VFD load wiring

[rivecar44]

Trying to figure out if 3 conductors, 250kcmil, XHHW, rated 90 C, inside a conduit is acceptable to use for a motor controlled by vfd. VFD manufacturer recommends wiring rated at 75 degrees. motor full load amps is 225 amps. Table 310.16 shows 290 amps @ 90 degrees, and 255 amps @ 75 degrees. I understand that with the 125% multiplier per NEC the 250kcmil will not be suitable, yet the ampacity rating of the conductors limited by the terminations' temperature rating (75 degrees) is still above the full load amps of the motor.

 

[petersonra]

Code would require you to provide conductors to the motor with a minimum ampacity of 125% of the full load amps shown in the NEC motor current table T430.250. I don't see a motor in that table that says 225 Amps.

If the VFD instructions say to use 75 C wire, you can only use the ampacity of the conductors at 75 C. In most cases terminations are only listed for 75 C.

 

[kwired]

If motor full load is 225 amps, keep in mind for most motor applications this value needs to come from motor current tables in art 430, then minimum conductor ampacity almost always needs to be 125% that value. Intermittent duty motors would be one case where that 125% factor doesn't apply. You need conductor with minimum ampacity of 281 amps before any adjustments if necessary are even considered.

 

[ptonsparky]

125% of VFD rating , not motor for line side of VFD.

 

[Krusscher]

I'm thinking you would be ok to run 90 degree wire but it would have use the 75 degree column due to termination ratings.

 

[garbo]

If motor full load is 225 amps, keep in mind for most motor applications this value needs to come from motor current tables in art 430, then minimum conductor ampacity almost always needs to be 125% that value. Intermittent duty motors would be one case where that 125% factor doesn't apply. You need conductor with minimum ampacity of 281 amps before any adjustments if necessary are even considered.

Have to disagree to a point. The 430 current tables vast majority of the time are higher then any plain Jane NEMA frame motor that are used in well over 95% of situations that at least I came across during my 50 years in the trade. Another point every time that I ordered a new or replacement VFD the extremely knowledgeable sales department that I usually dealt with to!d me VFD'S are sold by output ampere and not horsepower. Case in point several 100 HP 480 volt VFD'S that I purchased were rated for 125 to 127 Amps and the off the shelve plain jane four pole motors ( 1770 RPM ) . NEC should be adjusted for drives. Also any drive with at least a 5 second ramp up time do not draw high in rush currents.Mechanics told me that the 40 to 150 HP belt driven fans that had the 20 to 60 second ramp up time that I usually used made the belts last a lot longer. Therefore if I install a drive rated for 125 amps I believe that you should only need to size drive feeders 125% ( excluding uprising for voltage drop on long runs ) or 156.25 amps.

 

[petersonra]

The code does not care what you believe. It requires you to provide conductor ampacity of 125% of whatever the table says the full load current is.

 

[garbo]

Difficult to keep up with energy saving & other new products. Can remember reading maybe eight years ago somebody building a large office building wanted to use smaller guess 480/277 volt transformers to supply an all LED building but were not allowed.They had to install a dozen or more way over sized transformers to meet an old code article. With the high price of copper it does not make sense to install feeders from a motor chart that might be from the 1920's. Large place that I retired from had over 500 drives and can not remember coming across a single one that was not a standard two, four or six pole off the shelve NEMA or IEC three phase motor that were all had motor nameplates that were probably 15% lower then the NEC tables.

 

[kwired]

Difficult to keep up with energy saving & other new products. Can remember reading maybe eight years ago somebody building a large office building wanted to use smaller guess 480/277 volt transformers to supply an all LED building but were not allowed.They had to install a dozen or more way over sized transformers to meet an old code article. With the high price of copper it does not make sense to install feeders from a motor chart that might be from the 1920's. Large place that I retired from had over 500 drives and can not remember coming across a single one that was not a standard two, four or six pole off the shelve NEMA or IEC three phase motor that were all had motor nameplates that were probably 15% lower then the NEC tables.

Motor current charts are for motors running at usual speeds, with normal torque characteristics. They assume the worst power factor and worst efficiency of available NEMA general use motors for the most part.

 

[ptonsparky]

Motor current charts are for motors running at usual speeds, with normal torque characteristics. They assume the worst power factor and worst efficiency of available NEMA general use motors for the most part.

That's why we use the motor label for overcurrent protection. IDR if I've ever seen a label and the table come close to each other.

 

[kwired]

That's why we use the motor label for overcurrent protection. IDR if I've ever seen a label and the table come close to each other.

On a general purpose motor they are always more efficient. I've run into OEM/definite purpose motors that still may not be as bad as what is in NEC tables but with higher nameplate current than a general purpose motor of same rating. And of course you and I know there are some definite purpose motors out there for the AG industry that don't fit in the NEC tables. 12.5 HP air over is kind of common, 16 HP single phase T base motors have become kind of common the past 15-20 years as well, but you pretty much never see them outside of the grain handling industries.

