Calculating Panel FLA: Use VFD input current or motor table FLA?

Tip DS

Tip DS

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Using a VFD to drive a 5HP 480VAC, 3-Phase, 60Hz motor.

Motor FLA from NEC Table 430.250 is 7.6A
VFD Manufacturer states 480VAC Mains input current is 9.3A

Is the expectation to use the NEC or OEM value in calculating panel FLA? Where is that codified?
 
For load calculations we have to look at Article 220. The relevant section for feeder and service calculations is 220.50, which says (2020 NEC): "Motor loads shall be calculated in accordance with 430.24, 430.25, and 430.26 and with 440.6 for hermetic refrigerant motor-compressors." Since that doesn't mention 430.122, I'd say you don't need to consider the VFD rating.

Cheers, Wayne
 
wwhitney said:
For load calculations we have to look at Article 220. The relevant section for feeder and service calculations is 220.50, which says (2020 NEC): "Motor loads shall be calculated in accordance with 430.24, 430.25, and 430.26 and with 440.6 for hermetic refrigerant motor-compressors." Since that doesn't mention 430.122, I'd say you don't need to consider the VFD rating.

Cheers, Wayne
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Well considered response; thank you. Here's the mind bender... Consider an application where you have a single phase service supplying a VFD that drives a three phase motor.

Concrete example: 120VAC, 1-P, 60Hz service to VFD driving a 1HP 230VAC, 3-P motor. VFD input is 16.6A, motor FLA is 4.2A
If that's the only load in the panel, do you label the panel as a 120V, 1-P, 60Hz panel with FLA 4.2A, with largest motor 4.2A, or do you label it as a 120V, 1-P, 60Hz panel with 16.6A FLA - largest motor 4.2A or 16.6A?

Then do the same panel, but with 10 VFD/motor combinations within.
 
ptonsparky said:
Use hp Kw instead of amps, then convert.
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If I understand what you're proposing, that won't give you a valid solution. You can take a motor's HP from the NEC table, use it's kW value to calculate the amps of the motor - that amp value will be far lower than the table value. (It doesn't account for power factor or efficiency.)
 
Tip DS said:
Concrete example: 120VAC, 1-P, 60Hz service to VFD driving a 1HP 230VAC, 3-P motor. VFD input is 16.6A, motor FLA is 4.2A
If that's the only load in the panel, do you label the panel as a 120V, 1-P, 60Hz panel with FLA 4.2A, with largest motor 4.2A, or do you label it as a 120V, 1-P, 60Hz panel with 16.6A FLA - largest motor 4.2A or 16.6A?
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You have a 1 HP motor. Where the supply to that motor is 120V, single phase 2-wire, as at the panel, you'd use Table 430.248 and have an FLC of 16A. The downstream details of the VFD and how it supplies that motor and how that motor is wound don't matter for the panel load calc.

You could argue that the NEC is slightly non-conservative here, as it provides no allowance for the losses in the VFD. And so 220.50 should in fact reference 430.122. I wouldn't disagree, although the difference is not large (16.6A vs 16A).

Cheers, Wayne
 
FLC for a control panel is not the same thing as either the feeder calculated load, or the minimum ampacity of the feeder conductors.

This is what UL508a says.

49.2 The full-load current rating of the panel shall, at a minimum, include the sum of the current ratings
of all loads that are able to be operated simultaneously plus the primary current rating of all control
transformers connected to the input voltage.
Click to expand...

So you don't get a choice if it is a UL508a panel. You add up the max load of all the things attached to each power source and that is the FLC for that power source. The max load of a VFD has nothing to do with the motor attached to it. You have to use the rated input current of the VFD, which incidentally might not be real obvious because sometimes they are rated differently depending on whether it is "normal duty" or "heavy duty".

Note that UL says "at a minimum". Presumably, you can make the FLC anything you want above what it adds up to.
 
petersonra said:
You have to use the rated input current of the VFD, which incidentally might not be real obvious because sometimes they are rated differently depending on whether it is "normal duty" or "heavy duty".
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430.122 says you have to use the rated input current of the VFD for sizing the conductors supplying the VFD, but is there any language in the NEC that says you have to use that value for load calcs?

Cheers, Wayne
 
wwhitney said:
430.122 says you have to use the rated input current of the VFD for sizing the conductors supplying the VFD, but is there any language in the NEC that says you have to use that value for load calcs?

Cheers, Wayne
Click to expand...
The FLC calculation for a control panel is not covered by the NEC at all and has nothing to do with NEC calculated loads.
 
wwhitney said:
430.122 says you have to use the rated input current of the VFD for sizing the conductors supplying the VFD, but is there any language in the NEC that says you have to use that value for load calcs?

Cheers, Wayne
Click to expand...
Nope. It only applies to the conductor sizing.

By the way, most of the time, the input current rating of the VFD is actually LOWER than the output current, because the VFD going to essentially correct the power factor of the motor. So if you just use the motor current you are doing it the way it was intended.
 
petersonra said:
The FLC calculation for a control panel is not covered by the NEC at all and has nothing to do with NEC calculated loads.
Click to expand...
OK, but why are we talking about control panels? (Maybe it's that VFDs are listed as control panels?) Post #4 is about how to do the NEC load calc for a panel supplying a VFD that supplies a motor, for the case where the motor FLC on the load side of the VFD is less than the VFD rated input current.

Cheers, Wayne
 
Jraef said:
Do you label a distribution panel with load values at all? I guess we all just assumed control panel, because that’s something that needs labeling.
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Ah, I didn't know that about control panels. My responses have been based on "panel = panelboard." Perhaps the OP will clarify if that is what they meant or not.

Cheers, Wayne
 
wwhitney said:
Ah, I didn't know that about control panels. My responses have been based on "panel = panelboard." Perhaps the OP will clarify if that is what they meant or not.

Cheers, Wayne
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A panel board would have a feeder and that feeder load would be calculated according to nec rules.
 
petersonra said:
FLC for a control panel is not the same thing as either the feeder calculated load, or the minimum ampacity of the feeder conductors.

This is what UL508a says.

...

So you don't get a choice if it is a UL508a panel. You add up the max load of all the things attached to each power source and that is the FLC for that power source. The max load of a VFD has nothing to do with the motor attached to it. You have to use the rated input current of the VFD, which incidentally might not be real obvious because sometimes they are rated differently depending on whether it is "normal duty" or "heavy duty".

Note that UL says "at a minimum". Presumably, you can make the FLC anything you want above what it adds up to.
Click to expand...
Yes, this is pretty much exactly where I was leaning.
 
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