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Old 02-19-2009, 04:41 PM   #1
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Craftsman air compressor blowing breaker


Hi all,
I'm new to this forum but have been reading some of the posts. It seems like there are a bunch of knowledgeable people here, so here goes my question.
I have a Craftsman air compressor (model 919.165190). It is 5.5 max H.P. I have had it for years and have always had trouble with it blowing breakers.
I put it in my garage on the original circuit and blew once in awhile. This didn't surprise me because it was a 15 amp circuit that had other things on it. The manual calls for a minimum 15 amp dedicated circuit. I put up with this for years until last year I decided to fix the situation.
I ran a new line from the main breaker box. The reason it took so long is because there was noway to run it thru the house. The basement is finished, so I can't run wire there, plus the living room between the garage and basement is on a slab. The house has two different level roofs with no easy way to go between them either. So, I ran the line out the basement wall in the ground along the front of the house and into the garage.
I used 12-2 wire in a PVC conduit with a 20 amp Square D breaker and a 20 amp outlet. The air compressor is the only thing on this circuit.
The compressor now works better but it still blows 50 percent of the time. I started having trouble with the contacts in the pressure switch of the compressor. They got really corroded from sparking during on/off cycles. I ordered a whole new pressure switch assembly from Craftsman and put it on last night. I also checked the check valve to make sure it was working and it was fine. I can spin the motor easily by hand, so nothing is wrong there. I had a friend check out the capacitor and he said it was good. I put it all back together and started it up with the drain valve open and it immediately blew the breaker. I went downstairs and put in a new 20 amp breaker and started it again. This time it ran OK. It ran for several cycles without blowing but I'm just waiting for the next time.
What else can I do other than running another line? If I do this I'm gonna run a 50 amp breaker. Thanks in advance for your suggestions.

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Old 02-19-2009, 05:41 PM   #2
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Craftsman air compressor blowing breaker


Plug the thing into a long extension cord, more or less of the AWG size specified in your saw's manual. See if this improves your odds as to tripping the breaker.

I guess if there is a high resistance anywhere upstream of the motor, it will demand more current. You could check the voltage drop in motor wiring from the motor input to the compressor plug while it's running, and calc. the expected resistance from the wire size. Closed contacts at rated current should read less than 100 mV across them.


Last edited by Yoyizit; 02-19-2009 at 05:46 PM.
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Old 02-19-2009, 05:45 PM   #3
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Craftsman air compressor blowing breaker


Quote:
5.5 max H.P
This number is pretty much just made up.
Quote:
What else can I do other than running another line? If I do this I'm gonna run a 50 amp breaker.
Bad idea. If it's tripping a dedicated 20 amp breaker instantly, it's basically a short circuit. A 50 amp breaker will likely trip just as fast.
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Old 02-19-2009, 06:00 PM   #4
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Craftsman air compressor blowing breaker


Quote:
Originally Posted by Frankenberry View Post
blew once in awhile.
on startup or later?

The compressor now works better but it still blows 50 percent of the time.
on startup or later?

it immediately blew the breaker. I went downstairs and put in a new 20 amp breaker and started it again. This time it ran OK.
Maybe the tripping with the first breaker was a fluke, and both 20A breakers are good, but you might need a different trip curve (if it takes minutes to trip the breaker).

Your motor may have developed shorts that are partially shorting the windings that weren't there when you ran the original setup.

This HP at 120v is 34A; if the motor draws temp. high current lasting seconds it might squeak by with a 15A breaker. Home welding machines drawing very high current depend on the welding process lasting only a minute or so.

Last edited by Yoyizit; 02-19-2009 at 06:08 PM.
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Old 02-19-2009, 06:16 PM   #5
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Craftsman air compressor blowing breaker


Yoyizit.

Sometimes it blows right away, sometimes it blows after a minute, sometimes it blows after several cycles.
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Old 02-19-2009, 06:29 PM   #6
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Craftsman air compressor blowing breaker


Quote:
Originally Posted by Frankenberry View Post
Yoyizit.

