Disconnect Switch on Air Compressor

[Anonymous-0]

I have 5 HP IR 60 Gall. air compressor that I am hooking up. From the factory, it has a pressure switch and motor has internal overload protection (no magnetic starter). It is hooked to a 40 amp CB in the panel. I am considering adding an enclosed safety switch box that is fused right at or near the compressor. Is there any problem with having a fused switch and a circuit breaker setup? Can I use the safety switch box as my "daily" on/off switch so I don't have to mess around in the electrical panel? I though about mounting the safety switch box directly to the air compressor where a magnetic starter box would attach on a bigger compressor, but I am wondering if there would be an vibration issues causing loose connections? Any input is appreciated.

 

[John T]

Its been yearsssssss since I was a secondary power distribution design engineer (so no warranty, check with you local authority), but can recall basically how how I dealt with compressors and their branch circuits and disconnecting means.


1) Theres no problem whatsoever in a circuit breaker in the panel PLUS a fuzed safety switch disconnecting means within sight and 50 feet of the compressor motor. In fact the motor branch circuit needs overcurrent protection right at the panel/source to protect the feeders. Think about it, you sure wouldnt run unprotected feeders all the way from the panel out to the compressor.

2) You are indeed correct in having a safety switch for a disconnection means (within sight and 50 feet) near the compressor, you obviously dont want to have to go back to the panel and use the circuit breaker as a switch, thats NOT their purpose, their purpose is to provide overcurrent protection for the feeders from the panel out to the motor. SO YES YOU CAN AND INDEED SHOULD USE THE LOCAL SAFETY SWITCH AS THE NORMAL ROUTINE ON/OFF DISCONNECTION MEANS

3) As far as the motor branch circuit, the code permits an oversize of the circuit breaker IF NECESSARY TO ALLOW THE MOTOR TO START. Normally if you use wire rated for 40 amps then you would have a 40 amp circuit breaker but theres an exception for a motor branch circuit that might allow the use of a larger circuit breaker if needed to allow the motor to start.

4) If you use a fuzed safety switch, then you can use dual element (K5???) time delay fuses which will allow higher start up current which are greater then the normal full load running currents.

5) The purpose of the motors internal thermal overload protection device is to save the motor from overheating and destroying the windings while the purpose of the circuit breaker in the main panel is to protect the feeders.

6) If it were me Id mount the safety switch OFF THE COMPRESSOR and to further prevent transmission of vibration Id run flex conduit from the compressor to the switch.

If Im incorrect or missed anything hopefully the good gents who are more current in the trade then myself can add to this.

NOTE I did NOT calculate nor know the FLA for your compressor and only took your word about the 40 amp breaker so I make no statement about the branch circuit wire size or breaker size you mentioned.

John T Tooooo longgggggg retired EE

 

[circus]

A magnetic motor controller makes life easier. The on/off switch can be a normal 110v wall switch. A second pressure switch and a two way switch makes choosing between 120 lbs or 180 lbs air pressure possible.
May not be suitable for your situation.

 

[John T]

For sure and as best I recall, even a cord with a plug and receptacle (versus a switch) can serve as the means of disconnection (but I wouldnt, and prefer a switch)...Of course, he would need 2 pole switch etc for a 240 while a single pole would suffice for a 120.

In similar situations absent other overload protection, I often specified a "Combination Starter" which provided thermal overload protection PLUS a means of disconnection.

Take care yall

John T

 

[dixon boy]

it will work but it will really pull at start up since pressure switch at time will not let the bleed off air happen when does full cycle shutoff its hard on motors when starting compressor unde load, i done it but installed pressure relief valve so when turn it offf it will let air pressure off like it does when pumped full cycle

 

[Anonymous-0]

As long as I make sure the air compressor has cycled itself and the pressure switch shut it off, and then pull the lever on the fused disconnect, would that avoid having the load on the motor? I understand if the compressor was running and I pulled on the lever at the disconnect, the next time it would go to start, the motor would have load on it. Anything I am missing?

 

[Anonymous-0]

We had two compressors in the tire shop, ran them with that type shut off over 20 years with no problem.

 

[Charles (in GA)]

Go to Home Depot or Lowes or Menards, and buy an air conditioner disconnect box. Look at them and get one with a switch in it (some have pullouts). They are cheap ($10-$12) and usually rated for 60 amps (some are 30 amp, so check before you buy).

