HELP! Compressor now trips breaker on start up??

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Graminizer

Graminizer

The "Big G"
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OK all of you electricians out there....LOL

I have a 15 year old 120 v 5 hp Craftsman compressor with a 33 gallon tank. I added a switch to the circuit so I can shut the compressor off when I am locking up the shop. It's on it's own 20 amp circuit. Now, the breaker trips sometimes when I turn the new switch on or the compressor tries to start, but doesn't, then trips the breaker, or I turn the switch off.

Any ideas? I thought it would work well with the switch, but it is obvious that the compressor does not like it. Any help on this would be great. Thanks
 
What switch did you wire in? Are we talking a 15a or 20a light switch. A 5 hp is probably at the limit of 20a as most 5hp are wired 220 and might be more than switch(or its putting just enough additional resistance in the line) can handle. I still have one of the same compressors never could get it to run on 120. Inside the motor cover is the schematic to change to 220 wiring and ran it on a 30a 220v circuit. I think the rule of thumb for wiring and switches is stay under 75% of rated amps throughout the circuit and everything in the circuit has to meet that rating. Ie 20 amp wiring, 20amp switches and 20a receptacles. Also try a new breaker in the box. They can get weak. Not sure if I helped or not. By chance the Craftsman made by Devilbiss? I still keep mine hanging around as its been flawless for 20 yrs. My OL fires it up to run staplers and brad nailers. Also check the wiring/contacts in the pressure switch.
 
My guess would be wire gauge too small for the wired in (s)witch.
 
^^ Can't be the "witch"...Halloween is over :eek:ops:...Kidding :)
 
here's some basic electric wiring/code specs for the u.s. - i don't know if these are the same in "canook" land.

house lights can be wired with "14-3" size wire but all outlets should use "12-3". 12-3 wire is a heavier wire and can be used for a 15 amp breaker or a 20 amp breaker. 10-3 size wire is used for up to 30 amp draw appliances. some wall outlets are 20 amp. you can tell these from standard 15 amp outlets as they have a little "cross" opening on one of the slits where the plug goes in. a 20 amp plug has a similar copper cross shaft. if you have a 20 amp wall outlet, that circuit has to have a 20 breaker. most houses use 15 and 20 amp breakers for most things. electric stoves, hot water heaters and dryers use 30 amp breakers. outside air conditioner pumps, electric furnaces and "hot tubs" generally use 60 amp breakers.

your 120volt air compressor should run on a standard 15 amp outlet. however, there are two measurements of electrical draw on electric motors - "startup" draw and "running" draw. your compressor probably draws 13-14 amps when it first starts up and then drops to 10-11 amps when it is running. the same thing happens with your refrigerator. this makes a difference if you have any other appliances hooked to that circuit. for example, if you have garage lights or any other outlet with something plugged into it wired to the same circuit where your compressor is, the other devices may be drawing 1,2,3, etc. amps and when you go to turn on your compressor, the "start-up" draw of 13-14 amps trips the breaker. try and find an outlet that you know has nothing else drawing from that circuit and plug your compressor into that. if it starts up without tripping the circuit, then the total amprege draw is probably the problem. i have a 120v compressor in my garage and it is plugged into a dedicated compressor only 15 amp outlet.

another possibility is that the breaker that is tripping could be weak. breakers do develop fatigue over time. you could also try replacing the subject breaker with a new one - but get someone to do that for you if you are not familiar with electrical work as you usually have to take the cover off of your electric box to change breakers. if you have a main shut-off breaker for the entire house, you could switch that to off and then all the electric to your box will be off and you could change the noted breaker.

one last thing to remember. there is more pressure - and more amprege required - when a compressor starts when there is high pressure air in the tank. so if you are turning on your compressor with 100 lbs. of air in the tank your motor is going to draw more amps than if the air tank is empty.

finally, you should also check for lose wires/bad connections on the motor. a loose connection will cause an arc or interfear with correct amp draw.


ps. i didn't see your comment about the 20 amp circuit ... my other comments might be helpful
 
all the above is good info, also that motor should have a start capacitor check for loose connections or posibly a bad capacitor.
 
The way I read the OP, the compressor was and has been working fine until the switch was added.

Could be the switch, could be wired incorrectly, OP will have to let us know.
 
