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New Compressor Amp Requirement?

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New Compressor Amp Requirement?

Unread postAuthor: clemsonguy1125 » Fri Jun 04, 2010 6:16 pm

My new 4 gallon .75 horsepower craftsman compressor needs 15 amps to operate properly. Whats the best way to find out the amperage of my garages outlet.
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Unread postAuthor: jrrdw » Fri Jun 04, 2010 6:45 pm

Look at the breaker switch, it will have a number on it, 20 amp, 30 amp etc...

15 amp is the start up, it will draw less amperage 5 seconds or so after it turns on.
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Unread postAuthor: clemsonguy1125 » Fri Jun 04, 2010 6:53 pm

I checked and there was no amperage listed, it said 120/240 VAC
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Unread postAuthor: Technician1002 » Fri Jun 04, 2010 7:08 pm

The regular 120 volt outlets are 15 amp. What you have to watch is how much power is drawn on the same circuit breaker or fuse. Some homes have 15 amp circuits and some have 20. The outlets are usually 15 amps max per outlet. The things to watch for are freezers and such on a shared circuit breaker. The compressor is less than one HP so most of the time you have little to worry about. Plug it in and put it to work. The limits get harder to meet as you go to higher HP rated compressors.

In your paperwork, it should list the motor run amps. A 3/4 HP compressor will often draw less than 5 amps running and 3X that starting.
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Unread postAuthor: clemsonguy1125 » Fri Jun 04, 2010 7:51 pm

The papers say you need at least 15 free amps and the box says you need a minimum or 8 amps
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Unread postAuthor: ramses » Fri Jun 04, 2010 9:01 pm

That depends on your breaker. If it is a "fast" breaker, you need it to be rated according to the 15 starting amps. If it is a "slow" breaker, it won't trip from the starting surge, and you can use the run current rating.
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Unread postAuthor: Technician1002 » Fri Jun 04, 2010 11:15 pm

Most homes have a thermal/magnetic breaker. They have a time delay to allow the starting of electric motors such as freezers, vacuums, saws, drills, etc. If overloaded there is a small delay before they trip unless the fault current is very high such at 100X the rating for a fast short circuit trip.
Home breakers are designed to prevent overheating of the wiring.

Magnetic instant trip is seldom used in a home environment. They cost more and are often used to protect electronics.
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Unread postAuthor: Gippeto » Sat Jun 05, 2010 12:11 am

Technician1002 wrote:Plug it in and put it to work.



Sometimes a simple and to the point answer is the best one.

If you blow the breaker, then unplug the compressor, and RESET the breaker as you don't likely know what else is on that circuit (could be the freezer as mentioned).

Then try a different outlet. It's not rocket science, and shouldn't be made out to be. The outlets in your garage were INTENDED to be used, and if their "use" (tools) was thought about at ALL by the guy wiring the garage, they should be lightly loaded or higher amperage (20amp) circuits. That means there are fewer outlets/lights on the circuit than would otherwise be the norm. (Up to 12 "boxes" are allowed by code here.)

If you do have 20 amp circuits, you should have 20 amp plugs. One of the "slots" will be T shaped.


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Unread postAuthor: Technician1002 » Sat Jun 05, 2010 12:35 am

Some branch circuits are 20 Amp, but with a string of outlets on one breaker. The 20 Amp outlet is pretty much reserved for single itom 20 Amp service such as a Microwave oven or window air conditioner.

Behind the wall, the 20 Amp circuits use 20 AWG size wire (not sure the metric size for non US) and 15 Amp circuits use 14 AWG size wire. There are exceptions to the above rule when the distance is far from the breaker to keep voltage drop under load within the maximum allowed voltage drop.

The special outlet shown above is to prevent plugging a 20 A appliance into a receptacle that will melt with a 20 Amp load or a branch shared with the fridge, freezer, lights, stereo, vacuum cleaner, blow dryer, etc. Most 20 A receptacles are the only outlet on the breaker that feeds it. The electrical code requires it that way in some locations.

It is common for houses to have the smallest breaker be a 20 amp but use lots of 15 amp outlets.

It is common to wire 2 or 3 bedrooms on one 20 amp breaker with about 2 outlets per room for a total of about 6 15 amp reciptcals on the string. These often get overloaded if each room gets a window air conditioner.

Normal uses such as alarm clocks, lamps, a TV, bookshelf stereo, etc are no problem in this type of use.
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Unread postAuthor: jrrdw » Sat Jun 05, 2010 5:46 am

clemsonguy1125 wrote:The papers say you need at least 15 free amps and the box says you need a minimum or 8 amps


15 amp start up, 8 amp continuous... It will be like running a vacuum cleaner.
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Unread postAuthor: clemsonguy1125 » Sat Jun 05, 2010 11:17 am

Thanks for all the help, I just didnt want to damage the motor like the papers said you could if you didnt have the proper amperage, the outlet isnt the t shaped one but ill plug it in later today and see what happens.
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Unread postAuthor: Technician1002 » Sat Jun 05, 2010 11:34 am

The important thing with that size compressor is to never use it with a long light weight extension cord. They don't like the drop in voltage. A 50 foot 12 AWG cord would be OK, but a 50 foot 16 AWG cord or a 100 foot 14 AWG would be looking for trouble.
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Unread postAuthor: clemsonguy1125 » Sat Jun 05, 2010 11:39 am

I have heavy duty extension cord for my 2 hp leaf blower that should work fine.
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Unread postAuthor: mark.f » Sat Jun 05, 2010 12:53 pm

Technician1002 wrote:The important thing with that size compressor is to never use it with a long light weight extension cord. They don't like the drop in voltage. A 50 foot 12 AWG cord would be OK, but a 50 foot 16 AWG cord or a 100 foot 14 AWG would be looking for trouble.


Exactly. Plug the compressor into the wall with the cord it comes with and use a longer air-hose if you need air somewhere further away. I wouldn't use an extension cord at all.
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Unread postAuthor: clemsonguy1125 » Sat Jun 05, 2010 1:27 pm

I got it working great, Im uploading some videos of a airsoft machine gun running off it, youtubes slow for some reason today.
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