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Wiring for 240V Air Compressor???

K

keepup

Active Member
Although I haven't purchased the air compressor yet, I think I'm sold on the 240V vertical variety. In typical fashion I over-design everything for the just-in-case factor. So I've run #10 Romex to the compressor location but I want to be doubly sure that's big enough.

Only problem is: I can't find the current draw for any of the systems out there. I spent an hour of fruitless online search last night and the only info with current draw posted online that I found was for Porter Cable. Nothing for Ingersoll Rand, Campbell Hausfeld, Husky, Kobalt, Craftsman, etc.!!!! :mad::mad::mad: Porter Cable's largest 80 gallon dual stage (which is probably bigger than I need or want even if I will paint the plane myself) is rated at 30 amps. But is this continous or startup?

So will you check my math for me?

According to the calculator on http://www.southwire.com (makers of Romex) #10 wire at 240V will have 3% voltage drop (recommended max) at 30 amps over 114 ft.

But my run is only 50 feet so according to Method #1 I should be good for 30 amps.

Method 2: Again using the southwire calculator for minimum conductor size: #10 wire at 240V at 30 amps over 50 feet will sustain a 1.32% voltage drop. So I'm good there.

Method #3: Good ol' Electricity 101: 240V x 30amps = 7200 Watts = 9.6 electric horsepower according to http://www.onlineconversion.com

What I don't know about method #3 is that most of the larger compressors will state 7 hp continuous. But do they have a higher power requirement (translated higher current draw) during startup? For the life of me, I can't find this information out online.

So for now, it looks like my #10 wire should be good for 30 amps at 240V at my 50 ft. But the question still remains: What is the maximum current draw during startup on these 80 gallon dual stage (worst case) compressors????

Or do I need to pull out my #10 wire and use #8 instead?
 
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I have a unit just like you described, mine is listed at 5 HP, current draw 22 amp max.

No 10 wire, 30 amp breaker is sufficient for this setup.
 
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ingersoll rand

I have a 5 HP 80 gallon dual stage IR running on a 30A breaker with about 60' of #10 wire. Never tripped the breaker, and runs as cool as can be expected.
 
You're OK

The #10 AWG wire is rated for 30 amps continuous based on limiting voltage drop and consequent heating of the wire. It can carry more than 30 amps safely for short periods, including the inrush current at startup of an electric motor.
 
I robbed the circuit from the dryer.

My wife thought I was going insane. But I had a plan. It was going to be difficult to run a new circuit to the garage. The dryer was old and I had a gas line already plumed, so I bought a new gas dryer.
I agree #10 and 30 amp.
 
FYI-breakers protect the wire.

So, you ran 50' of #10. With a 30amp service you are protected.
Now this limits you to <9hp, so much for the 15hp Industrial Ingersol-Rand.
Next go out and buy the nicest 5, 6, or 7hp compressor you think you need.
 
I had a continuous run compressor that I wanted setup with an 80 gallon tank so I had North West Compressor (541-752-5366) come to my shop and install one. While they were here I learned a few things. Compressors draw comparatively low amps when starting. Amperage becomes an issue when the motor bogs down from the load of compressing higher pressure air. My new tank was rated for 200 psi, while being adjusted they checked the line inductance (I think that's what it's called) and adjusted the shut off switch to the point of the continuous run amp limit of the motor shown on the motors data plate. So I can comfortably reach only 175 psi without over stressing the motor. The air pressure load is removed from the compressor itself in order for the motor to have a chance to spin up relatively load free. That's the hissing you hear for a moment when a compressor stops. So it seems to me that rather than worry about the generality of which wire is absolutly correct, so long as the circuit itself is being adequately protected, it would be better to check for how many amps are being drawn at pressure for a given piece of wire and adjust to that.
 
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You should be fine with your proposed set up. However be VERY wary of advertised horse power. Manufacturers are very generous with their ratings. Most compressors with "7 hp" motors generally put out MAYBE 5 hp. Check the current draw. Some people even advertise 5 hp compressors that run on 110v. Not likely! That would take a LOT of current.
 
lots of air

A compressor of that size should keep a crew of three or four drilling, cutting,and riveting all at once. Should only take two, maybe three months to complete your airplane(including paint). :D:D
 
Method #3: Good ol' Electricity 101: 240V x 30amps = 7200 Watts = 9.6 electric horsepower according to http://www.onlineconversion.com

What I don't know about method #3 is that most of the larger compressors will state 7 hp continuous. But do they have a higher power requirement (translated higher current draw) during startup? For the life of me, I can't find this information out online.
Click to expand...

The inrush current of an induction motor of this size is probably between 100 and 200 amps. But, it is for a short time and the circuit breakers are designed to handle that. I think most, if not all, compressors have some sort of compression relief during startup also.

1 Hp = 763 watts. However, you can't simply multiply current times voltage in order to get watts with a/c induction motors. You need to multiply by about .8 to take into account the phase angle between the current and voltage. So, (240 x 30) * 0.8 = 5760 electrical power which is about 7.5 hp. Motor efficiency effects will pile on top of this.

For continuous loads, even though #10 wire is rated at 30 amps max, you will not be to code unless you de-rate by (IIRC) 0.8, or 24 amps continuous in this case. Someone on this list who remembers the code better than I should chime in.
 
AlexPeterson said:
For continuous loads, even though #10 wire is rated at 30 amps max, you will not be to code unless you de-rate by (IIRC) 0.8, or 24 amps continuous in this case. Someone on this list who remembers the code better than I should chime in.
Click to expand...

763 watts (theoretical transfer efficiency) for a 5hp motor is 15.9 amps at 240 volts. The standard draw for 5hp is rated at 22 amps for startup, which gives a margin for both inrush current and efficiency losses. A 30 amp breaker gives plenty of margin, and a #10 wire gives margin above that for the breaker dropout error. A #10 wire will carry a couple hundred amps for the few tenths of a second it will take for even a wildly-out-of-spec breaker to trip.
 
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