Yeah, start-up can typically pull 5 to 10 times as many amps as when running,depending on load. 5 hp is 5x749=3745 watts. 3745/120 volts=31.2 amps.So trying to start under load, on a 20amp breaker is pretty iffy.You need to drop the air pressure in the tank below the set-point of the cut-in device. Then the compressor will start like it used to.
-Also, you probably shouldnt try to pull those 31 amps through a typical light switch, unless maybe if youre switching the neutral side.
-I think putting a ball-valve in the airline at the tank is a better idea, and leaving the beast electrically connected. And,of course,eliminating all pre-valve leaks.
-Oh yeah,Typically the stated hp on the motor, may not correspond to the math exactly.The best idea is to get an induction amp probe, and find out exactly how many amps your motor is actually pulling; both when starting, and when running. Your motor may normally only pull 5 amps when running but pull 25 or 30 or more during start-up.Typically the start-up period is so short that the breaker doesnt respond to it. However when attempting to start under load the time period is/may, be long enough to trip it.If you treat the breaker this way for too long it will "get old" and eventually fail.
-It would be like trying to start your car engine, in 4th gear(m/t), with the brakes on.Theres nothing wrong with the system, except the load.
-Hope this helps, AJ
 
Yes, the compressor worked fine until I wired in the switch. I used a 20 amp light switch and 20 amp receptacle. 12/3 wire. Breaker is brand new. Also hooked up a 30 amp breaker, and the same thing happened. I pulled the capacitors out, but they look good. I will get them tested on Monday and replace if needed.
 
I How far away is your compressor from your breaker panel and how far away from the panel is the switch?
 
I had this same problem with my compressor kicking breakers on start up.
Motor was about the same age as yours also. Replaced the capacitor and
It now starts easier than it ever did. The capacitors must go bad from age or drawing
moisture?
 
Here in the US the black wire is the ;hot; side and the "white" is the neutral side. Mack sure you didn't get them reversed.
 
Wow I am amazed the all wiring God has not chimed in he knows who he is .KNOW IT ALL, were the manual.
 
Here in the states, only the black wire is interrupted by a switch. White wire goes straight through the box uninterrupted.
 
What "I" would do is un wire the switch. Connect an amp meter to the motor and see exactly what it draws on startup. I can tell you that motor is NOT a true 5HP motor. It will probably be more like 3. But by measuring the start up amperage draw, you can get an idea of the size motor it really is and what size wiring and switch you need.
 
RustyRatRod said:
What "I" would do is un wire the switch. Connect an amp meter to the motor and see exactly what it draws on startup. I can tell you that motor is NOT a true 5HP motor. It will probably be more like 3. But by measuring the start up amperage draw, you can get an idea of the size motor it really is and what size wiring and switch you need.
Click to expand...

^^^^^Yes x2
This is the first thing the guys I use to pay to check my 5hp well/irrigating motor did. I bought a used Fieldpiece and do it myself now. Get specs on start/run amps and check them. Might have capacitor/motor issues. If amps are in spec, double check your wiring. My 2cents...full refund if not completely satisfied. Lol Good luck!
 
mod5v said:
Here in the US the black wire is the ;hot; side and the "white" is the neutral side. Mack sure you didn't get them reversed.
Click to expand...

It's the same here...that's how I have it wired.

I will be checking the capacitors tomorrow and replacing them if needed. Hopefully that is what the problem is. I cannot find a switch larger than 20 amps. It did the same thing with a 30 amp breaker.
 
Graminizer said:
It's the same here...that's how I have it wired.

I will be checking the capacitors tomorrow and replacing them if needed. Hopefully that is what the problem is. I cannot find a switch larger than 20 amps. It did the same thing with a 30 amp breaker.
Click to expand...

Curious, if you leave the new switch in the on position and use the 20 amp breaker to start the compressor, does it still trip?
 
The capacitor should have the rating in microfarads stamped on it. Usually expressed as 10mf or 12.5uf. Some of them (usually black plastic case) have a range like 125-180 mf on them. You'll just need a meter that reads microfarads to test. There are switches rated for higher amps . They look like a breaker without a value on them. They fit in an enclosure like a disconnect or single breaker housing. Whatever you do, don't exceed the capacity of the wire with a breaker that is rated too high. 12 ga wire needs a 20 amp or lower breaker.
 
RustyRatRod said:
What "I" would do is un wire the switch. Connect an amp meter to the motor and see exactly what it draws on startup. I can tell you that motor is NOT a true 5HP motor. It will probably be more like 3. But by measuring the start up amperage draw, you can get an idea of the size motor it really is and what size wiring and switch you need.
Click to expand...


Finally, the voice of sanity (go figure).

If it worked before you added the switch....start by removing the switch and any other mods done at the same time.

I agree a true 5 HP motor probably draws way more than 20A (unless it's super, super efficient). Probably closer to 30A.

IIRC the startup draw is a "delta" or close to three times the running draw.

My true 5HP motor is the size and weight of two small block heads and requires 30A and #10 wire.

"AmProbes" are kinda pricey, and finding a 30A capable consumer VOM to put in series might be as well.

You could always install a $10 disconnect box with a 20A breaker near the compressor, and use that.
 
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