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I have to install a 7 1/2 HP, 3 phases, 230 v. motor compressor. Design B.
From 430.250 I found the FLC as 22A.
From 310.16 I selected 10 AWG wire for 30 A.
From 430.52 I found the breaker not to exceed 60A.
If I match the breaker with the wire (30A) I might be too low for the starting current. If I go to 60A breaker then the wire could have not enough protection at 40A or 50A if that would be the case.
Also at 441.51(3)(A) I found that the breaker for an air compressor for refrigeration "shall be rated at no more than 125 percent of FLC.(is this only for A/C? does it apply to any compressor?)
I will appreciate any commnets
 

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Master Of Disaster
Generator installs and service , temp and emergency power
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How about nameplate data that's always a good place to look as well
Im too lazy to do the math but depending on your distance 10 seems kinda small. I dont deal alot with 3 phase comps but for 5 horse single for vd and such I usually end up with 6.
 

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Master Of Disaster
Generator installs and service , temp and emergency power
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AOE-LLC said:
Is there any sort of motor starter with thermal overloads or a VFD inside the unit?
Motor starters generally have the thermals in it. As for a vfd im not very familiar with them all of the stuff I deal with starts across the line. I think jreaf and mdshunk are really sharp with vfds
 

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I have to install a 7 1/2 HP, 3 phases, 230 v. motor compressor. Design B.
From 430.250 I found the FLC as 22A.
From 310.16 I selected 10 AWG wire for 30 A.
From 430.52 I found the breaker not to exceed 60A.
If I match the breaker with the wire (30A) I might be too low for the starting current. If I go to 60A breaker then the wire could have not enough protection at 40A or 50A if that would be the case.
Also at 441.51(3)(A) I found that the breaker for an air compressor for refrigeration "shall be rated at no more than 125 percent of FLC.(is this only for A/C? does it apply to any compressor?)
I will appreciate any commnets

Is this home work or are you prepping for a test? Air compressor motors are not the same as a A/C refrigeration compressor.
 

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I tend to think you are correct with the 10awg and 60A as long as the wire length is not excessive and you have overloads in the starter or the motor itself, but will hang around an see.
 

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You are correct. #10 wire and max 60 amp breaker. You did the calculation right. What concern do you have? Remember, this is a motor calculation and in many instances the breaker far exceeds the wire ampacity for a reason. "Starting current or Locked Rotor Current".
 

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I'm in agreement with the other answers.

The circuit breaker provides ground fault and short circuit protection only. It is up to the installer to insure that overload protection is supplied by other means. As pointed out by others, this is usually a motor starter or drive.

If you want to figure the voltage drop at starting, look for the code letter on the nameplate, then look it up in Table 430.7(B). Using the maximum value, multiply by the motor horsepower then use the formula locked rotor current = locked rotor kva/(1.73 x kv), the answer will be in amps which you can plug into a voltage drop calculator.

If you do plug this starting current into a voltage drop calculation, you could probably use 10% voltage drop as that is usually within normal voltage tolerance for the motor and it only lasts a very short time.
 
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