# Sizing compressor breaker



## jwjrw (Jan 14, 2010)

What is the correct way to size the breaker for a compressor? Which code article applies? The name plate has 20.9 FLA and the amps were between 12 to 21. It is on a 25 amp now. Compressor company says compressor ok? Maybe it needs a 30a breaker? But it ran 4 years on a 25 with no issues.


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## BBQ (Nov 16, 2010)

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## jwjrw (Jan 14, 2010)

BBQ said:


> Thanks for posting on ElectricianTalk.com. The Moderators of this forum would prefer if you post Do It Yourself related topics on our sister site www.DIYChatroom.com
> 
> ElectricianTalk.com is designed for electrical industry professionals to discuss issues and topics related to the electrical trades and related industries. Many of our professionals are also members at DIYChatroom.com and are looking forward to assist you with your needs.
> 
> ...





:lol::lol::lol:

Now answer my question!


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## backstay (Feb 3, 2011)

Sounds like the coil is dirty and unit is running high pressure. Doesn't the condensor list the breaker/fuse rating?


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## jwjrw (Jan 14, 2010)

backstay said:


> Sounds like the coil is dirty and unit is running high pressure. Doesn't the condensor list the breaker/fuse rating?



Nope. Compressor serviced recently. All filters etc. Compressor looks like new and is indoors.
Most compressors I've seen do not say what breaker to use. I had to call compressor company last time.


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## BBQ (Nov 16, 2010)

jwjrw said:


> Now answer my question!


If all you have is an FLA I would multiple it by 1.25% and round up to the next breaker.

In your case it comes out to 26.125 amps so code wise I think you should have a 30 amp breaker but as you say it has been running fine on a 25.

Did it suddenly start tripping it?


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## HARRY304E (Sep 15, 2010)

jwjrw said:


> What is the correct way to size the breaker for a compressor? Which code article applies? The name plate has 20.9 FLA and the amps were between 12 to 21. It is on a 25 amp now. Compressor company says compressor ok? Maybe it needs a 30a breaker? But it ran 4 years on a 25 with no issues.



If it 20.9 FLA then a 25 amp breaker is fine.


*
240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers.​*​​​​The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601. The use of fuses and inverse time circuit breakers with​
nonstandard ampere ratings shall be permitted.


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## jwjrw (Jan 14, 2010)

BBQ said:


> If all you have is an FLA I would multiple it by 1.25% and round up to the next breaker.
> 
> In your case it comes out to 26.125 amps so code wise I think you should have a 30 amp breaker but as you say it has been running fine on a 25.
> 
> Did it suddenly start tripping it?



Yes it started tripping over the last few days. I went out and took amp readings. Had between 12 and 21.6 at times. Compressor company was out servicing it a few months ago. Came out today and said no issues with it. But I want to know why I was getting over 20.9 fla at times.


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## BBQ (Nov 16, 2010)

jwjrw said:


> Yes it started tripping over the last few days. I went out and took amp readings. Had between 12 and 21.6 at times. Compressor company was out servicing it a few months ago. Came out today and said no issues with it. But I want to know why I was getting over 20.9 fla at times.



What voltage is the motor and what voltage did you have?

Is this three phase and are the voltages / amperages pretty close from phase to phase?

Small 3 phase voltage imbalance can cause high readings and hot motors.




> The NEMA standard for electric motors and generators recommends
> that the maximum voltage unbalance be limited to 1%.
> 
> When the voltages between the three phases (AB, BC, CA)
> ...


 *Motor Protection Voltage Unbalance and Single-Phasing*


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## jwjrw (Jan 14, 2010)

BBQ said:


> What voltage is the motor and what voltage did you have?
> 
> Is this three phase and are the voltages / amperages pretty close from phase to phase?
> 
> ...


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## nitro71 (Sep 17, 2009)

Without a manufactures recomendation I'd size it like any motor. Breaker at 250%. Overloads/fuses at 125%. Or close depending on a few variables.


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## backstay (Feb 3, 2011)

jwjrw said:


> Yes it started tripping over the last few days. I went out and took amp readings. Had between 12 and 21.6 at times. Compressor company was out servicing it a few months ago. Came out today and said no issues with it. But I want to know why I was getting over 20.9 fla at times.


