220V, 1-Phase, SF 36 AMP Motor Trips Circuit Breaker

Electrical code specifies the minimum breaker size of 125% x FLA which puts you at 45A and you would round up to a 50A breaker would be the minimum size breaker and many posts recommend 60A. Your capacitors are well withing spec. which is normally has much more variability (+/- variance). My guess is that the breaker was marginal to start with and they can get a bit finicky with time. As far as wire size, on a 50A breaker I use #8 to my compressor, with a 7.5Hp I am a bit surprised at the use of #8. It also depends on the temp rating of the wire, if pulled through conduit vs. plug, etc. Leave that to the electricians. I will say that a 40A breaker is too small for the motor rating, panel breakers tend to be inexpensive, so could try a 50A with the #8 wire you have. I have a Champion 2-stage HR5-8 compressor which my manual specifies #8 wire for 5Hp single phase, #6 wire for 7.5Hp single phase model, and for longer runs #4. They do not give a breaker size, but it must be upsized to handle a motor start load and with your SFA of 1.15 you could be pulling over 40A as it gets to higher pressures.
 
mksj said:
Electrical code specifies the minimum breaker size of 125% x FLA which puts you at 45A and you would round up to a 50A breaker would be the minimum size breaker and many posts recommend 60A. Your capacitors are well withing spec. which is normally has much more variability (+/- variance). My guess is that the breaker was marginal to start with and they can get a bit finicky with time. As far as wire size, on a 50A breaker I use #8 to my compressor, with a 7.5Hp I am a bit surprised at the use of #8. It also depends on the temp rating of the wire, if pulled through conduit vs. plug, etc. Leave that to the electricians. I will say that a 40A breaker is too small for the motor rating, panel breakers tend to be inexpensive, so could try a 50A with the #8 wire you have. I have a Champion 2-stage HR5-8 compressor which my manual specifies #8 wire for 5Hp single phase, #6 wire for 7.5Hp single phase model, and for longer runs #4. They do not give a breaker size, but it must be upsized to handle a motor start load and with your SFA of 1.15 you could be pulling over 40A as it gets to higher pressures.
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Thank you, I'll try your suggestion first.
 
Agree with Mark above, caps are OK. Problem is elsewhere
-M
 
I undersized my wire gage when I first installed my air compressor. Worked great for a couple months then had same issues. Opening all the wire box covers and found melted wire nuts. Unfortunately it shorted out real good and started the motor on fire. That was a fun evening......not.
I'd double check the wiring and go 6 gauge been considering a motor starter.

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Since my last post, all of you inspired me to better understand how electric motors work. I dove in head-first, pulled the motor, and disassembled it. Other than it being a little dirty, it looks good, including the centrifugal switch. The bearings also look and feel good, though I'm debating to replace them since I have it apart. Thoughts? I'm confident I found my start and run coils. They measure 1.6 OHMS and .5 OHMS, respectively, and should be good. All wires tested against the frame were OL, so I don't have anything shorted. My start and run caps continue to test good. I found no evidence of heat anywhere, and all contacts look good.

As you said, my problem is the undersized wiring and breaker. I picked up 6 AWG this morning and plan to run it this weekend. Of all that I've read, 8 AWG THHN would be sufficient and would take a 50 or 60 amp breaker, but I'm going with #6. I have a 50A breaker, so I'll run it first, and upgrade it if necessary. Thanks for your help, and I'll send an update when I get it back together and tested.
 
johneezreno said:
I undersized my wire gage when I first installed my air compressor. Worked great for a couple months then had same issues. Opening all the wire box covers and found melted wire nuts. Unfortunately it shorted out real good and started the motor on fire. That was a fun evening......not.
I'd double check the wiring and go 6 gauge been considering a motor starter.

Sent from my SM-G950U using Tapatalk
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Problem solved - It was pretty much the same as yours, minus the evidence of heat damage.

I replaced both bearings since I was in there, and I now understand motors much better. I ran #6 THHN and upped the breaker to a 50A since I already had one. It works just fine now.

And now to admit to fall on the sword. I'm glad it failed the way it did as it could have been much worse. While I thought I had ran #8 THHN originally, that was for my welder circuit. I only ran #10 THHN. I believe my thinking was if the motors SF was 36 amps, I would be safe with #10 THHN's 40 amp rating. I think had I initially ran #8 on a 50 amp breaker, I would not have had the problem. #6 was probably overkill for the 25' run, but I'm good with it. I went ahead and upgraded my #8 welding circuit to #6 with a 70A breaker as well. This should allow me to run my recently acquired Precision Tig 185 at full throttle, though I doubt I'll ever have a project requiring that.

Thanks everyone for your input.
 
Gaffer said:
Happy Veteran's Day everyone!

I have a 7.5HP air compressor you've helped me with before. It recently began tripping the circuit breaker upon a cycle start - 135 psi I believe. If I let the air out of tank, it worked fine on initial fill cycle, but upon refill, the breaker tripped every time. The mag starter and pressure switch are new, and coincidentally the problem began soon after I replaced them. Now it trips the circuit breaker every time.

I was able to hand turn the pulley and they turn freely with negligible resistance. I removed the drive belt to isolate the pump from the motor, and it still trips the breaker. I'm hoping my problems is simply defective start capacitors. I have minimal experience with this and why I need your help. I believe the capacitor on the top of the motor is the run capacitor. There are two cylindrical capacitors on the side of the motor, and I think they are the run capacitors - and what I hope is my only problem.

I have not put my meter on them, nor tired to discharge them. From what I've read on this site, I should discharge them with resistor, and this is the part that scares me because I've only dealt with small, low voltage capacitors in the past. Note in the pictures the pair of black capacitors have resistors soldered in place from the factory.

How should I proceed? I've read that a 20k OHM, 5 watt resistor is fine for discharging, but would like an expert's opinion. I have a Fluke 87V to test them. If the capacitors are the problem, should I replace all 3 to be safe? What brand/supplier do you recommend? I don't mind spending a little more for quality, as this compressor needs to be reliable.

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Sounds to me like the unloaded is malfunctioning.
If the motor will work fine with the tank empty but overloads when the tank is pressurized then compressed air is going from the tank to the compressor.
Most have a small air line that goes to the unloaded with the line in the outlet of the compressor.
Quincy used a small valve on the crank case operated by pump oil pressure.....
 
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