VFD Input Conductors & NEC 240.4(G)

[Flapjack]

Are VFD input conductors permitted to have an ampacity rating that is less than the rating of the protective device upstream?

NEC 240.4(G) states "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G)."
Table 240.4(G) calls out motor conductors - Article 430 Parts II through VII.
Adjustable speed drives fall under 430 Part X.
NEC 430.130(A)(1) for circuits containing power conversion equipment references 430.52(C) in Part IV to size the protective device.

So if the VFD input conductors are sized per 430.122, the upstream SC & GF protective device can have a higher ampacity than the conductors as long as it doesn't exceed the rules in 430.130, correct?

 

[ptonsparky]

Look at your installation instructions. You will find the suggested fuse protection is considerably higher than the conductor ampacity.

 

[PD1972]

Yes - VFD input conductors per 430.122 and OCPD per lesser of 430.130(A)(1) & 430.130(A)(2).

Most of the time that means that your minimum conductor size is less than or equal to that of your OCPD. However, there might be strange cases where your input conductors end up being larger than that required per 430.130 if you are oversizing your VFD. Note that 430.122 requires 125% of your drive input current where as 430.130 is based on FLC of the motor load. If you put a 1HP pump on a 10HP VFD, the conductors are sized per the 10HP VFD input current and the OCPD per 1HP motor FLC.