Electrical Wiring Question

I know there are several electricians on here, so hopefully I can get some good answers. I have a 6.5 (claimed) HP 60 Gal compressor that I need to wire up tonight. I don't know the model number or motor specs off the top of my head, but from my research it seems these are almost always rated at 15 amps, and I know it is 220V. Assuming this is true ( I will double check when I get home), I am thinking I can remove the 50 amp breaker the previous owners put in for their hot tub, put a 20 amp 240V double pole breaker in, run 12 GA 2 conductor (with ground) wire to a 620R type 20 amp recepticle in the wall and then put a matching plug on the wire coming out of the compressor. Does this sound correct, or is there something I am missing? Also, do I need any special type of box to mount the recepticle in, or will a run of the mill type box work?

 

Make sure to cover the ends of the wire off of the 50A breaker, you don't want that getting in the bus. Then you'd have a hot plug somewhere with no disconnecting means.

 

You're looking at more like 30A for that load.

 

I just checked the motor and it says 15 amps. Am I ok with 12 GA wire and a 20 amp breaker?

Oh, and thanks for the reminder to tape off the 50 amp wire. Definitely don't want those going to the wrong place. If they weren't in conduit I would actually pull them out of the box.

 

That's the most efficient 6.5HP motor I've ever heard of.

15A max amp draw would require a 20A circuit, so #12 copper it is.

You can use a standard 20A 240v plug & receptacle, or twist-locks. I prefer the twist-locks on stuff like that personally. :up:

 

Thanks for the help Darren. I am highly skeptical that the motor is actually a 6.5hp motor. It doesn't look anywhere near big enough. Would a motor that size need to be 3 phase anyway? From what I have seen on the internet the marketing guys have some formula they use to exaggerate the HP rating to make it look better.

I will check out those twist lock plugs.

 

Yeah, something doesn't sound right or that motor isn't 6.5hp. I have a 80gal compressor that has a 5hp motor. I hard wired with a 40amp breaker (8awg) since the FLA was 28amps.

 

Yeah, I agree, there is no way the motor is 6.5 hp. Especially since it is a single stage compressor that I bought at Lowes. I bet the motor is somewhere around 2-3 hp.

 

Do the math...

220V x 15A (what it says on the motor) = 3.3Kw or 3300 watts...

1 hp = 754Watts 3300/754 = 4.37, lets round it off to 4.5hp.

best thing to do is wire it up and measure the amp draw while running...that will tell you if its drawing more or less than 15A. and yes if its 15A, you overrate the breaker by 125% and a 20A breaker is called for it.

 

@100% efficiency.

You could corner the market with that motor.

Most of 'em come in between 60-70% efficiency.

 

Does the nameplate state an amperage and an overcurrent protection rating?

 

It just says 15A

 

Marketing BS. The fine print probably says "peak HP" somewhere, or it should. If you do the math on the wattage at peak vs rms voltage at 230v it comes in at about 6.46hp, assuming the 15 is an FLA rating. Be lucky they didn't go peak to peak and call it a 13HP motor.

 

Ok, so I took out the 50 amp breaker and put the 20 amp in. I actually pulled the conduit apart in the garage attic and used part of the conduit to run the 12-2 wire up the wall and then routed it with the rest of the wiring for the house. I ran that across the garage attic and into a 20 amp outlet in the wall on the other side. I hooked up a plug on the compressor, flipped the breaker, plugged it in and turned it on for a couple seconds and everything seems to be working.

Now, the engineer side of me started overthinking things and I have a couple questions. First, I have two power wires and a bare ground wire. Is it possible to hook the two power wires up backward? It seems obvious that you can't but I want to be sure. Second, I saw somewhere on the internet that I needed to pigtail the ground wire to both the metal box and the ground terminal on the outlet. Is that necessary or am I good just hooking the ground wire up to the outlet. Finally, this goes back to question number 1, is it possible to hook things up such that the motor will turn backward? I only ask because for the couple seconds I had it running it sounded funny, but then again, that could be because the tank is empty so there isn't much resistance. Not to mention I haven't heard this compressor run in probably 5 years so it could be normal for all I remember.

 

You're good on the power wires, it doesn't matter, the capacitor on the motor gets it spinning in the right direction. Only on 3 phase do you need to be concerned with the wiring. On those motors you can bump start it and see which way it's spinning if it's in the wrong direction just flip flop 2 wires
As far as the ground goes you should tie the metal j-box to the ground. If the wire wore thru some where in a hot lead and charged up the metal box that potential energy doesn't have any place to go

 

Thanks GRH. Sounds like I have everything set up right except grounding the box. I will add a pigtail to the ground, change the oil in the compressor and I should be good to go.

Edit: So if I had used a plastic box I could have just hooked up the three wires and been done with it correct?

 

Yep.

Good job on your install. :up:

 

Thank you for the help Darren. I haven't done much electrical work and it made me feel a lot better running my plans by some licensed electricians.

 

I wish more people would.

Been helping a friend on a project, and in cleaning up/moving circuits, discovered that the "handyman" that wired his basement 15 years ago completely screwed the pooch on his basement lighting circuit, stealing neutral from another circuit because he didn't run 3-wire between his 3-way light switches to carry the neutral for the new circuit he ran. Took a while to track that down and figure out why a circuit that was turned off was showing 5.3 amps of current over the neutral. :down:

 

Yeah, I don't want to be that guy. I understand the theory enough to know that there is a reason codes are written the way they are, yet I don't understand enough to just start hooking stuff together without planning it out and making sure it is right. House wiring is definitely not a place to take short cuts. That reminds me. I need to put some black tape on the white wire in the outlet when I fix that ground.