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Friday, October 25, 2019
Question:
I have done considerable research on this code requirement and it seems clear that you have to size conductors feeding the VFD panel at 1.25 x published max vfd amps.
This is a real problem in two scenarios. First, vfd’s often require oversizing as the set carrier frequency is increased above 3kHZ. This has nothing to do with current and everything to do with having a larger bridge to handle additional heat losses at higher carrier frequency.
The second scenario is when special configured wide constant hp motors ie rpmac have high output amp requirements but at low vfd output voltage. In this situation the actual output watts reflected back through the rectifier result in much lower drive input amps. In one case, the output amps at 310 vac is 600 amps, but at 460 vac is 400 amps.
Following nec 430.122 results in massive cable oversize in the vfd enclosure as well as the cable from the main down stream supply. Very expensive for both the VFD panel builder and end user!
In the past, I have seen people just protect against this by limiting current by sizing the incoming vfd enclosure breaker to 1.25 x motor fla (at 460 vac in the case of a 460 vac supply). This seems to make more sense to me! Please let me know if you have any input on this.
Thanks! Motorman
A
Answer:
Hey Motorman thanks for your question. You are correct, the minimum conductor ampacity must be 125% of the rated input current to an adjustable speed drive (ASD) as stated in 430.122(A). There are no exceptions but if the ASD manufacturer specified a larger conductor size that would be required as stated in 110.3(B). For the 2020 NEC additional wording has been added to clarify that sizing the adjustable speed drive output conductors is based on 125 % of the motor full load current. If you have sufficient substantiation to allow smaller input conductors, consider submitting a Public Input for the 2023 NEC