Just keep putting a bigger breaker in until the compressor has issues(I hate to tell them, but I think the commpressor is crying out for help)


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## Jlarson (Jun 28, 2009)

jwjrw said:


> Nope. Compressor serviced recently. All filters etc. Compressor looks like new and is indoors.
> Most compressors I've seen do not say what breaker to use. I had to call compressor company last time.


What type of unit is this with the compressor indoors?


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## jwjrw (Jan 14, 2010)

Jlarson said:


> What type of unit is this with the compressor indoors?


Screw type in a dry cleaners. Kaeser 7.5 horse.


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## Jlarson (Jun 28, 2009)

How does it behave, does it come up to pressure quick or slow? What about the oil, level is good, it's oiling and what not?


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## jwjrw (Jan 14, 2010)

Jlarson said:


> How does it behave, does it come up to pressure quick or slow? What about the oil, level is good, it's oiling and what not?


Since the compressor company was involved and they had serviced it recently and it was in use while I was there I assume it is ok. I'm going to put a 30a breaker on it and see what happens.


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## electures (Oct 23, 2008)

jwjrw said:


> BBQ said:
> 
> 
> > What voltage is the motor and what voltage did you have?
> ...


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## rnr electric (Jan 29, 2010)

i have seen faulty capacitors cause this problem once,does not supply enough kick to get motor to proper rpm and therefore straining the motor and pullig a higher amperage


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## Electric_Light (Apr 6, 2010)

rnr electric said:


> i have seen faulty capacitors cause this problem once,does not supply enough kick to get motor to proper rpm and therefore straining the motor and pullig a higher amperage


3 phase motors don't use capacitors.


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## MDShunk (Jan 7, 2007)

If it's drawing over nameplate, it could be as simple as what phase you put your amp clamp on. In a perfect world, every winding would be in balance with the others, but not always. Especially if the motor is failing.


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## Wirenuting (Sep 12, 2010)

Check to see if the unloading valve is sticking. That can cause it to trip intermittently.


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## racerjim0 (Aug 10, 2008)

Something else close by could be kicking on causing a temporary low voltage condition, raising the amps. It being a screw compressor , I'll assume the motor runs constantly and the unloader valve cycle as necessary. The 25 amp breaker is probably just getting old. I wouldn't think twice about putting a 30 amp breaker in it.


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## nitro71 (Sep 17, 2009)

Guess I missed something here. It sounded like the motor fluctuates on amperage up to it's FLA. That breaker is way to small for that motor unless the manufacturer has other specs.


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## jwjrw (Jan 14, 2010)

Manufacture says it should be on a 30 amp breaker.


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## Electric_Light (Apr 6, 2010)

If a motor is 208-230 rated, what voltage do they assume when they rate the FLA? Obviously under same load, it will be more at 208.


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## frenchelectrican (Mar 15, 2007)

Electric_Light said:


> If a motor is 208-230 rated, what voltage do they assume when they rate the FLA? Obviously under same load, it will be more at 208.


 
If that motour run on 208 triphase it will draw little more than what it will draw at 240 volts.


Jwjrw.,

The only way you can get the screw compressor overloaded quick is close the discharge port very fast { if it don't have bypass valve in there } and few case wrong oil or too much oil will result the simauir reaction.

But if you close the suction valve the current drawage will drop a bit to near unloaded value { pretty close to half of the FLA } but watch the oil level during that time.

BTW is this is a 1800 or 3600 RPM motour ? if 1800 you should be ok with the breaker keep it at either 25 or 30 but no more than that. for 3600 RPM use 30 or 35 amp breaker.

The other thing you can test is make a discharge port for air compressor and ball valve and throttle the discharge port and adjust it when you get steady flowage and pressure { the pressure should be near the cutoff or unloader valve kick in } and read the current drawage the screw will be highest when you are running near max flowage at max pressure that where I find the curpits.


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## jwjrw (Jan 14, 2010)

This was a 208 motor. Did not appear to be dual voltage rated. The nameplate said 208 not 208/230.


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