BREAKERS, 175% OR 225%

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fifty60

fifty60

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I am sizing breakers for hermetic compressors. NEC 440 says that should not size larger than 175%, but am allowed to size to 225% if needed. For some of my compressors, sizing at 175% gets me about 4.4x's under the LRA rating. Using D-curve breakers, this will give me 1 second worse case on the curve.

The LRA rating is called "LRA HIGH" meaning that this is the highest voltage rating of the compressor. These compressors are 230V, but can be operated at 208VAC. If they are operated at 208VAC, then the the LRA will increase as well. This creeps me ever closer to 5 times beneath the LRA. At 5 times under LRA, my D curve fuse will last about .75 seconds.

I am trying to build these machines to last. I do not want the customer to have to up the size of the breaker as the compressors age. Am I allowed to go to 225% if, for example, the fuse I get a 175% is 4.4 times under? Where do I want to be at on the breaker trip curve so that I can be confident that my breaker will hold every time.

Should I be confident in the 175% number? Can I go straight to 225% and not get hassled by the UL,CSA, or CE inspector? Should I go straight to 225% and not bat an eye?
 
fifty60 said:
I am sizing breakers for hermetic compressors. NEC 440 says that should not size larger than 175%, but am allowed to size to 225% if needed. For some of my compressors, sizing at 175% gets me about 4.4x's under the LRA rating. Using D-curve breakers, this will give me 1 second worse case on the curve.

The LRA rating is called "LRA HIGH" meaning that this is the highest voltage rating of the compressor. These compressors are 230V, but can be operated at 208VAC. If they are operated at 208VAC, then the the LRA will increase as well. This creeps me ever closer to 5 times beneath the LRA. At 5 times under LRA, my D curve fuse will last about .75 seconds.

I am trying to build these machines to last. I do not want the customer to have to up the size of the breaker as the compressors age. Am I allowed to go to 225% if, for example, the fuse I get a 175% is 4.4 times under? Where do I want to be at on the breaker trip curve so that I can be confident that my breaker will hold every time.

Should I be confident in the 175% number? Can I go straight to 225% and not get hassled by the UL,CSA, or CE inspector? Should I go straight to 225% and not bat an eye?
Click to expand...

Are these Copeland hermetic compressors?
 
fifty60 said:
They are not Coplematic, not to my knowledge anyway.
Click to expand...

My knowledge and experience is with the Coplametic compressors.
Coplametic units that I have used extensively have not had long life at 208 V.
Most experienced hermetic burnout well before the warrany expired.
When we pushed the issue with the manufacturer they disclosed that 208 was low enough to cause the burnout and a buck-boost was required to bring up the voltage.
 
I have seen Copeland documentation that says these particular compressors can operate at the +/- 10% nominal. I know they do not operate for long outside of this range though, and have had to put autotransformers on several of them.

My issue now is with sizing breakers. I want to do it correctly. The information I have is the RLA, the LRA, breaker curves, and the NEC. The NEC tells me to size the breaker at 175% max, but that I can raise that up to 225%. The 175% allows me to cut down on conductor sizes and also breaker frame size. I do not, however, want the breakers failing at startup a year from now.

I believe that the declared LRA is not actually what I see at startup, but that it is actually about 75% of the LRA that I see for about the first 10 cycles of AC. 10 cycles would be enough to trip the instantaneous on a D curve breaker if the current were between 10-20 times the nominal breaker rating.

What I am seeing here is LRA's that are about 4.4-4.6 time the nominal breaker rating. This puts them well outside of the instantaneous trip of the breaker. For a worst case breaker on the curve they could take 4.4 times for about a second before tripping. Is this enough for a compressor?

Now, I am taking the D curves at their worst case. The instantaneos trip, for example, is set to occur between 10 and 20 times the nominal breaker rating. This is a huge window in my opinion. I imagine the actual trip point would be somewhere in the middle, or closer to the high end most of the time.

My main question is what distance from the LRA is far enough. When should I consider going above 175%?
 
