Min Ampacity - VFDs

[ptonsparky]

The informational note to 430.122(A) states "Power conversion equipment can have multiple power ratings and corresponding input currents."

Typically I look at the label on the drive and use that current. Looking at the fine manual I find those different currents.
In this case the label max current is 24*1.25=30. That is at 200V. Knowing that my system voltage will be 240 I should be able to use 20.2*1.25=25.25. Correct?

Splitting hairs on this example but it may make a difference on other installs

 

[kwired]

View attachment 2575536
The informational note to 430.122(A) states "Power conversion equipment can have multiple power ratings and corresponding input currents."

Typically I look at the label on the drive and use that current. Looking at the fine manual I find those different currents.
In this case the label max current is 24*1.25=30. That is at 200V. Knowing that my system voltage will be 240 I should be able to use 20.2*1.25=25.25. Correct?

Splitting hairs on this example but it may make a difference on other installs

I say yes you are looking at 25.25 needed for ampacity of conductor and basis for overcurrent protection as well.

Whether or not I'd use 12 AWG vs 10 AWG conductor - might depend on details of the application.

This is apparently a 3 HP drive, if I were powering a smaller motor for some reason....

I have a customer we put a 100 HP motor on a 250 HP drive simply because it was laying around as they don't use the equipment it was connected to - that was previous owners operation in a now unused portion of the plant. It hasn't burned the place down yet and has been operating at least 10 or more years on that 250 HP drive with only 100 HP sized conductors and 100 HP sized overcurrent device. Reality is it likely should work on even lower rated overcurrent device. Original breaker was 200 amp, probably would been fine on a 125, or since they never load it to 100 HP likely would hold on maybe even a 90 amp breaker.

 

[infinity]

In this case the label max current is 24*1.25=30. That is at 200V. Knowing that my system voltage will be 240 I should be able to use 20.2*1.25=25.25. Correct?

Correct, you can use the nameplate current of 20.2 amps because that corresponds to your input voltage.

 

[ptonsparky]

I say yes you are looking at 25.25 needed for ampacity of conductor and basis for overcurrent protection as well.

Whether or not I'd use 12 AWG vs 10 AWG conductor - might depend on details of the application.

This is apparently a 3 HP drive, if I were powering a smaller motor for some reason....

I have a customer we put a 100 HP motor on a 250 HP drive simply because it was laying around as they don't use the equipment it was connected to - that was previous owners operation in a now unused portion of the plant. It hasn't burned the place down yet and has been operating at least 10 or more years on that 250 HP drive with only 100 HP sized conductors and 100 HP sized overcurrent device. Reality is it likely should work on even lower rated overcurrent device. Original breaker was 200 amp, probably would been fine on a 125, or since they never load it to 100 HP likely would hold on maybe even a 90 amp breaker.

I see nothing that requires I supply BC or SC protection at the max allowed by the VFD. The conductors should be sized appropriately. Yes the manual may say the type of fuse, breaker or MCP and the combination of.

 

[kwired]

I see nothing that requires I supply BC or SC protection at the max allowed by the VFD. The conductors should be sized appropriately. Yes the manual may say the type of fuse, breaker or MCP and the combination of.

I don't see anything either. Still can't convince me in my case of the 100 HP motor on a 250 HP drive that the OCPD originally sized for a 100HP motor won't protect the conductors also sized for a 100HP motor. The drive will trip on overload, the breaker will trip on short circuit/ground fault, just like it does with a magnetic starter as the controller. We only did that because we already had the drive. If it ever gives up it will be replaced with a 100 HP drive. That drive is an Allen Bradley drive that was new in probably about 1995 or so. It certainly doesn't owe anybody anything anymore.

 

[Jraef]

The OCPD values that drives typically showed has traditionally been maximum only, meaning if you wanted to use smaller, that’s your business. But with the recent changes in UL 61800-5-1 listings that require fuses in a lot of cases, they are now specifying MINIMUM fuse sizes as well. I only recently started seeing this, because drives that were listed under the old UL508 system didn’t need to consider that.

