Basic Sizing Question

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kwired said:
My issue with said rule is when you have a drive that is rated for more than the motor connected to it. Technically you have to size input conductors to drive input, I have had a few cases where an oversized drive just happened to be available and why not use it. Like a 250 hp drive on a 100 hp motor. Drive is never going to draw anywhere close to it's input rating in this instance, if it does chances are it is also going to have a fault code on it in virtually no time and will shut down anyway.
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If a disconnect ws put in the middle of this run how would that be sized? Is that an overload?
 
horsegoer said:
So one needs either the VFD or motor current . Switch size no bearing on conductor sizing?
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That's correct unless you're talking about an OCPD. The size of the switch doesn't really matter. You could have a 200 amp switch feeding a 125 amp circuit.

VFD's usually have an OCPD size selected by the manufacturer. The conductors are sized according to the drive input current * 125%. For elevator controllers with VFD's we typically size the conductors to 100% of the drive input current.
 
infinity said:
That's correct unless you're talking about an OCPD. The size of the switch doesn't really matter. You could have a 200 amp switch feeding a 125 amp circuit.

VFD's usually have an OCPD size selected by the manufacturer. The conductors are sized according to the drive input current * 125%. For elevator controllers with VFD's we typically size the conductors to 100% of the drive input current.
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So the NEC doesn't require to 125% od the VFD current?
 
horsegoer said:
So the NEC doesn't require to 125% od the VFD current?
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I asked a similar question a few years ago the consensus was that the elevator controller VFD did not require the 125% factor for conductor sizing.

Elevator VFD Conductor Sizing

How would you determine the minimum conductor size for this VFD for an elevator motor? Seems simple to me 85 amps. Is there a different formula for elevator drives? input current * 125% 67.31 amps * 125% = 84 amps Someone is saying otherwise.
forums.mikeholt.com
 
infinity said:
I asked a similar question a few years ago the consensus was that the elevator controller VFD did not require the 125% factor for conductor sizing.

Elevator VFD Conductor Sizing

How would you determine the minimum conductor size for this VFD for an elevator motor? Seems simple to me 85 amps. Is there a different formula for elevator drives? input current * 125% 67.31 amps * 125% = 84 amps Someone is saying otherwise.
forums.mikeholt.com
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That was a good read an informative. Thanks.

Lastly, so the drive could be PART of the controller? I know with stand-alone VFD's you have conductors between the VFD and the motor. If the VFD was part of the controller would it be internally/factory wired or would those conductos need to be field installed? Thanks.
 
horsegoer said:
That was a good read an informative. Thanks.

Lastly, so the drive could be PART of the controller? I know with stand-alone VFD's you have conductors between the VFD and the motor. If the VFD was part of the controller would it be internally/factory wired or would those conductos need to be field installed? Thanks.
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A VFD is a motor controller whether a stand alone drive or part of a more complex assembly.
 
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