Sizing two circuits for two 200 hp compressors

[cowman79]

I am currently looking at a job where I have two compressors to install both are 200 amp compressors one has a vfd and one does not. I am a little confused on sizing the wire and breakers for this application. We will have breakers that will feed fused disconnects next to the compressor and the farthest compressor is 50 feet from the switchgear.

first compressor with a VFD
480 volt 284 amp FLA
reccommended cable 500 kcmil
reccommended fuses fast acting 500 amps.

Second compressor non VFD
480 volt 275 amps fla
reccommended cable size 400 mcm
reccommended fuse size 350 amp time delay

 

[Jraef]

One tricky part is that for the one powered by the VFD, you must size the circuit for 125% of the VFD's max amp rating, not the motor FLA. So to do it right you will need the nameplate data from the VFD, sometimes they have significantly higher max. amp ratings than the motors you connect to. Right now at 284A you are fine for 500kcmil* but if the VFD is rated more than 304A, you may need to go to 600kcmil. For example a Constant Torque 200HP Hitachi drive is 290A, but a 200HP ABB CT drive is 316A. So depending on what they selecterd, you may be over the edge.

* I'm not doing the VD cals for you, that's your job

 

[cowman79]

The amperage is the rating is from the spec sheet for the compressor.

 

[Jraef]

The amperage is the rating is from the spec sheet for the compressor.

I would still check, they are not above making mistakes and it's you that would have to re-pull the cable!

 

[augie47]

welcome to the Forum.
If you have lurked here long, you already know some of the best advice you can find on motors comes from Jraef so I would certainly heed anything he says.
From the FLA it looks as if you have more than just a 200 HP motor (such as
fan motors,controls, etc). Without that info one can't say for sure about the non-VFD compressor. The 400 kcmi is probably fine, but cutting it close depending on other loads.
On the VFD unit Jraef's advice is what I would have cautioned.

 

[cowman79]

Thanks for the advice. When sizing the breakers for these feeds would they need to be 300 percent of fla. This seems to be the confusing part that the breakers and fuses could have an ampacity larger than the cable it is protecting.

 

[skeshesh]

Regarding your last question, read the on-going current discussion here: http://forums.mikeholt.com/showthread.php/138947-Motor-Feeder-Overload

Here's another older thread that discusses this issue: http://forums.mikeholt.com/archive/index.php/t-136920.html?

Basically the conductors are protected by the motor overload and your breaker or fuse is providing short-circuit/ground-fault protection.

Welcome to the forum and have a good weekend!