One Recepticle only giving 15 amps on 20 amp circuit

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Old 12-01-14, 09:20 AM
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One Recepticle only giving 15 amps on 20 amp circuit

My house had a fairly new 20 amp circuit that was wired directly to the garage (20x20). There was one receptacle previously on this circuit on the back wall. I double checked the wire and parts to be 20 amp 12/2 etc just for verification purposes. However, I have been running my air compressor for 2 years with no issues. The compressor needs around 18 amps to start so clearly it will not work on a 15 amp circuit.

This weekend I bought a total of 50 feet of 12/1 wire. I started at the original receptacle on the back wall. We will call it receptacle A. I then used pigtails and ran an additional wire about 22 feet to the back of one side of the garage and installed a 20 amp receptacle. We will call this receptacle B. From receptacle B I also used pig tails to run another 22 feet to the opposite side of the garage to create receptacle C with a single and ending termination. I first used my wet/dry vac to test all outlets and it worked with no issues, this to me proves that I am getting 15 amps at each receptacle (A + B +C). I thought well this is great, everything is working. I proceeded to test my air compressor. It worked perfectly in receptacles A + B but did not work in receptacle C. I tried many other 15 amp devices and everything worked. However, when I tried my compressor it blew it's internal breaker and acted exactly like when I have tried to plug it into a 15 amp circuit. Please let me know what I did wrong and how I can correct the issue. I did re verify that receptacle C is 100% without a doubt a 20 amp receptacle.

Thanks!
 
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Old 12-01-14, 11:48 AM
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Receptacles have no means to limit the amperage delivered as there is no over current device in them. The only way you could say there was a limit was due to the slot configuration in that a 15 amp device will not accept a 20 amp cord cap.

You need an amp meter to see what the compressor is drawing on startup. I suspect voltage drop or a poor connection may be causing the problem.
 
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Old 12-01-14, 11:59 AM
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This weekend I bought a total of 50 feet of 12/1 wire.
What the heck is 12/1 wire?

However, when I tried my compressor it blew it's internal breaker and acted exactly like when I have tried to plug it into a 15 amp circuit.
What is this "internal breaker" that is part of the compressor unit? I suspect it is an overload device that is tripping because of voltage drop on your supply wiring causing the compressor motor to draw excessive current.
 
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Old 12-01-14, 12:51 PM
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The compressor needs around 18 amps to start so clearly it will not work on a 15 amp circuit.
Why? Almost all breakers are by nature slow trip. So long as the FLA is 12* amps or less a momentary 18 amps load should not trip the breaker. What is the full load amps running?

*Even if the FLA was grater than 12a but 15a or less if the motor doesn't run for more then fifteen minutes it probably won't trip.
 
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Old 12-01-14, 01:35 PM
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What are the spec's on this compressor?
Geo
 
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Old 12-01-14, 01:55 PM
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Thanks for all of the replies.

I was just confirming that I am using 12/2 AWG wire rated for 20 amp circuits (apologies, I said 12/1 in my post). As for the air compressor, it takes 18 to start and runs constantly at 15. It has an internal breaker and a reset button. It sounds like it's just a coincidence that 15amp devices work since and there simply isn't enough voltage.

I will double check my connections again tonight but I wanted to make sure that I didn't do anything wrong up to this point. Is it possible that voltage drop would occur for an additional 22 feet?
 
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Old 12-01-14, 02:02 PM
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Is it possible that voltage drop would occur for an additional 22 feet?
If it was borderline at the point where you started the additional run then yes, it is possible.

What is the total length of the run from the circuit breaker panel to the last receptacle? Remember also that most home-shop air compressors use special motors that are not all that efficient. These motors often have higher than usual starting currents.
 
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Old 12-01-14, 02:24 PM
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It would not be unusual for a compressor to have a start current of 2-3 times or more of the run current. Your two figures are very close which is kind of unusual.

A breaker will hold over 125% of its rating for 2 hours, less if the current is higher.
 
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Old 12-01-14, 02:31 PM
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Rule of thumb. Add hose, not extension cords to a compressor. Air at the tip is the same at 20' as it is at 120'. We never connect a compressor to a cord beyond its own. Always directly into a receptacle.
 
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Old 12-01-14, 02:46 PM
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To answer the question about the total run - The total run to the last receptacle (C) would be roughly 85 from the breaker box in the basement of the house. Everything is working correctly at receptacle B, which is about 65 ft from the breaker.
 
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Old 12-01-14, 03:01 PM
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A whole 20 X 20 garage wired with just one 12-2 incoming line!
Seen it done way to many times.
Just makes no since to me.
Someone at some point is going to be adding on to the needed load.
Cost more but just makes total since to me to always run more power then you think you will need so you do not have to go back and redo the whole thing.
I build a garage for someone and I always use a at least a 60 amp. 220 volt panel.
One 15 amp. circuit for lights and future garage door opener, one 20 amp. for outlets.
Still leaves you plenty for a compressor, window A/C ECT.
 
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Old 12-01-14, 07:28 PM
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As for the air compressor, it takes 18 to start and runs constantly at 15
And how exactly did you measure the current draw?
 
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Old 12-02-14, 09:25 AM
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Someone measured the amperage for start and posted this on a review for the item, but the specs say that it uses 15 amps constant.

Also, the comment about this being the only run to the garage was pointless and irrelevant. I never said that and it was a pretty strong assumption. There is a 15 amp circuit for lights and garage door opener and two other wall receptacles. This 20 amp was dedicated to running a compressor or large equipment only.

I redid the pigtail connections at receptacle B and it kicked on and worked normally once I tried to test again. So there seems to be an issue with the voltage drop if the connections aren't perfect. Feel free to correct my thinking. I originally wanted to make sure that I did not do something wrong or unsafe. Any feedback if appreciated.
 
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Old 12-02-14, 10:07 AM
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Sounds like you found the problem. Good job. Thanks for letting us know. It may help others.

Just a not the claim of 18 amps start current is suspect at best if the run amps is really 15. If the run amps is 15 the peak start amps would be expected to be 30-45 amps or more. A 20 amp circuit will handle a short term load like that without tripping. However if the plug on the compressor is a 15 amp plug (NEMA 5-15p) It is unlikely 15 amps is the run current despite the data plate. (You would expect the plug to be at least slightly higher rated then the expected long term run amps.)
 
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