Sometimes it blows right away, sometimes it blows after a minute, sometimes it blows after several cycles.
I'd say the compressor is intermittently failing, but you might want to measure current draw with a clamp-on meter, but this will not show high current spikes. For that you need an oscilloscope and a shunt.

Also measure motor ohms, disconnect the meter and turn the shaft and measure again. Average a few readings. Don't turn the shaft with the ohmmeter connected.

As a coarse troubleshooting aid only, you might wire it to breakers considerably larger than 15A.
If the compressor intermittently shorts due to vibration it should probably blow these larger breakers also.

This is about as hard as it gets. The randomness bespeaks of great evil within this device!
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Old 02-19-2009, 06:56 PM   #7
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Craftsman air compressor blowing breaker


Just thinking out loud here. But is there a one way check valve on the compressor that keeps the tank pressure from back feeding back into the compressor? If so and it was allowing air in the tank back it could make the compressor work harder or deadhead on startup.

Gary
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Old 02-19-2009, 07:51 PM   #8
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Craftsman air compressor blowing breaker


Can he just install a 30 amp breaker according to article 440 ??

They are telling me at Mike Holts) that I can install #12's on a 40 amp breaker to a motor load (AC). Is this compresor viewed differently from an AC compressor or other motors in 440?

I don't have a code book with me. Maybe it's reserved for motors with certain protection built in.
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Old 02-19-2009, 07:54 PM   #9
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Craftsman air compressor blowing breaker


Gary,
It does have a check valve and I made sure it was working correctly when I replaced the pressure switch last night.
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Old 02-19-2009, 08:33 PM   #10
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Craftsman air compressor blowing breaker


The reason the breaker trips occasionally is because the starting current of the motor is very close to the magnetic tripping current of the breaker. Basically, the breaker is seeing the motor starting current as a fault.

These compressors are notorious for extremely high starting current. One solution, if your panel is Square D QO (not homeline) is to get a high-magnetic 20 amp breaker. The model# is QO120HM.

The 5.5HP rating is not the actual running HP, it's the maximum HP the motor will produce before it stalls. It would produce 5.5HP for less than 2 minutes before it burns up. The actual running HP is more like 2 HP or so. Look on the motor nameplate, I bet is says for amps (FLA) something around 20.

If the compressor is hard-wired (not cord-and-plug connected), according to article 430 (motors) in the code, the maximum breaker size is 250% of the full-load current. Usually, we use a table in the codebook that gives amps vs. HP, but I bet this motor doesn't list a HP on the nameplate. It very likely says HP...SPL. The minimum wire size is 125% of the full-load current.

Therefore, you could use a larger breaker than the wire is normally rated for, but in no case can you use a larger breaker than the receptacle is rated for. So the compressor must be hard-wired, and it must be the only thing on the circuit.

Rob
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Old 02-20-2009, 10:44 AM   #11
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Craftsman air compressor blowing breaker


Does the compressor start under full load (i.e., high inrush)?

Last edited by Yoyizit; 02-20-2009 at 10:47 AM.
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Old 02-20-2009, 04:48 PM   #12
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Craftsman air compressor blowing breaker


I'm interested in what micromind said. Does this mean if I cut The plug off, take out the outlet and wire it directly I can use a 30 amp breaker with 12 ga wire? This will be a dedicated circuit.
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Old 02-20-2009, 05:03 PM   #13
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Craftsman air compressor blowing breaker


In your owner's manual it says a 15 amp circuit but it can be changed to a 20 if needed. There's something wrong, and I would try to assess that vs. up sizing the breaker. Centrifugal switch perhaps?
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Old 02-20-2009, 05:09 PM   #14
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Craftsman air compressor blowing breaker


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Originally Posted by wirenut1110 View Post
There's something wrong, and I would try to assess that vs. up sizing the breaker.
Amen.
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Old 02-20-2009, 05:49 PM   #15
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Craftsman air compressor blowing breaker


Well, I did cut off the plug and wired it directly in the box. I left the 20 amp breaker in. I have a job tomorrow that I will use it with, so I'll see how it's working.

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