VFD Wire Size

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MR.DEI

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I am replacing a 75hp VFD with same size. The motor is a 480 volt with a 68 FLA. The manufacture install manual indicates to install a 125amp OCP. The existing wire is only #2 shouldn't this be #1 wire? Any help would be appreciated.
 
Do you have the rated current of the VFD ??
It's possible 240.4(B) would allow the #2 but without knowing the load there no way to tell (see 430.122).
 
The user manual only gives me the OCP at 125 amps. I called tech support they told me the Drive is rated up to 125 amps and that's what is required for the OCP even if my motor is only rated at 68 amps. The motor is feeding a chiller tower so I think I am ok with the #2 wire by going with 240.4(B). Thank you and Merry Christmas!
 
As Gus stated you need to know the input current of the drive then size the conductors at 125% of that number. The OCPD size is typically decided by the drive manufacturer.
 
Was existing originally a listed assembly? Was probably listed as it was.

Replacing drive on such a thing maybe is tricky if not replacing with OEM components.

Strictly going off NEC a 75hp 480 volt three phase motor needs a minimum 1 AWG copper conductor, NEC doesn't care what motor nameplate says you must use tables in 430 for FLA.

VFD supply conductors can be different though - for one thing they don't carry same power factor as the load conductors.
 
Jeffry said:
Your VFD vendor has a minimum wire size on a table that is published.
125a is an overload condition and is not continuous.

use this table for #2 wire amps
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/


Think you are ok here with #2.

Jeff
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The conductors feeding the VFD are sized base on the input current rating of the VFD. Since according to post #5 that rating is 97 amps the conductors ampacity must be at least 121 amps (97*125%). As Augie stated the #2 conductor is too small.
 
Jeffry said:
Your VFD vendor has a minimum wire size on a table that is published.
125a is an overload condition and is not continuous.

use this table for #2 wire amps
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/


Think you are ok here with #2.

Jeff
Click to expand...
430.122 requires that the line side conductors feeding power conversion equipment (such as a VFD) are to be sized at 125% of the rated input current of the device. This is not interpretable based on actual load.

You can’t use the 90C column for general ampacity, you only use that when doing de-rating for VD or # of conductors. So in the 75C column, #2 is rated 115A, his minimum rating requirement per 430.122 is 121.5A, hence the #2 is inadequate.

125A is not an “overload condition”, it is the size of the OCPD they selected and while I would have probably gone a little higher, it’s acceptable. The VFD mfr likely provided a MAXIMUM size of the OCPD based on 250% of the expected motor FLC, but there is nothing saying you can’t go smaller than that and people often do to save space/cost. The difference here might be the difference between a 125AF and a 250AF breaker, which can be significant when designing a panel.
 
Besoeker said:
Ah, I hadn't thought of that.
Thank you.
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I didn't think about it either, that changes things. Technically one must still supply the VFD with conductor based on input rating - which I think is stupid if driving a lesser than rating motor, as long as you also reduce overcurrent protection accordingly.

Output side of drive - if you have motor overload set correctly you can't overload a conductor properly sized for motor. Standard 60 HP motor only needs 3 AWG conductor before any adjustments would come into play.
 
Thank you for your support! Since the VFD is considered a piece of equipment I need to go by the input rating +125% to size the conductors. which is #1 wire. I don't like this rule only because the motor is rated at 68amps! If I would of looked at the options for the ABB drive I could of saved myself some headache! They make another model with a 87 amp input rating with the same 125amp OCP. Now I know when replacing an existing VFD research the options on the input rating with the existing conditions.
 
MR.DEI said:
Thank you for your support! Since the VFD is considered a piece of equipment I need to go by the input rating +125% to size the conductors. which is #1 wire. I don't like this rule only because the motor is rated at 68amps! If I would of looked at the options for the ABB drive I could of saved myself some headache! They make another model with a 87 amp input rating with the same 125amp OCP. Now I know when replacing an existing VFD research the options on the input rating with the existing conditions.
Click to expand...
As I mentioned before I think the need to supply the VFD with input rating when used on a smaller than rated motor is pointless. It will never draw more power then overload setting will allow. On top of that input side will be less amps than output side because of power factor. Input may have distortion power factor but will not have displacement power factor, that only exists between the motor and the drive.
 
kwired said:
As I mentioned before I think the need to supply the VFD with input rating when used on a smaller than rated motor is pointless. It will never draw more power then overload setting will allow. On top of that input side will be less amps than output side because of power factor. Input may have distortion power factor but will not have displacement power factor, that only exists between the motor and the drive.
Click to expand...

Maybe the thinking is that someone might put a 75 hp motor on that 75 hp VFD at some point in the future.
 
Besoeker said:
Maybe the thinking is that someone might put a 75 hp motor on that 75 hp VFD at some point in the future.
Click to expand...
or they may put a 40 Hp on it. When that time comes whoever does so must determine if what is there is sufficient for what they want to do. If driven machine is rated 75 HP one would think it would already have a 75 HP motor on it.

Maybe they derated the drive for some reason? Lots of starts, breaking, accelerating, reversing?
 
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