Vfd

[Scotta9669]

I have a vfd. I assumed that the feed to the motor would be rated according to the motor fla. The VFD has a 100 amp breaker. How does the code apply to this main feed and what size wire do I need? Its about 100 foot run to a 100 amp diconnect. Then 10 feet to the VFD which will be run with SO.

 

[weressl]

I have a vfd. I assumed that the feed to the motor would be rated according to the motor fla. The VFD has a 100 amp breaker. How does the code apply to this main feed and what size wire do I need? Its about 100 foot run to a 100 amp diconnect. Then 10 feet to the VFD which will be run with SO.

It maybe helpful in getting answer if you present your question in an orderly fashion. Usually it is best to follow the circuit in a linear fashion, from item to item, giving the size or other technical information, stating your question concerning that item, then onto the next circuit element.

 

[Scotta9669]

Got you. The control wiring and motor feed was already there is this a used piece of equipment. They intend on going from the service panel on a 100 amp breaker to a 100 amp disconnect, a distance of about 100 feet. Then from the 100 amp disconnect to a 100 amp breaker in the vfd to supply power to the VFD. I was thinking of ordering cable based on the 100 amps. Is this correct. I believe 4 or 3 awg. 480 volt system

 

[don_resqcapt19]

The code requires that the feeder to the VFD be sized at 125% of the rated input amps for the VFD. The conductors between the VFD and the motor are required to be at least 125% of the full load amps as found in the tables at the end of Article 430.

 

[Scotta9669]

So in the future I treat it as an ordinary motor circuit but the FLA is dependent on the VFD and not the motor it is controlling in a nut shell.

 

[ramsy]

The 2005 NEC has an example VFD feeder calc. in appendix D

Example D10 Feeder Ampacity Determination for
Adjustable Speed Drive Control [see 215.2, 430.24,
430.24, 620.13, 620.14, 620.61, and Table 430.22(E), and
620.14]

It appears to me, with an assumed 0.85pf for motor loads, the feeders would need to be at least #1 Awg to stay within 75c operating limits.

 

[Jraef]

So in the future I treat it as an ordinary motor circuit but the FLA is dependent on the VFD and not the motor it is controlling in a nut shell.

The slight difference is that the cables feeding the VFD are based on the VFD max amps, the cables going to the motor are sized as they always would be, based upon the MOTOR amps. So you can end up with two different size cables in the circuit. The breaker feeding the VFD then would of course be sized to protect those cables.

 

[Besoeker]

AC Variable speed drive currents and cables

AC Variable speed drive currents and cables

AC Variable speed drive currents and cables

The input and output currents are not the same. This is primarily as a result of the different power factor between the input and the output of the VSD.
The output power factor is determined by the motor displacement power factor.

The input power factor depends on the VSD input configuration. On all but the smallest units, this is most often a six-pulse diode bridge (with or without a dc choke) which draws non-sinusoidal current from the supply*. The degree to which it is non-sinusoidal is affected by supply reactance and the size of the dc choke. It is typically 0.95 or worse. This is distortion rather than displacement power factor.

Note also that the output current will be the lesser of maximum VSD current or motor rated current. The maximum continuous output current from the VSD cannot exceed motor rated current, even if the VSD has a higher rating than the motor. In this, case the cable need not be rated at VSD rating.

But after all that, I can?t recall ever putting in different input and output cables on a single motor drive.

 

[benaround]

The maximum continuous output current from the VSD cannot exceed motor rated current, even if the VSD has a higher rating than the motor. In this, case the cable need not be rated at VSD rating.

.

Besoeker, 430.122(A) requires feeders be 125% of the rated input of VFD.

 

[weressl]

But after all that, I can?t recall ever putting in different input and output cables on a single motor drive.

I do. The ASD feeder was within the MCC @ maintained 25C, while the ASD fed a motor outside where the temperature was 40C.

I have also used a 40HP ASD on a 30HP motor to eliminate a unique size drive and sized the cable according to the motor size.

 

[don_resqcapt19]

Since Part X of Article 430 does not specify the rating of the OCPD, and Part X says that Parts I-IX apply unless modified, then is it correc to say that the supply conductors to the VFD can have overcurrent protection per the rules in 430.52?

 

[benaround]

Since Part X of Article 430 does not specify the rating of the OCPD, and Part X says that Parts I-IX apply unless modified, then is it correc to say that the supply conductors to the VFD can have overcurrent protection per the rules in 430.52?

Don, 430.52 Rating or setting for Individual Motor Circuit, in Part X of Article

430, are we dealing with a motor circuit ?, or an Adjustable-Speed Drive System.

 

[Besoeker]

Besoeker, 430.122(A) requires feeders be 125% of the rated input of VFD.

Ah, OK. Fair point.
Being a Brit, I'm not familiar with that.
We use generally use BS 7671 (IEE wiring regulations) for rating cables.
Apart from current carrying capacity, it takes into account type and arrangement of cable, type of protection, installation method, voltage drop etc. As far as I am aware, it has no set requirement to size in accordance with VFD rated input current.
As a matter of good practice, you wouldn't normally use a cable that is close to being marginal and the next size up will give some spare capacity.

VFD inverters are generally rated by output current rather than input current. And the rms input current will, to some extent, depend on supply characteristics.
In relation to supply cables, this is in the manual for one of the bought-in VSDs we are currently using:
"The cables and fuses must be dimensioned in according to frequency converter nominal OUTPUT current which you can find on the rating plate....
.....because input current never significantly exceeds the output current."
Their use of upper case.

 

[weressl]

Ah, OK. Fair point.
Being a Brit, I'm not familiar with that.
We use generally use BS 7671 (IEE wiring regulations) for rating cables.
Apart from current carrying capacity, it takes into account type and arrangement of cable, type of protection, installation method, voltage drop etc. As far as I am aware, it has no set requirement to size in accordance with VFD rated input current.
As a matter of good practice, you wouldn't normally use a cable that is close to being marginal and the next size up will give some spare capacity.

VFD inverters are generally rated by output current rather than input current. And the rms input current will, to some extent, depend on supply characteristics.
In relation to supply cables, this is in the manual for one of the bought-in VSDs we are currently using:
"The cables and fuses must be dimensioned in according to frequency converter nominal OUTPUT current which you can find on the rating plate....
.....because input current never significantly exceeds the output current."
Their use of upper case.

FYI: This Forum concerns itself with the NEC and electrical practices and installations in the ANSI world.