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szicree

Well Known Member
My old compressor has a motor data plate that says 240 volts and 15 amps, so I wired a dedicated circuit with 35 feet of #12 Romex and a 20 amp breaker. I've never had any problems with it. Today I upgraded to a compressor with a motor that says FLA 21.5. My understanding is that this means the motor draws 21.5 amps when making it's full rated power (5 HP). My question is whether I have to rewire with #10. I know that #12 is only used for up to 20 amps, and the new motor can draw 21.5, but I also know that 35 feet is pretty short and the motor probably draws far less than the full 21.5 amps. I've looked at the voltage drop calculations and I get 1.22 volts versus .76 volts. It hardly seems worth the time and money to change it out. I guess I'll also add that the wiring run is inside conduit on an exterior wall and that I will be moving in the near future and removing it anyway. I'd love to hear from some knowledgeable electrical folks. Thanks.
 
You'll be fine

As long as you don't install a breaker larger than the 20 amp you will be safe.
You probably knew that.

So the answer is no, you don't have to rewire.
If your compressor was really loaded and started tripping the breaker, then you would need to rewire with #10 and a 30 amp breaker.

Mark


szicree said:
My old compressor has a motor data plate that says 240 volts and 15 amps, so I wired a dedicated circuit with 35 feet of #12 Romex and a 20 amp breaker. I've never had any problems with it. Today I upgraded to a compressor with a motor that says FLA 21.5. My understanding is that this means the motor draws 21.5 amps when making it's full rated power (5 HP). My question is whether I have to rewire with #10. I know that #12 is only used for up to 20 amps, and the new motor can draw 21.5, but I also know that 35 feet is pretty short and the motor probably draws far less than the full 21.5 amps. I've looked at the voltage drop calculations and I get 1.22 volts versus .76 volts. It hardly seems worth the time and money to change it out. I guess I'll also add that the wiring run is inside conduit on an exterior wall and that I will be moving in the near future and removing it anyway. I'd love to hear from some knowledgeable electrical folks. Thanks.
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put an amp probe on it, see what it really pulls then go from there. it will likely be ok.
 
wire is cheap

Like plumbing, when you try to squeeze too many amps thru a wire too small or marginal, you create heat and waste electricity. 35 feet is not short.

If you want the motor to last longer and wish to conserve electricity, oversize is better than not. Wire is cheap, motors are not especially when you are in the middle of painting and the compressor dies.

Barry
 
The motor is rated at 21.5 Full Load Amps - but that assumes a full 5 horsepower output. The motor is going to be slightly larger than required for the full-pressure compressor load, it's likely the compressor will only be drawing 4 or 4.5 horsepower, not 5, which will decrease somewhat the electrical draw on the motor.

If you have an amprobe, use it to determine your actual full-load draw. Personally, I wouldn't change it out until/unless you start tripping the 20 amp breaker. DO NOT put a 30-amp breaker on the circuit unless you upgrade the wire. In my experience, you're probably going to be tripping it at 20 amps with a 5hp compressor, I set mine up from the start on a 30-amp breaker.
 
the LRA is likely 3 times that of the running current.( locked rotor amperage ) you will see this on start up every time and the more pressure in the tank the higher it will be..)
you can set your cut in pressure lower to decrease this if it is at the threshold of being to much.
 
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