VFD Primary Conductor Sizing

[timm333]

I have a question regarding the primary conductor sizing of VFD. NEC 430.122 states that the conductor is to be sized for 125% of the rated input current of VFD; but it is applicable when there is power conversion equipment (rectifier) on the primary terminals of VFD.

In our case VFD has rectifier, but before the rectifier there is a “phase shifting isolation transformer” which feeds the rectifier. Will NEC 430.122 still be applicable if there is a “phase shifting isolation transformer” (instead of rectifier) on the primary terminals of VFD? Thanks

 

[winnie]

All common VFDs have an input rectifier, as part of their input current rating.

A phase shifting transformer won't change the VFD input current, unless it is only feeding part of the rectifier (eg a high pulse rectifier setup)

I don't know if 430.122 applies directly to conductors which feed a transformer which then feeds a VFD, but the same current is flowing (unless the transformer changes voltage) so I would expect a corresponding requirement to apply to the transformer feeder

Jon

 

[timm333]

Can NEC Table 450.3(B) be used for sizing the breaker for isolation transformer of VFD?

 

[petersonra]

I have a question regarding the primary conductor sizing of VFD. NEC 430.122 states that the conductor is to be sized for 125% of the rated input current of VFD; but it is applicable when there is power conversion equipment (rectifier) on the primary terminals of VFD.

Power conversion equipment does not mean rectifier.

 

[petersonra]

Can NEC Table 450.3(B) be used for sizing the breaker for isolation transformer of VFD?

The short answer is yes. It is just a transformer subject to all the NEC rules UNLESS it is part of a listed assembly.

The important question is why do you think you need an isolation transformer on a VFD? There are some cases where it is required, or maybe just a good idea, but normally it is not especially useful.

 

[timm333]

It is manufacturer’s standard design to include isolation transformer for harmonic mitigation.

 

[petersonra]

It is manufacturer’s standard design to include isolation transformer for harmonic mitigation.

If the manufacturer supplies the transformer as part of the listed assembly then it will come with instructions on what OCPD to use and the code requires you to abide by what those instructions tell you.
430.130(A)(2)

 

[winnie]

Is this a '12-pulse' rectifier where an input transformer is used to create 6 phases from the 3 phase supply, which then goes to the rectifier?

If so I would consider the phase shifting transformer to be part of the VFD, and base everything on the input rating of the rectifier system of the VFD.

-Jon

 

[Jraef]

Is this a '12-pulse' rectifier where an input transformer is used to create 6 phases from the 3 phase supply, which then goes to the rectifier?

If so I would consider the phase shifting transformer to be part of the VFD, and base everything on the input rating of the rectifier system of the VFD.

-Jon

Correct. 12 or 18 pulse VFDs use that transformer to create 6 or 9 phases going into the VFD rectifier. But from an installation standpoint, WE still have to size the feeder conductors to the primary of that transformer per 430.122.

If you were questioning the use of smaller wires going from the transformer to the rectifier, that is of course not your concern because it would be part of the drive listing design, but also the current flowing INTO each rectifier “pulse” is 1/12th or 1/18th that of the the 3 phases going into that transformer.