# 3phase compressor service sizing.



## HARRY304E (Sep 15, 2010)

Rdumas32 said:


> I am installing 2 compressors and 2 dryers. Each compressor is required by the manufacture to be fused at 175 amps. The current draw per the service manual is 117 amps each. The two dryers current draw is 4.9 amps each. I plan to install a 400 amp breaker to a main breaker panel and comming off of that with two 200 amp disconects with 175 amp fuses and two 15 amp breakers. My plan was to run 2 sets of 4/0 from the 400 amp to the new panel then run 3/0 to each disconect. 14 awg will be ran to the dryers. Am I going to big or am I right on the money. By the way from the main panel to the new breaker panel is only 75' Any insight is appreciated.


That looks about right to me.

Welcome to the forum..:thumbup:


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## Going_Commando (Oct 1, 2011)

Rdumas32 said:


> I am installing 2 compressors and 2 dryers. Each compressor is required by the manufacture to be fused at 175 amps. The current draw per the service manual is 117 amps each. The two dryers current draw is 4.9 amps each. I plan to install a 400 amp breaker to a main breaker panel and comming off of that with two 200 amp disconects with 175 amp fuses and two 15 amp breakers. My plan was to run 2 sets of 4/0 from the 400 amp to the new panel then run 3/0 to each disconect. 14 awg will be ran to the dryers. Am I going to big or am I right on the money. By the way from the main panel to the new breaker panel is only 75' Any insight is appreciated.


What is the HP of the compressors? You size the OL protection based on the nameplate FLA, and OCPD and motor feeder sizing is based on 430.250.


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## ponyboy (Nov 18, 2012)

I've done the two compressors two dryer thing tons of times and I believe you'd be better off having each compressor on its own breaker. Especially if the 400 amp you intend to use has ground fault protection.
In most of the two compressor setups I've done one of the two is not pushing full CFM and is there to supplement the first and serve as a fail-safe if the other goes down. Someone would be looking for blood if the back up compressor went down with the primary one for no good reason other than they shared a CB. Jmo, I've seen 100 people sent home because of this same scenario.


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## Rdumas32 (Mar 1, 2013)

ponyboy said:


> I've done the two compressors two dryer thing tons of times and I believe you'd be better off having each compressor on its own breaker. Especially if the 400 amp you intend to use has ground fault protection.
> In most of the two compressor setups I've done one of the two is not pushing full CFM and is there to supplement the first and serve as a fail-safe if the other goes down. Someone would be looking for blood if the back up compressor went down with the primary one for no good reason other than they shared a CB. Jmo, I've seen 100 people sent home because of this same scenario.


I am installing a 400 amp breaker in the main feed coming into the building. Off of that I am running two sets of 4/0 to a breaker panel with a 400 amp main. I will then install 2 200 amp breakers one for each compressor. I will not be sharing breakers. Also each compressor has a 100 hp motor. Thanks for the fast replies.


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## ponyboy (Nov 18, 2012)

Rdumas32 said:


> I am installing a 400 amp breaker in the main feed coming into the building. Off of that I am running two sets of 4/0 to a breaker panel with a 400 amp main. I will then install 2 200 amp breakers one for each compressor. I will not be sharing breakers. Also each compressor has a 100 hp motor. Thanks for the fast replies.


Ten four. You could get by with paralleled 3/0s though. I'm sure these units have reduced starting ability of some kind or VFDs but keep in mind the locked rotor current on a 100hp 460v motor is like 500 amps. You could feed each motor with a 350 amp breaker and still be right, assuming your overloads are correctly set.


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## Rdumas32 (Mar 1, 2013)

ponyboy said:


> Ten four. You could get by with paralleled 3/0s though. I'm sure these units have reduced starting ability of some kind or VFDs but keep in mind the locked rotor current on a 100hp 460v motor is like 500 amps. You could feed each motor with a 400 amp breaker and still be right, assuming your overloads are correctly set


So your're suggesting I come off my main panel with a 400 amp breaker and feed my panel for my compressor which I plan on having a 400 amp main. But to not use 200 amp breakers for each compressor use 400 amp breakers because of the locked rotor current? Am I correct?

