Compressor Shopping

A three-prong 115V circuit can be 20 amps, but it has to be the 20A rated socket, and the plug for 20A has one prong rotated 90 degrees. Plugs and sockets with two parallel prongs are rated for 15A - these are lighting outlets.

1 HP is about 750 watts, so 1500 wats or 13A at 115 V is the minimum current for 2 HP at 100% efficiency. But 100% efficiency is physically impossible, so Quincy specs 19 A, which is about 70% efficient. This is possible, I guess, but seems rather optimistic to me. Maybe ...

 

Do they tell you what amp circuit you need? 5 HP is about 16A at 100% efficiency, but I think they are probably lying about the HP rating. With 50% efficiency, you'd need 32 A ... A 30 A circuit should be ok. You can do that with the orange Romex that is available cheaply at the home center. No plug/socket. Use a 10 ga dryer whip to go between the terminal box on the wall and the compressor. You want stranded wire (like the whip) for the transition from the wall to the compressor - solid wire like Romex will fail from the vibration.

I think 30A should be enough, but HF calls for a 40A circuit from their stated 23A peak load. If you go with 40A, that's 8ga wire and an 8ga whip. Wire prices seem to make a bog jump from 10 to 8, and you probably won't find 8 ga Romex at your local home center. The 10 ga wire is mass-marketed and seems a lot cheaper.

BTW that Kobalt is a good option - I read the ad that it is "like" the Home Depot unit. Kobalt is definitely from Lowes. The current compressors from Lowes are made by Campbell-Hausfeld, like this one Air Compressor, 80-Gallon Vertical Two-Stage 14CFM 5HP 208-230V 3PH (CE3001) They say 12.8A, which is impossible for 5 HP at 230V. 12.8A at 230V is 3.9HP at 100% efficiency.

 

Mmm. You could run it at 115 on a dedicated circuit at 30A. There is a different plug and socket for for each of those ratings. Given the 19A rating from Quincy, it should have the 5-20 NEMA plug. 20A and 30A receptacles will accept that plug, 15A won't.

 

Tim, this compressor you noted require 3 phase power. Highly doubtful that is available at a residents. Not a good comparison.

 

Yes, my mistake. I should have linked to this one. Air Compressor, 80-Gallon Vertical Two-Stage 14CFM 5.5 HP 208-230V 1PH (CE3000)

The current needs of the 3-phase motor is going to be lower than singlephase, so my claim about the HP is wrong. With a single-phase motor, it's spec'd at 22A. That is 6.74 HP or 5.5HP at 82% efficiency. Still seems unlikely, but it's not blatantly unphysical.

 

Gosh I can only guess. There will be a steady-state demand where the compressor runs 3/4 of the time. If the specs are honest, that should be the same as the spec for how much air the compressor can deliver. For example, when the compressor is delivering, say, 15CFM at 90 psi continuous, the motor should run for 7.5 minutes and be idle for 2.5 minutes. If you knew the compressor set points, the tank volume, and the demand, you should be able to calculate the running time of the nominal duty cycle. But it's an integral equation, because the effective volume of the tank changes as the pressure drops. Certainly if the compressor can't keep up without some significant occasional pause, you are exceeding the duty cycle.

Thinking of a way to make a WAG... I'll get back to you.

 

All the duty cycle says is that the pump, working into the tank volume and set points, can deliver 33% more air than the spec. Thus it only needs to run 75% of the time to achieve the spec. The larger the tank and the further apart the set points are, the longer the cycle will be, both active and idle.

They would claim 100% duty cycle if the pump delivered exactly the spec.

If the spec is 15 CFM at 90 psi, then the pump delivers 15*1.33=20CFM at 90psi. The pump delivers the air at a much higher pressure, then that higher pressure gets diluted by whatever is in the tank. You can use the basic gas law P1 V1 = P2 V2, but that does not consider temperature (of the gas) - this is a big issue.

Motor temperature is another issue. For me, I never get into the regime where I worry about duty cycle - I just never work that hard. But I don't have a blast cabinet, and I've never painted a whole car at once.

 

Yeah, according to my interpretation. In theory, a compressor can deliver any volume of air (limited by the tank size and standing pressure) over a short enough time. All you need is a bigger hole for more air. The only rating that makes sense is the one it can maintain.

Of course, I'm just reasoning my way into this, so I could be mistaken. The specs could mean something else.

 

Champion is good stuff. Gardner-Denver Corp. Horizontal tank takes a lot of room though.

Not real impressed with the pump - how about Bare Compressor Pumps & Replacement Pumps | Eaton Compressor