Use nameplate ratings on motor or VFD??

[ControlThis]

I have A VFD capable of running a 1 hp motor but will only be using a 1/2 Hp.

I'm running 208vac, 3 phase, 60 Hz

Motor nameplate states 2.21A @ 208vac
VFD nameplate states 8.2A @ 200-230vac

For the purpose of load calculations and sizing supply conductors, OCPD, and other peripherals, would I use the nameplate ratings of the motor or the VFD? I would usually use the larger, but where there is a large discrepancy between the two, I'm looking for insight on the proper way.
Thank you!

 

[DickDV]

I'm not an expert in NEC rules but, I believe that the input leads to the drive are sized by the drive rating and the motor leads from drive to motor are sized based on motor nameplate.

Is that right?

 

[waross]

The NEC requires motor conductors to be sized by Hp and the appropriate code table. The minimum is #14 AWG.
The supply conductors for equipment (VFD) is based on nameplate current. The minimum is #14 AWG.
Load will be based on nameplate current of the VFD. The starting surge and the over load current of the VFD will be based on the nameplate values.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ControlThis]

Thank you for the responses!

The application is an industrial machine with multiple motors/vfd's with one control cabinet. I sould have included more information as to where I was headed with this.

1.For the supply conductors to the machine, which rating would I use to calculate the load? I know to use 125% of the largest motor in this calc but of which value, 8.2A or 2.21A?

Questions on the control cabinet nameplate data i need to display on the cabinet:
2. For full load current rating of the machine, would I add the above mentioned motor as 8.2A or 2.21A?
3. Say this was the largest motor. Would I specify largest motor rating on this nameplate as 8.2A or 2.21A?

Thanks Again!

 

[waross]

Define or describe "machine". Does this include the VFD or is it supplied by the VFD?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ControlThis]

The machine includes 2 VFD's with PLC and low voltage controls.

 

[rockman7892]

For OCPD of a feeder to the the input of a VFD should the input current rating of the VFD be used or should 125% of the input current rating be used?

 

[ozmosis]

The feed to the VFD should be rated to the VFD capacity +local regulation overload rating. I think NEC states 1.25 or 1.2 overload.(If a VFD has the potential to operate at 8.2A then that is how you should design the system for input protection)
Depending on the type/make of VFD, the settings you input of the motor nameplate data will determine the necessary protection the VFD can provide to the motor, if it is a lower rating to the VFD. Some VFD's will have electronic thermal overload (built in) that will be certified by UL for single motor protection.This is wholly based on the user putting the correct motor details into the VFD.
If you do not do this, then you must use an additional thermal overload for the motor to ensure you have motor protection, or if you have multiple motors on the output then you need to have additional form of thermal overload and cannot rely on the in-built protection the VFD can provide.

 

[ControlThis]

Questions on the control cabinet nameplate data i need to display on the cabinet:
2. For full load current rating of the machine, would I add the above mentioned motor as 8.2A or 2.21A?
3. Say this was the largest motor. Would I specify largest motor rating on this nameplate as 8.2A or 2.21A?

Thanks Again!

 

[jraef]

As waross said, for the conductors going TO the VFD, the NEC requires using 125% of the MAX. INPUT CURRENT rating of the VFD, not the load connected to it; so 125% of 8.2A. The OCPD is sized then to protect the conductors. But in your specific case, regardless of the values you use, you will be using minimum 14ga wire anyway.

But it's all relative as well. If the VFD requires an OCPD that is SMALLER than what would protect the conductors, you must go with that. That would be stated in the VFD installation guidelines. So for example, you may use 14ga wire that is technically rated for 15A, but the VFD manual may say that you must have a 10A fuse in front of it. If that's the case, the 10A is not violating the NEC because it is LOWER than the conductor protection, so no problem. If on the other hand the VFD manual said to use a minimum 30A fuse or breaker, and you ran 14ga wire, you cannot protect that wire with a 30A breaker, you must use a 15A. Either that or run 10ga wire. The 10ga wire is of course a LOT larger than the minimum 125% of the VFD input current, so no problem there.

For the conductors going TO the motors, follow the standard NEC rules for motor conductors, i.e. 125% of FLC of 2.21A. The Motor Thermal Overload protection of the VFD is protecting those conductors.

If you have more than one motor behind the VFD, you technically need new Short Circuit protection for the conductors as well. You were not clear as to how many VFDs vs how many motors you have.

For the FLC rating of the machine, you would need to base it on the FLC rating of the VFD, the 8.2A. but for the "largest Motor" rating, I would use the actual motor value. That information is somewhat useless anyway, so just follow the exact wording.


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