50 HP Compressor discrepancies

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tim89s

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An existing 50 HP compressor and controller states it "rated amps" is 82.
The new 50 HP compressor with VFD states its "rated amps" is 136.
They are both 480 volt 3 phase units.
The major difference between the compressors is the VFD. Would a VFD cause this big of a discrepancy in the rated amps? Since these are package unit do I size the wire off the of rated amps instead of the HP rating?


Thanks for any and all help

Tim
 
tim89s said:
An existing 50 HP compressor and controller states it "rated amps" is 82.
The new 50 HP compressor with VFD states its "rated amps" is 136.
They are both 480 volt 3 phase units.
The major difference between the compressors is the VFD. Would a VFD cause this big of a discrepancy in the rated amps? Since these are package unit do I size the wire off the of rated amps instead of the HP rating?


Thanks for any and all help

Tim
Click to expand...
When you are given the input current to a VFD, you need to size the conductors off that rather than by any information (either current or HP) of the controlled motor.
 
tim89s said:
Would I then take 125% of the input current as stated in 430.122 (a)? 136 amps x 1.25% = 170 amps.
Click to expand...
That is not my area of expertise, sorry....
The MCA for a motor already had the 1.25 (not 1.25%) built in, but I do not know how the VFD input current rating is assigned.
 
tim89s said:
An existing 50 HP compressor and controller states it "rated amps" is 82.
The new 50 HP compressor with VFD states its "rated amps" is 136.
They are both 480 volt 3 phase units.
The major difference between the compressors is the VFD. Would a VFD cause this big of a discrepancy in the rated amps? Since these are package unit do I size the wire off the of rated amps instead of the HP rating?


Thanks for any and all help

Tim
Click to expand...
A 50 Hp 480 volt 3 phase motor according to NEC should use a FLA of 65 amps for calculations. Now minimum conductor ampacity needs to be 125% of FLA which is 81.25 amps - close enough to the 82 on the existing controller that that is probably how that figure was derived.

Chances are your VFD is rated for 136 amps is why the new one has that figure marked on it. Perhaps the drive itself it is rated for more than 50 hp is the reason why it is such a high rating? They derate drives for many reasons, like frequent starting or frequent reversing - though neither of those are likely to apply to a compressor motor, they may still have some reason to install an oversized drive with the intent of getting more life out of it, maybe high ambient temp is a factor?
 
tim89s said:
Thank you Kwired. I am still confused if 125% needs to be applied to the rated amps.
Click to expand...
The supply conductors to the VFD must have an ampacity of at least 125% of the rated input amps for the VFD.
430.122 Conductors ? Minimum Size and Ampacity
(A) Branch/Feeder Circuit Conductors. Circu it conductors supplying power conversion equipment included as part of an
adjustable-speed drive system shall have an ampacity not less than 125 percent of the rated input current to the power conversion
equipment
Click to expand...
The conductors from the drive to the motor must have an ampacity of at least 125% of the motor current as shown in the tables at the end of Article 430.
 
I will probably go to electrician hell for this, but I have a customer that happened to have a 250 hp drive that they no longer use the equipment it was driving (belonged to prior owners of the plant, but this owner doesn't use that equipment or even that section of the plant where it was located). They had a need for a VFD on a 50 hp motor so we used the 250 hp drive. Supply conductors and overcurrent device are no where near what would be required for the drive rating, and has been that way for around 15 years:angel:
 
Since these are package units does that make a difference if you apply 125% or not. For example
one unit has a rated amperage of 82 amps. As pointed out earlier this number is very close
to 65 amps (from table 430.250) x 1.25 = 81 amps. On package units do you multiply the rated amps
by 125%?
 
tim89s said:
Since these are package units does that make a difference if you apply 125% or not. For example
one unit has a rated amperage of 82 amps. As pointed out earlier this number is very close
to 65 amps (from table 430.250) x 1.25 = 81 amps. On package units do you multiply the rated amps
by 125%?
Click to expand...

Is this refrigeration compressor or something else like an air compressor?

If refrigeration equipment the unit should have a MCA on the nameplate and just follow that information, the MCA already has continuous factor of 125% figured into it.

If an air compressor or something similar, it probably still has a nameplate but may or may not mention minimum conductor ampacity. You need to find out exactly what any values marked on it mean. You can help us speculate:) by telling us exactly what is listed on the nameplate.
 
It is an air compressor. Here is the info on the nameplate
supply volt 460
phase/hertz 3/60
rated amps 82
max H.P. 50
control volts 120

this is exactly how the nameplate reads.
 
kwired said:
I will probably go to electrician hell for this, but I have a customer that happened to have a 250 hp drive that they no longer use the equipment it was driving (belonged to prior owners of the plant, but this owner doesn't use that equipment or even that section of the plant where it was located). They had a need for a VFD on a 50 hp motor so we used the 250 hp drive. Supply conductors and overcurrent device are no where near what would be required for the drive rating, and has been that way for around 15 years:angel:
Click to expand...
That is not uncommon.

The code also does not require that the OCPD be sized at 125% of the input amps, only that the supply conductors be so sized.
 
tim89s said:
Since these are package units does that make a difference if you apply 125% or not. For example
one unit has a rated amperage of 82 amps. As pointed out earlier this number is very close
to 65 amps (from table 430.250) x 1.25 = 81 amps. On package units do you multiply the rated amps
by 125%?
Click to expand...
It is based on what is used as the motor controller.

If a starter is used as the controller, the conductors are sized based on 125% of the motor full load current as found in the tables in Article 430.

If a VFD is used as the controller, the conductors are based on 125% of the rated input amps to the VFD.
 
I will try to be calm. To size conductors on the controllers is nuts. VFD not withstanding. When I was a plant engineer, we standardized on a few VFD sizes to cut down on inventory. It was not uncommon to have a drive larger than the driven load. WE logically sized conductors on the ACTUAL motor used! Who came up with this requirement, the wire and cable cartel? And, what about conductors from the VFD to the motor??? Why stop at VFD's, what about combination starters, huh? Too many of the "code" requirements obviously line the pocket of manufacturers. And let's not forget the "zero risk" mentality type. I really question the 136A rating of the VFD, talk about absurdly over sized!!!!! I must be in a sour mood today. RC
 
Ragin Cajun said:
I will try to be calm. To size conductors on the controllers is nuts. VFD not withstanding. When I was a plant engineer, we standardized on a few VFD sizes to cut down on inventory. It was not uncommon to have a drive larger than the driven load. WE logically sized conductors on the ACTUAL motor used! Who came up with this requirement, the wire and cable cartel? And, what about conductors from the VFD to the motor??? Why stop at VFD's, what about combination starters, huh? Too many of the "code" requirements obviously line the pocket of manufacturers. And let's not forget the "zero risk" mentality type. I really question the 136A rating of the VFD, talk about absurdly over sized!!!!! I must be in a sour mood today. RC
Click to expand...

I hear you. Just look at my example mentioned earlier of the 250 hp drive supplying a 50 hp motor. We left the existing 100 hp breaker supplying that motor and reused the existing 4AWG conductors (well we did have to extend some of them, a 250 hp drive doesn't exactly fit the same footprint as a size 3 starter)

A 250 HP motor would require 500 kcmil conductors minimum, not sure what the drive required but would have to be at least that size minimum. Only a couple dollars difference there in copper, and real easy to land on a 100 amp breaker:happyno:
 
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