Equipment run for new compressor and air dryer.

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Maintenance@privatelabelf

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New 100 hp compressor install, rated fla 125a. 200 amp breaker, 4/0 wire 200 amp fusible disconnect. I need a second disconnect for the dryer, 30 amp. I had planned to tap off the line side of the 200a disconnect.

Double tapping requirements came up from one of my techs. I can't find where this would apply to this situation. At least my interpretation.
 
There is no problem with "tapping" off your 4/0 to feed the dryer, however, you need to do so in a Code compliant manner.
In all probability the lugs on your disconnect are not rated for two conductors, especially a 4/0 & #10.
You might consider using a "Polaris" connector or similar.
 
Maintenance@privatelabelf said:
New 100 hp compressor install, rated fla 125a. 200 amp breaker, 4/0 wire 200 amp fusible disconnect. I need a second disconnect for the dryer, 30 amp. I had planned to tap off the line side of the 200a disconnect.

Double tapping requirements came up from one of my techs. I can't find where this would apply to this situation. At least my interpretation.
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I would question your figures you mention before commenting on you dryer circuit.
While I don't know all the details of your compressor and type of starting you may need to rethink this. For example, if this is a garden variety piston type compressor with across the line starting your conductors would only need to be 2/0. But the 200 amp breaker is likely to small for this machine to start reliably. The NEC would allow a 300 amp breaker or even a 350. This may be important as when you go from 200 amp to, say, a 300 the frame size will change and you must be able to accommodate this.
Pending more details, I would likely choose to use 2 branch circuits in the same raceway and just adjust the conductor ampacity for 6 CCC. For the compressor that would still be less than a 4/0.
 
That size of compressor is probably a scroll or airscrew type. Those start virtually unloaded, then controls start the compression action. Also that size of motor would probably be two stage start.
 
hillbilly1 said:
That size of compressor is probably a scroll or airscrew type. Those start virtually unloaded, then controls start the compression action. Also that size of motor would probably be two stage start.
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I agree and that is why I was choosing my words carefully. My point was that there seemed to be a lack of understanding of code requirements. In fact, if this is a typical modern packaged screw compressor it will likely have minimum MCA and max OCPD listed in the specs.
 
Maintenance@privatelabelf said:
Thank you all for the information. This is a screw compressor on a VFD. The manufacturer calls out the 4/0 for this machine and fuse protection at 125a at 460v.
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Check the nameplate and instructions carefully. We have a screw compressor that (unclearly) called out similar specs. It could be wired for either 208V or 480V, and they only listed the wire size for 208V. You can likely install smaller conductors and save $$$ on conduit and wire.


SceneryDriver
 
SceneryDriver said:
Check the nameplate and instructions carefully. We have a screw compressor that (unclearly) called out similar specs. It could be wired for either 208V or 480V, and they only listed the wire size for 208V. You can likely install smaller conductors and save $$$ on conduit and wire.


SceneryDriver
Click to expand...
I installed one last year that was ordered 480, but came in labeled 208, called the distributor, and they said they changed it, but just didn’t relabel it. Checked it anyhow, and they did change it, but the motor leads were done very sloppily. One wire just fell out of the connectors they used! Phase loss monitor exploded twice, though it was rated and set for 480.
 
hillbilly1 said:
I installed one last year that was ordered 480, but came in labeled 208, called the distributor, and they said they changed it, but just didn’t relabel it. Checked it anyhow, and they did change it, but the motor leads were done very sloppily. One wire just fell out of the connectors they used! Phase loss monitor exploded twice, though it was rated and set for 480.
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Did it self repair between those two explosions?:cool:
 
augie47 said:
There is no problem with "tapping" off your 4/0 to feed the dryer, however, you need to do so in a Code compliant manner.
In all probability the lugs on your disconnect are not rated for two conductors, especially a 4/0 & #10.
You might consider using a "Polaris" connector or similar.
Click to expand...

Maintenance@privatelabelf said:
Thank you all for the information. This is a screw compressor on a VFD. The manufacturer calls out the 4/0 for this machine and fuse protection at 125a at 460v.
Click to expand...
Not exactly sure but 125 amp fuse should not protect a 100 HP motor that draws 125 amps. I prefer to use the lowest ampere fuse that I could get away with so in most cases only like to go 125% over motor FLA. For a motor drawing 125 amps and does not have a longer start to full speed like some air compressors and centrifuges I would use a quality 175 amp time delay fuse.
 
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