I have a mixture of 3 phase and single phase. The breakers on my single phase units, when sized at 175% RLA, all come out to be around 3.3-3.4 times less than the RLA. I am very comfortable with using the 175% sized breakers on these single phase compressor ( to clarify on single phase RLA is 3.3-3.4 times more than the Breakers when sized at 175% RLA)

The RLA is 4.4-5 times more than the breakers sized at 175% on the 3 phase compressors. Do you think this is intentional on Copleands part? I definitely see a pattern there. The three phase compressors overall draw less current, but do they have a larger locked rotor current draw? If so, should I size my breakers larger for 3 phase?

What does the inrush do at 50Hz with the voltage stepped down to 380V? Is the LRA affected?
 
fifty60 said:
I have a mixture of 3 phase and single phase. The breakers on my single phase units, when sized at 175% RLA, all come out to be around 3.3-3.4 times less than the RLA. I am very comfortable with using the 175% sized breakers on these single phase compressor ( to clarify on single phase RLA is 3.3-3.4 times more than the Breakers when sized at 175% RLA)

The RLA is 4.4-5 times more than the breakers sized at 175% on the 3 phase compressors. Do you think this is intentional on Copleands part? I definitely see a pattern there. The three phase compressors overall draw less current, but do they have a larger locked rotor current draw? If so, should I size my breakers larger for 3 phase?

What does the inrush do at 50Hz with the voltage stepped down to 380V? Is the LRA affected?
Click to expand...

Are you dividing the LRA by the RLA to get your inrush ratio?
 
No. I know that ratio is generally around 6. I am getting this ratio by dividing the LRA by the fuse that I have after multiplying the RLA by 1.75. For single phase compressors I consistantly get a ratio of 3.3. For 3-phase I am seeing higher LRA/Fuse ratio's of 4.4 -5.0. I am using D-curve breakers. A D-curve breaker can hold an inrush of 4.4x's its rating for 1 second (worse case). I am wondering what LRA/Fuse ratio I should be comfortable with, and when it is recommended to go larger than 175%.
 
fifty60 said:
No. I know that ratio is generally around 6. I am getting this ratio by dividing the LRA by the fuse that I have after multiplying the RLA by 1.75. For single phase compressors I consistantly get a ratio of 3.3. For 3-phase I am seeing higher LRA/Fuse ratio's of 4.4 -5.0. I am using D-curve breakers. A D-curve breaker can hold an inrush of 4.4x's its rating for 1 second (worse case). I am wondering what LRA/Fuse ratio I should be comfortable with, and when it is recommended to go larger than 175%.
Click to expand...

Devide LRA by RLA

5#For single phase compressors, multiply RLA by:1.6 if inrush ratio is 5 or less, or 1.7 if inrush ratio is more than 5.


6#Fore three phase compressors, multiply RLA by:1.5 if inrush ratio is 4 or less, or1.6 if inrush ratio is more than 4.

Use the next higher Time Delay fuse rating above the adjusted RLA value calculated in step 5 or step 6.
 
Actual LRA is dependent on the supply. High impedance source or excessive voltage drop on conductors is going to lower actual LRA.

Marked LRA is likely assuming a certain max source impedance or even infinite available supply, which is almost never going to be the case.
 
kwired said:
Actual LRA is dependent on the supply. High impedance source or excessive voltage drop on conductors is going to lower actual LRA.

Marked LRA is likely assuming a certain max source impedance or even infinite available supply, which is almost never going to be the case.
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Rated Load Current "Amperage" is 64.1 % of the MCC Maximum Continious Current that is determined by the compressor manufacturer.
 
Correct. Copeland gives me an MCC and then calculates 2 RLA's (MCC/1.4 for sizing contactors, and MCC/1.56 (which is 64%) for sizing conductors and OCPD's) They also give a MOC, but they state that that is primarily for IEC ratings. I appreciate the help guys, I am more confident going with the 175%.
 
Gregg Harris said:
Rated Load Current "Amperage" is 64.1 % of the MCC Maximum Continious Current that is determined by the compressor manufacturer.
Click to expand...

Could be, IDK how it is determined. But I wasn't talking about RLA I was talking about LRA. Any motor can not draw more locked rotor current than the source can deliver to it.
 
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