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Old 05-29-2008, 11:01 AM   #1
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Garage Circuit

My garage circuit is on a 20A breaker, however all the outlets minus the GFI outlet are 15A outlets. My compressor is a 15A rated motor, but at start up especially if its cold it trips its internal breaker. It doesn't do this if it is plugged into the GFI outlet, which is closest to the breaker box. I'm wondering if the 15A plugs are limiting the compressors ability to pull the Amps it needs at start up?




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Old 05-29-2008, 11:41 AM   #2
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Is the GFI outlet 20amps?


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Old 05-29-2008, 11:51 AM   #3
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My assumption is Yes...because it has the horizontal prong inlet.
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Old 05-29-2008, 08:08 PM   #4
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A typical capacitor-start induction motor (almost certainly the type on your compressor) draws about 6 times more current to start than it does to run. If the running amps is 15, it'll draw about 90 to start. This is normal. It's why the lights will dim somewhat when the washer, refrigerator, or garbage disposer starts.

Non-industrial (homeowner type) air compressors are notorious for loading motors to the gills. The colder it gets, the harder it is to turn a piston pump. The amount of starting torque (turning power) a motor can deliver depends largely on the voltage at its terminals.

Anytime there's current flowing in a wire, there's voltage drop across it. For example, a 2 amp load on a #12 wire 2' from the breaker will 'lose' about .015 volts. The same circuit 2000' out will 'lose' about 15 volts.

Now your compressor isn't a 2 amp load, nor is it 2000' from the breaker. What I'm getting at here is the further from the panel the load is, the less voltage there'll be left at the end of the wire.

An induction motor, when running, has the unique ability to draw more current if the voltage is lower, or the driven load is higher. (Within reason). This compressor will likely draw pretty close to 15 amps if the pump is at operating temperature (hot), and the voltage at the motor terminals is close to 115. If the voltage is lower, the current will be higher. If the pump is cooler, the current will be higher.

The overload in a motor is nothing more than a precision circuit breaker. It's tolerance is much closer than the one in a panel. It's designed specifically to protect a motor. The breaker in the panel only protects the wire connected to it. Remember, the further from the panel we get, the more voltage drop there'll be. That means less voltage at the motor terminals. Lower voltage=higher current; lower temperature=higher current. Both of these are ganging up on the overload. Raise the temperature, it runs OK; raise the voltage (by getting closer to the panel), it runs OK.

I doubt that there's any problem with the wiring, I think the compressor is designed operate right at its limit under ideal conditions. The further from the panel, and the colder it is, the less ideal the conditions.

As far as the outlets go, most if not all manufacturers use the same base and contacts for 15 amp ones as 20 amp. Just the faceplate is different. If you break one open, you'll see what I mean. Without the faceplate the contacts will accept either horizontal or vertical prongs. The rating is determined by the prong configuration of the faceplate.

It's completely legal, and even normal to put more than one 15 amp outlet on a 20 amp circuit. It's not legal however, to put a 20 amp outlet on a 15 amp circuit.


P.S. I'd bet the compressor won't run very well on an extension cord, even a short one. (Voltage drop).
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