VFD OCPD - GOVERNED BY ARTICLE 430?

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jeremy.zinkofsky

Senior Member
Location
nj
I understand that it is not a good design practice to treat the line side wiring of a VFD as a motor circuit. Makes perfect sense as VFD input conductors will never see the inrush current on motor start-up.

My question is "Do i have a choice whether or not to ignore article 430 when sizing the OCPD feeding the VFD?"

It is clear to me that 430.120 Adjustable-Speed Drive Systems requires table 430.52 to be used when sizing the OCPD, no consideration or exception that limits the size of the OCPD to 125% of VFD rating.

So, IMO it is totally legal to choose the rating of the OCPD feeding the VFD to be greater than 125% of VFD rating as long as the values in table 430.52 are not exceeded.

Let me know if you agree or not.
 

kwired

Electron manager
Location
NE Nebraska
I understand that it is not a good design practice to treat the line side wiring of a VFD as a motor circuit. Makes perfect sense as VFD input conductors will never see the inrush current on motor start-up.

My question is "Do i have a choice whether or not to ignore article 430 when sizing the OCPD feeding the VFD?"

It is clear to me that 430.120 Adjustable-Speed Drive Systems requires table 430.52 to be used when sizing the OCPD, no consideration or exception that limits the size of the OCPD to 125% of VFD rating.

So, IMO it is totally legal to choose the rating of the OCPD feeding the VFD to be greater than 125% of VFD rating as long as the values in table 430.52 are not exceeded.

Let me know if you agree or not.
Code wise, no you must size supply side per the drive not the motor.

That said, a little dose of reality says if the front end rectifier of the drive fails, it needs replaced regardless of whether you complied with code or not when selecting overcurrent protection. Your choice of protection may change how spectacular that failure ends up being.
 

jeremy.zinkofsky

Senior Member
Location
nj
Code wise, no you must size supply side per the drive not the motor.

That said, a little dose of reality says if the front end rectifier of the drive fails, it needs replaced regardless of whether you complied with code or not when selecting overcurrent protection. Your choice of protection may change how spectacular that failure ends up being.

I'm approaching this without considering the VFDs role in the circuit, take the theory and design out of it. I simply am asking which section of the NEC governs the feeder to a VFD?
 

kwired

Electron manager
Location
NE Nebraska
I'm approaching this without considering the VFDs role in the circuit, take the theory and design out of it. I simply am asking which section of the NEC governs the feeder to a VFD?
430 partX, which is what you were looking at. But pay attention to what it says in 430.130(A)(2), that will apply most of the time and you will be protecting the drive with what the manufacturer specifies if you want to meet what NEC requires for short circuit/ground fault protection of the supply circuit. Minimum conductor size will usually be same if powering a motor same rating as the drive, overload protection you usually don't have to worry about other than to make sure you program the motor rated amps into the drive.
 

jeremy.zinkofsky

Senior Member
Location
nj
430 partX, which is what you were looking at. But pay attention to what it says in 430.130(A)(2), that will apply most of the time and you will be protecting the drive with what the manufacturer specifies if you want to meet what NEC requires for short circuit/ground fault protection of the supply circuit. Minimum conductor size will usually be same if powering a motor same rating as the drive, overload protection you usually don't have to worry about other than to make sure you program the motor rated amps into the drive.

Ok, thanks. I am still awaiting final VFD shop drawings and always match what the manufacturer recommends.

However, without that info i am allowed to not exceed the values in 430.52 for OCPD sizing. Correct statement?
 

kwired

Electron manager
Location
NE Nebraska
Ok, thanks. I am still awaiting final VFD shop drawings and always match what the manufacturer recommends.

However, without that info i am allowed to not exceed the values in 430.52 for OCPD sizing. Correct statement?
I don't believe many drives would allow the values allowed in 430.52, they don't have the same surge of current when starting so there is no need to have those high trip levels.

But back to what I said in my first reply, in the real world if the front end rectifier in the drive faults, those values in 430.52 are likely sufficient to allow for protecting the conductors, the rectifier is toast no matter what the protection level is. Most those drive manufacturers are going to want special fuses in the circuit and they are expensive yet financially you don't really gain anything by using them in most instances.
 
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