 

[Jraef]

IMHO arguing that the Code is wrong is a pointless exercise in the field. You can make your arguments to the Code Making Panel to try to get it changed, but your opinions on its merits have no bearing on what an AHJ will allow. The Code calls for the
Motor lead conductors to be minimum 125% of the NEC table tables for that HP, based on max. 75C conductor ratings if the device terminals are rated for that, less if not. It’s really not open for debate.

The conductors FEEDING the VFD are different, they must be sized at 125% of the rated INTUT current of the VFD, which is usually less than the output current rating.

The issue of sizing a VFD for the motor nameplate FLA instead of HP is because the higher the pole count, the higher the FLA. I have seen some really expensive mistakes because of this. One client bought 250HP drives for their “250HP” pumps, but the pumps were slow speed 10 pole motors, the FLA was more than 350A instead of the 302A rating of the VFD. All 6 drives had to be returned with substantial restocking penalties.

 

[garbo]

IMHO arguing that the Code is wrong is a pointless exercise in the field. You can make your arguments to the Code Making Panel to try to get it changed, but your opinions on its merits have no bearing on what an AHJ will allow. The Code calls for the
Motor lead conductors to be minimum 125% of the NEC table tables for that HP, based on max. 75C conductor ratings if the device terminals are rated for that, less if not. It’s really not open for debate.

The conductors FEEDING the VFD are different, they must be sized at 125% of the rated INTUT current of the VFD, which is usually less than the output current rating.

The issue of sizing a VFD for the motor nameplate FLA instead of HP is because the higher the pole count, the higher the FLA. I have seen some really expensive mistakes because of this. One client bought 250HP drives for their “250HP” pumps, but the pumps were slow speed 10 pole motors, the FLA was more than 350A instead of the 302A rating of the VFD. All 6 drives had to be returned with substantial restocking penalties.

Like I said in an earlier post every time that I ordered a new or rep!acement VFD our very talented drive salesman always asked for the motor ampere. He said VFD'S are made & sold for ampere output and not horsepower. Of course the label on his drives always had a HP rating after the ampere rating. In my 50 years never came across a 10 pole motor. Just for a pump to run that slow thinking it was for thick material. Heard that the average VFD produces 3% energy lost so always had a problem believing that if a drive was running @ full speed and producing 60 hertz & 480 volt the input power should be using more power then the output. I know that with IGBT'S drives can fire up to a 5 K Hertz output and could never receive a definite answer if this what we used to call chopping of the output frequency uses less energy to power a motor compared to a motor that is just powered straight from line voltage. We were fortunate to have an extremely talented drive tech that only worked on drives for over 25 years tell me that the same thing that you stated in that drive input currents are usually less then the output currents but could not explain why. His company only charged a coup!e hundred dollars to perform drive start ups that was a win win because it extended free parts & labor warranty to three years.

 

[LarryFine]

We were fortunate to have an extremely talented drive tech that only worked on drives for over 25 years tell me that the same thing that you stated in that drive input currents are usually less then the output currents but could not explain why.

Does that claim include less output power, or just output current? My mental logic and understanding of physics tell me that the voltage must be higher if the current is lower.

 

[petersonra]

Does that claim include less output power, or just output current? My mental logic and understanding of physics tell me that the voltage must be higher if the current is lower.

The output to the motor has a PF substantially < 1. The input PF to the drive is close to 1. So the input current will likely be less than the output current at a given power.

 

[garbo]

The output to the motor has a PF substantially < 1. The input PF to the drive is close to 1. So the input current will likely be less than the output current at a given power.

Thanks for bringing up power factor. In my 40 years working on drives never occurred to me about the PF affecting the output current. Now that you brought that up can remember when we added PF capacitors to six 100 HP motors that ran 24/5 that would trip out maybe twice a year in a 100 degrees room years ago.None of the six motors ever tripped out after I stalling the capacitors.

 

[bwat]

The output to the motor has a PF substantially < 1. The input PF to the drive is close to 1. So the input current will likely be less than the output current at a given power.

This is something I've known for years, but never really tossed this around in my mind and wrestled with it.

Assuming a perfectly efficient drive, I would have wanted to say that the real power in, is equal to the real power out. But is it actually MVAin is equal to MVAout? Perhaps both? I need to bust the calculator out and draw some vectors...

 

[petersonra]

This is something I've known for years, but never really tossed this around in my mind and wrestled with it.

Assuming a perfectly efficient drive, I would have wanted to say that the real power in, is equal to the real power out. But is it actually MVAin is equal to MVAout? Perhaps both? I need to bust the calculator out and draw some vectors...

VA is not power.

 

[bwat]

VA is not power.

It's not real power, but it's certainly apparent power. Not sure what you're getting at and how that answers my post?

 

[petersonra]

It's not real power, but it's certainly apparent power. Not sure what you're getting at and how that answers my post?

MVAin as you called it is roughly the same numeric value as power in with a VFD because pf is close to unity.

MVAout is going to be more than power out because VFD output pf <1.