You can mount this on the wall next to the compressor and use it for a disconnect. You really don't need additional fuses or breakers if you have properly sized breaker in the panel.

Charles

 

[circus]

More details about your compressor will get more detailed answers. For example, my compressor has a centrifugal unloader.

 

[bc]

Is this a 220v compressor you are talking about? I assume it is. The A/C boxes are all 220v. You may want to add a quick start capacitor like they use on A/C compressor to extend their life and not draw so hard on your system at startup. If any of the lights dim at startup, a jump start cap will help. They can't hurn anything.

 

[DaveK(IN)]

A local disconnect is a good idea although in most small shops the panel is "in sight" and less than 50 feet as code requires. Additional overcurrent protection is not required so a non-fused disconnect would be better and cheaper. Be sure that it is horsepower rated for 5 hp @ 220 V. I also think mounting it on the wall is a good practice for vibration isolation.

The circuit breaker and the A/C disconnect suggested are not listed for use as the starting device (ie: to regularly handle the inrush current pulled when the motor starts). They are meant to be used as the branch circuit disconnecting means when the circuit is not energized OR in an emergency stop situation.

The pressure switch must be listed for the hp involved but I bet it is marginal and won"t hold up over the long haul.

If you want to do it the best way (and most expensive) install a magnetic starter and use 120 volt control as suggested by others.

What ever you do, DON"T FORGET TO GROUND AND BOND EVERYTHING THAT IS PART OF THE CIRCUIT. This is the most important part of any electrical installation.

HTH- Dave K

 

[John T]

Many compressors have an automatic unloader, mine has whats caled a "Load Genie" located between the compressor and the receiver tank which acts BOTH as a check valve and then it also unloads head pressure so when the compressor re starts its NOT under pressure.

Whats hard and what causes higher start up currents is if you start it under pressure but the Load Genie or any other type of unloader valves prevents that.

Again, you need some sort of disconnect means within sight and 50 feet from the motor, be it plug and cord or fuzed or non fuzed safety switch and Id mount it off the compressor using flex conduit between it and the switch. You can even get by with a pull out disconnect like you see on some outside AC condensing units, but for no more then they cost Id prefer the convenience of an On/Off SWITCH........

Hope this helps

John T

 

[RodinNS]

I'd mount a disconnect to the wall, and a non-fused type at that. If that's all that's on the circuit then there's no need to a CB and a fuse. It does create a certain redundancy if the CB failed to open... but it also just adds a trouble shooting step when the fuses blow. Personally I hate fuses...
But do mount the box to the wall, not the compressor.

Rod

 

[buickanddeere]

Usually it's a disconnect within 10ft and line of sight from the machine.
Panel circuit breakers are not intended to be used as switches. The manufacture voids warrenty and declines any claims of ability to trip or accurate trip settings.
Most folk get confused with overload and over current protection.Not the same thing.A motor requires both.
A breaker or better yet fuses to rapidly blow for short circuit protection. Breakers are slow to trip at the best of times and will on occassion weld shut and not trip.
The breaker or fuse is not designed to protect a motor overloaded with for example 200% load.
Thermal overload heaters are what protect motors from overloads.
For this application a fused switched disconnect is ideal.
You would be amazed at how many cob jobs I've found where somebody used a single pole thermostat or pressure switch. On a 240V heater or motor with two live lines.Two pole switches on three phase equipment too.
They seem unimpressed as the "motor stops". They have no concern that the motor or heater is still electrically energized.

 

[bc]

I just had my a/c compressor fan quit tonight of all times. The same one I replaced the start cap on a few weeks ago. It was about froze up and the shaft would drop down a quarter inch. Flipped it over and oiled it real good till it was free but it still didn't run very long.Will have to suffer a couple days to get another one on Monday unless I can catch someone working a day off at the parts house.

The reference to this thread is:

I noticed on the disconnect that someone just put in 2 pieces of copper pipe (1/2" I think) instead of the fuses.

 

[buickanddeere]

Time to install some fuses in there. There is nothing else out there that can beat the short circuit protection and current interuption capacity of a fuse.

 

[36 Coupe]

My planer has a 5hp motor.I have a 2 fuse switch box with 2 30 amp time delay fuses in it.Dont use circuit breakers to start and stop motors.The points will wear fast or weld shut.My planer has been running on fuses since 1980, no problems.You could add a magnetic starter for 100 bucks.I had my switch box on hand so used it.