Quick question, Wiring a compressor.

[Crank1]

Hey everyone, I am currently having a 24x30 attached garage put on my house and while the guy is wiring it, I told him I wanted plugs for a compressor and a welder. The welder plug he has wired with 10-2 and a 30 amp breaker, but wired the compressor with 12-2 and a 20 amp breaker. I do not have a compressor yet, so I cant show him how much power it'll need, but in the future, I'd like a 80 gal capacity compressor, Looking now for a used Quincy or Champion vertical setup. Will the current wiring be sufficient or will it need to be bumped to the 10-2 and 30 amp service? I've read that is how most people have theirs wired up but is this mandatory or precaution? I'd just like to know before the sheetrock goes up. Thanks
Brandon

 

[MrMark]

I think the best option would be to run 8-2 NM with 40 amp breaker. That should do the job for just about any compressor. 10-2 is probably sufficient for 95 percent of compressors but you are mentioning some big stuff so I would go with the 8-2

 

[Steevo]

10/2 minimum and 8/2 is preferable. You can only run a 30A load on a 10/2 and a true 5HP motor will draw close to that on startup and possibly trip a 30A breaker.
The big, good quality units you have mentioned will likely have 5HP motors.

 

[MrMark]

Actually, a true 5HP inrush current on startup is very high, just a guess but maybe 150 amps or more for milliseconds. Inverse time delay breakers will not trip on inrush current. Their trip curves require sustained overload of several times rating for a second or more to trip.

 

[Steevo]

I stand corrected on the physics of electricity, and stand behind the need for 8/2 to be prepared for a real 5HP motor on a big compressor.

 

[PRH44]

Always go larger if its in the buget now is time while the walls are open and you have access. The best way is to install conduit, 3/4" would be fine as one can install up 6 #8 THHN conductors. But romex will get the job done just fine.
Article 430 of the NEC requires conductors to be sized at 125% of full load amps
5HP at 240 volts = 28 amps full load amps 28 *125%= 35 amps # 8 THHN at 60 degree C is good for 40 amps The Inverse time breaker can be up to 250 percent of the full load amp 28 * 250% = 70 amp. The breaker an wire will mismatch in this application, the motor needs to be thermally protected.

 

[sberry]

Replace the 12 with 10, most of the 5 hp 80 gallon units found in the box stores require a 10 wire.

 

[Crank1]

Yea, I'm on the lookout for a Wayne, Quincy, Champion or the like compressor. I don't wanna buy some cheap Craftsman compressor, so I'm keeping my eye on Craigslist, and trying to find a deal, but I'm thinking as long as I stick with a 5 hp compressor, the 10-2 should be good with a 30 amp breaker. This is a home garage, where I'll be working on projects, and farm stuff, so I want a bruiser of a compressor for big requirements, but I think the 5 hp will work for my needs. Thanks guys

 

[SteelHorseHD]

I have a Matco 60 gallon compressor. I am not sure what size motor is on it but It only pulls 14 Amps with 240V. I ran 12-2 wire to it with a 20A breaker. I would suggest looking around on the internet to find a compressor that is similar to what you would like to see what kind of power source you will need. I already had my compressor when I wired my shop so I just went with the 12-2. Like everyone else said, you can't go wrong with 10-2 for a compressor but depending on what you decide to get you might be okay with the 12-2.

 

[sberry]

For a wired now and add later trade the 12 for a 10. It will power a comp that meets your needs.

 

[PRH44]

Beware of HP rating vendors will use labels such as develops 5 HP or up to 5HP or 5 HP rated. The motor will only be a 3 HP but can develop burst of HP up to 5. Table saws, radial arm saws and compressors are notorious for this. Nothing really wrong with this if the equipment does what you want it to do. Just something to keep your eye one.

 

[elenahusky]

I ran 12-2 wire to it with a 20A breaker. I would suggest looking around on the internet to find a compressor that is similar to what you would like to see what kind of power source you will need. I already had my compressor when I wired my shop so I just went with the 12-2.

 

[hidollartoys]

For the "correct" answer to your question you should refer to Article 430 of the NEC. Motors are a "special" consideration in the code. Nameplate amps are not what you design a motor circuit to. Code requires you to design to "Table Amps" as found in Table 430.248 of the code. You can only use nameplate amps to determine "overload (heater)" size. Wire size is 125% of the table amps. Overload protection is allowed to be up to 800% of table amps depending on overload protection devices (see Table 430.52).

As it stands (per code) the 12ga wire is good for a 3hp motor. You can always upsize the wire to accommodate a higher rated motor and "fuse" to relieve any "start trip" that you experience.