I notice that many manufacturers will UL list a corresponding MSP (motor starter protector) as an OCPD for their VFD. The low end of the MSP FLA range always seems to be much higher than the input current of the VFD. Typically MSP's are selected based on motor FLA but in the case of VFD's are they are sized at 125% of the input current of the VFD?
Is there any reason not to feed a VFD with an MSP if its not specified by the manufacturer?
First, there are two main reasons why you see a lot of smaller VFDs listed in series combinations with MSPs*:
1) Because on really small drives, the smallest MCCB, 15A, allowable by rule because it protects the smallest wire anyway, is too large to allow for close enough protection for the VFD itself, which often NEEDS the closer protection to pass UL listing. So rather than rely only on fuses, they use the MSPs because they go lower than a 15A MCCB. This is especially true of 480V drives, because you can often get smaller MCBs (Miniature Circuit Breakers) but they are not rated for 480V Delta, so it makes it too complicated for casual users.
2) More importantly (to them), many VFD mfrs are also control device mfrs, but not all of them make MCCBs (that are UL489 listed), so they are listing their VFDs in series with THEIR OWN PRODUCTS, rather than having to test them with an MCCB made by a competitor.
Reason
not to use an MSP:
You cannot use any old MSP you find on the street corner, you must use the SPECIFIC unit that the VFD was listed with for the SCCR rating by the VFD mfr. That then means you must investigate the rules under which
THAT MSP unit can be used. Some have gone to the extra expense to be UL 489 listed as Branch Circuit Protective Devices, but many have not, meaning that they only have UL508 listings as Type E Self Protected Starters. That means you must have ANOTHER UL489 MCCB or UL98 disconnect with fuses ahead of it, with a maximum rating as specified. It's different from brand to brand, so you take on that added responsibility.
*I'm using the term MSP (Motor Starter Protector) as you (OP) did, but technically, that is a brand-specific term (Siemens). The more "generic" term for them in UL is Manual Self Protected Controller. Others call them "MCP"s, for Motor Circuit Protector, but that gets confused with the commonly used MCP that was brand-specific for Westinghouse (now Eaton) Magnetic-Only MCCBs. Still others call them MPCBs, for Motor Protection Circuit Breakers, or MMS for Manual Motor Starter.