What would your ideal instalation consist of?


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## Rdumas32 (Mar 1, 2013)

*So now I'm thinking*

Should my compressor panel be a 600 amp? with 2 400 amp breakers feeding my fused disconnects?


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## ponyboy (Nov 18, 2012)

That depends. What kind of starters are these compressors equipped with?


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## Rdumas32 (Mar 1, 2013)

ponyboy said:


> That depends. What kind of starters are these compressors equipped with?


I think its just a standard across the line start. I do not have the compressors in front of me they will not be here for another 2 weeks I only have the service manual which is where I have been getting all my information.


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## ponyboy (Nov 18, 2012)

Rdumas32 said:


> I think its just a standard across the line start. I do not have the compressors in front of me they will not be here for another 2 weeks I only have the service manual which is where I have been getting all my information.


Across the line...wow. ok so I'm guessing they will be sitting fairly close to one another? If so why not place the new panel in a location where its visible from both compressors and scrap the discos. If these are newer units they probably have overload contactors already in place


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## angryceltic (Feb 21, 2012)

It's very seldom that you would see a 100 hp d.o.l. They are doing part wind @ 7.5 hp and up nowadays due to load demand. I know most manufactures list the requirements for electrical in the Manual as its part of their listing requirements.


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## ponyboy (Nov 18, 2012)

angryceltic said:


> It's very seldom that you would see a 100 hp d.o.l. They are doing part wind @ 7.5 hp and up nowadays due to load demand. I know most manufactures list the requirements for electrical in the Manual as its part of their listing requirements.


Roger that I'd bet my boots its not line starting


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## Rdumas32 (Mar 1, 2013)

How can I attach a picture I took a screen shot of the electrical out of the manual? Or can I email the manual to one of yall and post it for me? Thanks


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## ponyboy (Nov 18, 2012)

Rdumas32 said:


> How can I attach a picture I took a screen shot of the electrical out of the manual? Or can I email the manual to one of yall and post it for me? Thanks


Whether they start across the line or with a soft start doesn't change the electrical requirements just maybe what size breaker you feed it with. If it were me and there was no way to know how they start I'd get two 350 amp breakers and be done with it, that way you're covered either way. The last two 100hp comps I did got put on 200 amp breakers but they had VFDs in them and had a 20 second ramp start


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## frenchelectrican (Mar 15, 2007)

100 CV (HP) on DOL that can be pushing the main system if not heeded with the set up.

But most of the modern compressours which I ran into do have VSD in there so they can ramp up the compressours without get the system in overload mode during start up.

The ramp up time the last unit was a screw compressour unit we set that for 15 seconds but the second unit is pistion unit we set that for 25 seconds ( the main reason is oil pump ) 

One of our European spec'ed compressour we have over here which they are wired for 415 volts triphase ( 100 CV ) can be used with either VSD or Wye-Delta starting system but I like the VSD better and we did use 300 amp breaker for this useage it hold up pretty good. ( it was pretty much max setting we can go with this moteur. )

Merci,
Marc


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## Cow (Jan 16, 2008)

Another way that may be cheaper is to run a 400 amp feed to a gutter/j-box. Install 4 disconnects tapped from dist. blocks in the gutter/j-box to feed your loads. No MB panel.


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## Rdumas32 (Mar 1, 2013)

*I have the machine in front of me now*

It has a wye start delta run system in it. So I should be okay with a 400 amp breaker feeding the gutter box with the disconnects tapped off. Am I correct? Thanks for all the feedback.


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## RePhase277 (Feb 5, 2008)

In a situation where coordination of these compressors is vital, wouldn't it be better to use 400 A fuses to feed the two 200-A breakers?


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