I'm going to purchase a air compressor which as a 7hp rating so I guess it's really a 5hp compressor. My shop has a 40amp 240v feed to my subpanel. Question, how many amp breaker is needed and typically what current does a compressor like this draw.
Regards
Ask the Vendor about the AMP rating of the motor is the only REAL way to know
Frankly, if this is more than a real 3hp motor, I would be very surprised
John
You need to look at the plate on the motor to see how many amps it draws.
It may be less than you think! I have a 7 HP Porter Cable compressor, it is in reality about 3 HP. The specs you need to know will be on the motor. You need to size the breaker so the motor draw as lower than 80% of the breakers rating. In my case the motor was rated at 15 amps@ 240 volts, I am running it on a double pole 20 amp breaker. Greg
You should be able to find the current specs from the vendor's web site. I would not be surprised to see it hovering around 20A.
Use a 2P-40A branch c'bkr and #8 AWG, 2 conductor /w/ ground to feed the compressor.
It's what I've used for a 5HP, 2 stage unit for the last 10 years.
HTH
Lew
I have an Ingersol Rand 60 gal and it only draws 15 amps @ 240v. I was only going to run it off our secondary clothes dryer feed (we no longer use it). It would have been an easy hookup, but my electrician said that the 40A could possibly be too much for it. For the price of a new breaker I have a better piece of mind that I am not going to overload the motor with 40 A when it only requires 15A. So I gave it a 20A breaker/
Rich
That is a common misunderstanding of the relationship between a c'bkr and a motor.
The c'bkr does NOT protect the motor, only the insulation on the wires feeding the motor.
There are conditions where a 20A c'bkr will nuisance trip on a 5 HP motor.
If you truly want to protect the motor, you must add an overload relay.
HTH
Lew
Most motors in the consumer market have internal overload protection. The allowable overcurrent device is generally allowed to be up to 250% of the FLA on the nameplate, rounded UP to the next standard breaker size. Legally you could put a 15a FLA motor on 14 ga wire (you CAN use 310-16 not
240.4(D)) with a 40a breaker (430.52). At that point voltage drop would be a bigger consideration than anything else.
If that's your electrician's understanding of electricity, you need a new electrician.
Kevin
Just got back from sears and HD. Sears had a 5hp which the motor was rated at 22.5FLA. What does FLA mean. HD had a advertised 7hp rated at 17.3amps. When I looked at the HD 2 weeks ago it was $100 less than tonight? So Saturday I'm going to Lowes an see what they have, but online I'm leaning to this one.#20849 for $637 in my area and if I open a new charge I think I can get 10%. Will check the specs and get back to you all.
FLA=Full Load Amps
17FLA is nominally 3hp motor at 230v 28FLA is 5 HPper 430.148 in the NEC
Well, if you're going to be doing some shopping around then take the time to go out to Campbell Hausfeld's site and look under Hot Buys. They have some great deals on reconditioned compressors. Much less than retail and fully warranted. I don't know who builds Sear's compressors for them, but both Home Depot and Lowes use CH compressors. CH does not build a spec compressor, so what you get from HD is the same compressor that you would get from Lowes, or directly from CH with the possible exception that the private label stuff might have some extras thrown in.
If your electrician said that, fire him. Amperage draw is determined by the motor, not the breaker.
Looks like some _dreadful_ efficiency numbers, or a humongous phase-angle.
3hp == 2237 watts.17A @ 230v is 3910 VA (or watts at 0 phase-angle.)
17A @ 240V is 4080 VA (or watts at 0 phase-angle.)2237::3910 equates to a 57.212% efficiency
2237::4080 equates to a 54.828% efficiency.Postulating 85% mechanical efficiency for the motor, that gets an input power of 2632 watts.
Takes a _substantial_ phase-angle to make 2600 watts equivalent to 3900 VA.
My 5HP motor is 21 amps at 240 volts. That said, the motor is a replacement for a three phase unit and I didn't know about "compressor horsepower". My motor is 5HP continuous duty, not intermittent. It's on a 30 amp breaker.
Not necessarily so. His electrician may have meant that he should protect both his wiring and his motor with the proper breaker. Yes, the breaker primarily protects the wire, but it also protects the device at the end of the wire. A failure in the motor that won't trip the breaker until it draws
40A will be more catastrophic than one that will trip the breaker at 20A. His electrician is correct in stating that he should size the breaker to the load that will go on it.I got 10% off the same model from CH's hot buy($580) for opening up a new account at lowes so the price was 570. It's rated at 4hp and 17.3 amps and so far runs great on a 15amp breaker, but I will upgrade to a 20amp later this weekend. See new post for air line setup.
You're missing a point. He said that that as "per 430.148 in the NEC". That is an electrical code table used for sizing circuits, not the actual rating or current draw of any particular installed motor.
FLA = Full Load Amps LRA = Locked Rotor Amps
1 HP = 746 watts so FLA x running voltage / 746 gives a pretty good idea of the real HP. Take advertised HP with a grain of salt. Especially from Sears.Woodchuck wrote:
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