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I have an air compressor for running tools in my garage. I recently moved the compressor to the basement and put an accumulator tank in the garage ceiling. I did this to reduce noise in the garage and outside.

It works very well, but now I'd like to control power to the compressor from the garage via my insteon or zwave home automation system.

Years ago, I made the mistake of plugging a compressor into an outlet with its switch in the 'on' position. (The manual advises against this.) After a major arc flash, part of the plug was missing, and I replaced the outlet and plug. So, I suspect that the initial inrush current may be some hundreds of amps depending on where the piston last stopped, and I'm reluctant to trust a standard 20 amp relay to the job.

How many amps should I plan for?

The compressor is "rated" at 120v, 14.5A. My compressor manual does not mention how I would like to choose a relay or switch that will show no appreciable wear after 5000-10000 cycles.

What type of relay should I use?

Here's a picture of the compressor: Front of compressor And one of the labels on the back Back of compressor

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  • A good rule of thumb is to assume inrush current for a motor is 10X the steady state one. Also this question is probably better on EE site.
    – got trolled too much this week
    Commented Sep 8, 2017 at 5:18
  • I've answered something similar there a while back.
    – got trolled too much this week
    Commented Sep 8, 2017 at 5:26
  • Do you have a horsepower rating for the motor?
    – ThreePhaseEel
    Commented Sep 8, 2017 at 11:36
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    a 40A zero-cross solid-state relay should be able to handle it. since it only turns on upon the instant when the AC is at 0v, you avoid the extremely high instant spikes, even if shorting the brushes.
    – dandavis
    Commented Sep 8, 2017 at 13:35
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    After further consideration, I'm only interested in control via relay driven by home automation. A manual switch would necessitate running 12ga wire from the basement to the garage just to control one appliance. I don't want to spend the time and money on running that wire, or incur the 'unnecessary' voltage drop associated with it. I also can easily see leaving that switch on by accident only to be woken up at night by the compressor kicking off, and with home automation I can shut it off from bed with my phone. My apologies to Retired Master Electrician.
    – Billy
    Commented Sep 9, 2017 at 3:46

3 Answers 3

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I would just use a standard air conditioning condenser relay rated for 30 amps or more to start with. Make sure it is rated for horsepower larger than your compressor. (To really calculate the inrush current you need the code letter from the motor nameplate under the LRA or locked rotor current. Then see Table 430.7(B) in the NEC. Multiply your horsepower by the number from the table and divide by one thousand. See this article here.)

Or just start with an A/C relay like I said above and if it wears out fairly quickly you could spend much more money for a higher rated motor contactor. An A/C relay probably costs $30 or less.

The other thing to consider is when you decide to upgrade your compressor, make sure it is a two-stage compressor. A two stage starts the motor unloaded and then engage the compressor after the motor is running. This significantly reduces the inrush current to the motor since it is starting unloaded. The single stage style is required to start with the compressor under pressure and consequently requires a large inrush current.

Good luck and stay safe!

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  • I'm going to assume my compressor's motor is no greater HP than 2.4 because it's got a NEMA 5-15 plug and 1800 Watts is 2.41something HP. Can I infer the code letter from that?
    – Billy
    Commented Sep 9, 2017 at 3:53
  • Could I infer the inrush current by measuring the ohms resistance from line to neutral on the compressor? Use ohms law to convert that and 120v to a number of amps? Am I likely to calculate some hundreds of amps measurement that way?
    – Billy
    Commented Sep 9, 2017 at 3:56
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    BTW -- most "compressor relays" are really definite purpose contactors, and should give able service in the OP's application
    – ThreePhaseEel
    Commented Sep 9, 2017 at 14:04
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I would suggest you use a motor rated switch. You need to make sure it is rated for the hp of your compressor motor. You can order them with their own enclosure.

Here's a link http://www.cooperindustries.com/content/dam/public/wiringdevices/BuyersGuides/AHBG/E/AHBG-E-Sec.pdf

Good luck

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  • Yes, for a manual switch you should use a safety switch. However, the OP wanted to bw able to control it from his home automation system and I assumed remotely from the garage. That is why I recommended a relay.
    – ArchonOSX
    Commented Sep 8, 2017 at 15:37
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You probably can't do this.

Some air compressors (like the one pictured) have to be shut down properly to be able to start up again. (Look for an overly bulky switch module with multiple air hoses entering it.)

The power switch on such a compressor accomplishes this by mechanically venting pressure between the compression piston chamber and a check valve. This is why you normally hear a 'hiss' when you shut the running compressor off via that switch.

Cutting electrical power to the unit will not cause that venting to occur, and the pressure pushing against the stopped piston head will prevent the motor from turning when power is re-applied. This causes the motor to draw far more than its normal power, its 'Locked Rotor Amperage', continuously and it will either melt the motor winding, or pop the breaker on the compressor.

To control the compressor pictured, you will have to improvise a mechanical means of rotating its provided power switch in either direction.

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5
  • So, the compressor would fail this same way if there was a power loss? Or if you just cut power to it off after it finished its cycle?
    – ArchonOSX
    Commented Sep 10, 2017 at 19:45
  • Yes. The compressor fails the same way if there is a power loss. I'm considering trying to return it because of how nuts that is to me, but I've had it for a while now.....
    – Billy
    Commented Sep 10, 2017 at 19:49
  • If you wait until a "cycle" is complete and only cut power after the compressor has gone silent, then you can avoid locking the rotor. But for instance, I would want to use a macro to 'enable the compressor for 2 hours', or cut power at 10PM every night. I don't have a sensor that would know if the compressor motor is running or not, and without that I'm uncomfortable with it being possible to get it 'stuck' in the state that will lock the rotors on the next start.
    – Billy
    Commented Sep 10, 2017 at 19:54
  • Yeah, that sounds like a bad design if it can't handle a power loss without locking up.
    – ArchonOSX
    Commented Sep 10, 2017 at 20:16
  • I tried to do it anyway, knowing that I'm likely to damage the compressor or pop its breaker if I apply power while it has head pressure. I have to remember I can't shut it off it's running. So I installed an air shutoff in the garage that lets it top its pressure off pretty quick. Knock on wood, it's worked OK. Still I feel like counting it as not doable, because it requires special handling every time to avoid damage.
    – Billy left SE for Codidact
    Commented Oct 23, 2017 at 21:52

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