 

[ptonsparky]

The OCPD values that drives typically showed has traditionally been maximum only, meaning if you wanted to use smaller, that’s your business. But with the recent changes in UL 61800-5-1 listings that require fuses in a lot of cases, they are now specifying MINIMUM fuse sizes as well. I only recently started seeing this, because drives that were listed under the old UL508 system didn’t need to consider that.

Good to know because I’ve certainly used 30 amp vs 60 amp Class J.

 

[Jraef]

If the drives were tested and listed prior to the change (2020), the manufacturer can leave them as is. New drives designed and listed since the change have to follow the new rules.

 

[ptonsparky]

If the drives were tested and listed prior to the change (2020), the manufacturer can leave them as is. New drives designed and listed since the change have to follow the new rules.

I looked at a PowerFlex 525 and saw the min max fuse for IEC applications. UL didn't have the option.
My GS20 has two input currents. One each for VT or CT and while it lists the input voltage range of 200-240 the manual shows no difference in current as did the older ATV in my OP.

RTFM because YMMV is not an understatement.

The EC I was helping wants to be shown how because of his poor reading and comprehension skills. Maybe with the old PowerFlex 4, but nothing since.

 

[kwired]

The OCPD values that drives typically showed has traditionally been maximum only, meaning if you wanted to use smaller, that’s your business. But with the recent changes in UL 61800-5-1 listings that require fuses in a lot of cases, they are now specifying MINIMUM fuse sizes as well. I only recently started seeing this, because drives that were listed under the old UL508 system didn’t need to consider that.

You can go less with OCPD but NEC seems to think you must have minimum conductor sized per the drive rating even if you aren't going to use the full capacity and still have OCPD that protects your conductor. Not very often does one put a much smaller motor on a drive like my example had, but there are occasions - we already had an oversized drive that wasn't being used so we used it.

 

[garbo]

I say yes you are looking at 25.25 needed for ampacity of conductor and basis for overcurrent protection as well.

Whether or not I'd use 12 AWG vs 10 AWG conductor - might depend on details of the application.

This is apparently a 3 HP drive, if I were powering a smaller motor for some reason....

I have a customer we put a 100 HP motor on a 250 HP drive simply because it was laying around as they don't use the equipment it was connected to - that was previous owners operation in a now unused portion of the plant. It hasn't burned the place down yet and has been operating at least 10 or more years on that 250 HP drive with only 100 HP sized conductors and 100 HP sized overcurrent device. Reality is it likely should work on even lower rated overcurrent device. Original breaker was 200 amp, probably would been fine on a 125, or since they never load it to 100 HP likely would hold on maybe even a 90 amp breaker.

The place I retired from would increase the VFD by one size for critical equipment. Example for a 60 HP plain Jane NEMA frame motor went with a 75 HP drive. Would have to use smaller ampere fuses on the drive fuse holder . Thinking was drive would last longer.

 

[kwired]

The place I retired from would increase the VFD by one size for critical equipment. Example for a 60 HP plain Jane NEMA frame motor went with a 75 HP drive. Would have to use smaller ampere fuses on the drive fuse holder . Thinking was drive would last longer.

Frequently started or reversed motors is one place where you usually are supposed to de rate a drive when selecting it to help lengthen life of the drive. May need to de rate the motor as well.

 

[don_resqcapt19]

You can go less with OCPD but NEC seems to think you must have minimum conductor sized per the drive rating even if you aren't going to use the full capacity and still have OCPD that protects your conductor. Not very often does one put a much smaller motor on a drive like my example had, but there are occasions - we already had an oversized drive that wasn't being used so we used it.

Many drives have multiple ratings that are found in the instructions and those are permitted to be used to size the input conductors. It does not always have to be 125% of the rated input shown.

The informational note following 430.122(A) was added in the 2011 code with the following substantiation.

This proposal clarifies current practice in that power conversion equipment may have multiple input ratings.
Equipment might have only one rating on the name plate. Additional ratings can be provided in the manufacturers instructions for use.

However there are many cases where the driven motor is much smaller than the smallest rating of the VFD.

