I just bought a 20 gallon 2HP air compressor (Ingersoll Rand P1.5IU-A9) and the manual says the breaker should be twice the rating of the motor. The motor is 15A, so this means a 30A breaker. This seems overkill to me. I just powered on the compressor for the first time a few moments ago on a 15A circuit (which was powering my cable modem and router) via a 10 ft extension cord and the breaker didn't trip. I will be running a dedicated circuit tomorrow, and planned on using 12/2 and a 20A breaker. FYI, the compressor has a 3-prong 110V plug. How do they expect the user to use a 30A breaker with 10 gauge cable and a standard 110V receptacle?

 

Bah, a 20 amp breaker will be fine for a dedicated line. Clamp on an amp meter with it running and you won't see near 15 amps steady state. There is probably a jumper on the compressor if you want to run it 220v.
-=Phyber

 

It isn't a dual voltage compressor. I'll go ahead with 12/2 and 20A. I'm sure it will be fine.

 

I hate to rain on your new tool but that motor will only deliver 2 horsepower in a highly overloaded state. Look at the nameplate and I'll bet it says "compressor duty" or something like that and also that it has a rotational speed of about 3450 rpm. A REAL 2 horsepower motor will draw approximately 24 amperes at 115 volts so that makes the 15 ampere maximum of your motor more in line with a one horsepower motor. The manufacturer can get away with the labeling because the motor WILL develop close to 2 horsepower for a limited time before it burns up the windings AND because it actually exceeds 2 electrical horsepower in its power consumption. Further, the "compressor duty" signifies that the motor is NOT rated to deliver full power except in a cycling operation.

But don't get too excited over this news as ALL compressor manufacturers do the same and it is even used as a marketing ploy with industrial compressors. The ONLY time the compressor will normally have to utilize the full starting torque of the motor is when it re-starts at the cut-in pressure of the pressure switch and even then most compressors will have a device (unloader) that relieves the tank pressure from the starting load.

The ONLY specification that is truly important is the delivered CFM at a usable pressure. That usable pressure is different for different tools. Most paint spray guns use a pressure between 40 and 60 psi while most tools need a minimum of 90 psi at the tool for proper operation. You will notice that as the pressure rises the delivered CFM will fall and it will fall significantly on a single stage compressor once the pressure exceeds about 100 psi. Over 100 psi a single stage compressor's volumetric efficiency drops off rapidly, as low as about 50% at 150 psi. For this reason multiple stages of compression are called for if your pressure needs exceed about 120 psi.

 

This is my first compressor and I realize it's not the best. It was a really good deal and should suffice for what I'm doing. If in time I find it isn't, I won't hesitate to upgrade at a later date.

So 12/2 and 20A breaker is fine?

 

I can't see why a dedicated 20 amp circuit would ever trip. It should be fine.

 

Here's a test. Plug the machine into a 20 ampere circuit that you can turn off any other loads, essentially making it a "dedicated" circuit. Pump the tank up to just under the point where the pressure switch would shut off the motor and while still running unplug the cord. Wait a few seconds and then plug it back in and if it starts without popping the circuit breaker then a 20 ampere circuit will be fine. At any rate, run your dedicated circuit with #12 copper wires and if the 20 ampere CB becomes problematic you CAN use a 30 ampere CB.

 

The NEC requires that motors have a "name-plate" that displays certain specific information ,such information including the HP rating , so I suggest you base your circuit design criterion on the motor name-plate info.