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Old 04-19-2011, 06:55 PM   #1
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Default Sizing compressor breaker

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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Old 04-19-2011, 06:56 PM   #2
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Old 04-19-2011, 06:59 PM   #3
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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.

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Now answer my question!
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Old 04-19-2011, 06:59 PM   #4
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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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Old 04-19-2011, 07:01 PM   #5
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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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Old 04-19-2011, 07:04 PM   #6
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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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Old 04-19-2011, 07:06 PM   #7
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Quote:
Originally Posted by jwjrw View Post
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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Old 04-19-2011, 07:08 PM   #8
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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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Old 04-19-2011, 07:16 PM   #9
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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.


Quote:
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)
are not equal (unbalanced), the current increases dramatically in
the motor windings, and if allowed to continue, the motor will be
damaged.
Motor Protection Voltage Unbalance and Single-Phasing
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Old 04-19-2011, 07:29 PM   #10
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[quote=BBQ;431253]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.



3 phase 7.5 horse 208V and voltages were all close to 208 and each other. It is a screw type compressor and amps would stay 12 then go to 20 to 21.6 for a minute or so then back to 12.
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Old 04-19-2011, 07:36 PM   #11
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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.

Last edited by nitro71; 04-19-2011 at 07:38 PM.
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Old 04-19-2011, 07:36 PM   #12
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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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Old 04-19-2011, 07:38 PM   #13
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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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Old 04-19-2011, 07:42 PM   #14
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What type of unit is this with the compressor indoors?
Screw type in a dry cleaners. Kaeser 7.5 horse.
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Old 04-19-2011, 07:54 PM   #15
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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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Old 04-19-2011, 07:56 PM   #16
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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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Old 04-19-2011, 10:08 PM   #17
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[quote=jwjrw;431262]
Quote:
Originally Posted by BBQ View Post
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.



3 phase 7.5 horse 208V and voltages were all close to 208 and each other. It is a screw type compressor and amps would stay 12 then go to 20 to 21.6 for a minute or so then back to 12.
WHat type of compressor is it? If it is an AC unit then article 440 applies. If it a motor compressor then article 430 applies. That being said;

430.6(A) 1 states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Table 430.248 indicates 24.2 amps for a 208V 7.5HP three phase motor.

430.22(A) General states;

(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of
not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).

24.2A x 125% = 30.25A

Table 310.16 indicates #10AWG is rated for 35A.

Next we look at sizing the ground fault, short circuit, overcurrent protective device.

430.52 states;

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for an inverse time circuit breaker protecting a three phase motor it is to be sized at 250% of the FLA as stated in Table 430.250.

24.2A x 250% = 60.5A

Based on the above calculation a 70A ITCB is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;

(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.

24.2A x 400% = 96.8A

Since we can’t exceed 400% of the motor FLA we round down to a 90A ITCB on a piece of #10 AWG conductor.

The FLA on the nameplate is only used for sizing the overload protetion, not the bc conductors, OCPD, ir disco. Generally speaking the overloads are sized at %115.

Hopes this helps.

Last edited by electures; 04-19-2011 at 10:11 PM.
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Old 04-19-2011, 10:18 PM   #18
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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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Old 04-20-2011, 06:58 AM   #19
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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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Old 04-20-2011, 08:04 AM   #20
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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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