 

[kwired]

Many drives have multiple ratings that are found in the instructions and those are permitted to be used to size the input conductors. It does not always have to be 125% of the rated input shown.

The informational note following 430.122(A) was added in the 2011 code with the following substantiation.

Going back to OP we have a rating for 200 volts and a rating for 240 volts input. You are stuck with using the rating for which ever voltage you input and essentially you still have one rating if you only have one voltage source to supply it with.

 

[ptonsparky]

Going back to OP we have a rating for 200 volts and a rating for 240 volts input. You are stuck with using the rating for which ever voltage you input and essentially you still have one rating if you only have one voltage source to supply it with.

Correct, but that same drive can be used on 208 or 240 with corresponding current.

My AD drive could require different conductors depending on how I am using them. VT or CT.

 

[Jraef]

You can go less with OCPD but NEC seems to think you must have minimum conductor sized per the drive rating even if you aren't going to use the full capacity and still have OCPD that protects your conductor. Not very often does one put a much smaller motor on a drive like my example had, but there are occasions - we already had an oversized drive that wasn't being used so we used it.

That’s the most common reason. I have a customer who buys VFDs in MCC buckets that he sizes the same as NEMA sizes, meaning he uses a 10HP drive for everything from 5-10HP, a 25HP drive for everything from 15-25HP, etc. that way if a drive goes down on a 5HP motor, and he has a spare 10HP bucket, he just swaps it out, fast because the entire bucket is the same size.

The weird one is when you run a motor that is BIGGER than the VFD, which you can do. The biggest differential I have done was a 20HP drive turning a 50HP motor. It’s a long story, but it worked for what they needed it to do. The thing was, the input conductors had to be sized per the INPUT current rating of the drive, but the output conductors had to be sized per the HP of the motor. So we had 8ga on the input side and 4ga from the drive to the motor.

 

[kwired]

That’s the most common reason. I have a customer who buys VFDs in MCC buckets that he sizes the same as NEMA sizes, meaning he uses a 10HP drive for everything from 5-10HP, a 25HP drive for everything from 15-25HP, etc. that way if a drive goes down on a 5HP motor, and he has a spare 10HP bucket, he just swaps it out, fast because the entire bucket is the same size.

The weird one is when you run a motor that is BIGGER than the VFD, which you can do. The biggest differential I have done was a 20HP drive turning a 50HP motor. It’s a long story, but it worked for what they needed it to do. The thing was, the input conductors had to be sized per the INPUT current rating of the drive, but the output conductors had to be sized per the HP of the motor. So we had 8ga on the input side and 4ga from the drive to the motor.

50 HP motor that maybe never ran over say 50 percent of normal speed of the motor, and likely not a high torque application or something kind of close to this?

I have a client with a 50 HP high pressure positive displacement pump that we run on a VFD they run it as low as about 30 Hz depending on product they are running through it. But it still draws near nameplate rating even at that speed because it is pretty high torque application, but if you look at what output voltage is and do the math, that motor is basically only doing around 25 HP of work.

 

[Jraef]

Fan that didn’t need to run at more than 40% speed. The fan was repurposed from another application because this was at a fish processing plant in Alaska, where it took weeks or months to get new equipment in by boat.

 

[garbo]

Frequently started or reversed motors is one place where you usually are supposed to de rate a drive when selecting it to help lengthen life of the drive. May need to de rate the motor as well.

I worked at a large slaughter house back in the 1970's where they up sized drum switches that were used to feed 120 volt coils on reversing starters. The hoist that lifted a steer up 20' in the air onto a blood rail and was operated over a hundred times a hour. The drum switches were mounted maybe 25' off the kill floor and had two chains thru a wood dowel. Once a year we had to replace the contacts. Drum switches were from the early 1950's and label said to lubricate contacts with a coating of Vaseline. The reversing starters for all of these hoist were also up sized. On conveyors that started & stopped frequently we used an electric clutch to stop motion while keeping the motor running continuously. Clutch was mounted between a C face motor & a gearbox.