timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
NEC 430.122(A) says minimum size of breaker for VFD is 125% of the rating of VFD. What is the maximum size of breaker that can be used for VFD?
VFD Breaker
- Thread starter timm333
- Start date
- Status
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
It doesn't say breaker, it says conductors. The conductors have to be at least 125%, but can be as large as you want.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
a vfd is just a motor circuit IMo. unless the manufacturers instructions say otherwise, I size it according to the normal way. many times the instructions used to limit the incoming overcurrent protection, but these days not so much.
shamsdebout
Senior Member
- Location
- Macon,GA
If I am thinking about this correctly, the MOCP published by the manufacturer is normally 175% of the FLA.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Actually, you will find that unless you are using VFDs sold by companies that also make MCCBs, the VFDs might not be UL listed for use behind circuit breakers at all, you might need to use fuses. So the only valid answer is to RTFM and see what the VFD manufacturer tells you is the correct size of breaker; if it is listed to be used with a breaker, it typically will only be listed with one. If they don't say anything, it's a safe bet that it's because they can't and you have to use fuses.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
I have not seen any VFDs in the last few years that are that way, but it may be just the brands we tend to use. There are some SCCR issues that can get in the way of using CBs with VFDS although the new line of AB drives all take on the AIC rating of the UL489 breaker that feeds it upto 100kA.
- Status