeder breaker feeing the drive is it necessary to have a breaker at the drive itself other then preference? Would the breaker at the drive be redundant?

If using two breakers should the one at the MCC be larger than the one at tht drive itself?

 

[Jraef]

Size the cables per the VFD rule (which to be clear is 125% of the MAXIMUM RATED input current, not just what you plan to use with the connected motor), then size the breaker per the cable protection. Most VFDs protect their LOAD from short circuits, but not itself and cannot really be protected by any breaker from a short circuit within itself. All you can hope for is to protect the cables so there is no fire.

You can ALWAYS use over sized cables if you don't mind wasting money or you want to plan for future expansion. Again, size the breaker for the cable.

The rules of redundant breakers would be no different from any other piece of equipment. A breaker at the drive would be redundant as an OCPD if the upstream breaker is sized correctly but you most likely need at least a local disconnect. Many people like to use "semiconductor fuses" at the VFD because they act much faster than breakers and might protect the drive components better. I personally have never seen that be the case; in the few VFD deaths I have seen, the VFD blows and takes out the fuses after the fact, so you have more expense. Some VFDs however, especially those made by 2nd or 3rd tier suppliers, must have fuses to meet UL. Read the installation manual carefully.