So I'm getting ready to wire up my Quincy 325 air compressor with a 7.5hp single phase 220v motor on it. I wired up a 220v single phase outlet on the wall, coming off of a 50a circuit breaker in the breaker box.
Before going any further, note that compressor manufacturers are NOTORIOUS for lying through their teeth about horsepower ratings. There was even a class action lawsuit over this issue a few years ago. I'll leave it to you and Google for the gory details.
That said,
IF your compressor REALLY has a 7.5HP motor on it, that breaker/socket/plug arrangement will NOT be adequate. In fact, AFAIK,
NO commonly available outlet/plug combo will be suitable, or meet code. You DO need to hard-wire it.
ok, i'm no electrician but the ground in my 220 outlet acts as my neutral. take that advice for what you paid for it though.
At least in theory, that can be done; and it used to be fairly common (including for other 220V/240V appliances, such as ranges/ovens, clothes dryers, etc.). However, it no longer meets more recent versions of the NEC. If there is ANY need for a 120V "tap" in the appliance in question, you MUST use a four-wire feed with separate Neutral and EGC conductors.
My motor had the wiring diagram under the wire box cover. 50amps seems really high, mine only needs a 20a.
Then there's no way that's really anywhere near 7.5 HP. Maybe 3 HP, at most.
1. I'd use a larger breaker, at least a 70 amp to start with.
2. You need to hardwire it. A 7.5HP motor on a 50 amp recep isn't right.
3. What size and kind of wire did you use?
4. You definitely need a starter with a 240 coil. They are out there. Every starter manufacturer makes one.
Agreed, on all counts.
I wired it to a 50a 220v receptacle so I can unplug the compressor, and hook a welder up to the same outlet.
As noted above, that's no good.
I'm going to use a 3-prong 50a rated dryer cord with 10ga wire.
Also no good, on multiple counts. You need
at least AWG 8, and AWG 6 would be a better bet. And you need to hard-wire it.
The running amps on the motor is around 20a, why would I need a 70a breaker?
You might not. But that's only because
IF that motor only pulls 20A, then it is NOT a 7.5 HP motor. You will probably find this:
http://www.buildmyowncabin.com/nec/nec2011_table430.248.html
useful.
It seems like every post I've found with guys that are running 7.5hp compressors, they all use 50 or 60 amp breakers.
Aceman covered this perfectly: A 50A breaker is "legal"; but more is better, as long as the feeder cable is up to snuff.
I actually have the receptacle wired up to a 50a breaker, in a sub panel...right next to where the compressor is, so I can use it as a disconnect.
Using the breaker as the local disconnect is not ideal; but it's probably workable in a pinch.
I have #6 wire with #10 ground going to the receptacle box. So that parts done.
So convert that "receptacle box" into a hard-wired junction box, using some appropriate BX/MC cable or similar to connect to the compressor itself.
I guess I'm confused....if the motor is rated to pull 30amps at full load, (but actually pulls around 20a when it's running the compressor), why would a 50a receptacle not be up to the task? The breaker, the wire, the receptacle, the plug....everything is rated for 50a.
Because (at least per code) plugs and receptacles have BOTH current ratings AND horsepower ratings, and neither one "overrides" the other. IOW, you need to meet BOTH specs in order to be code-compliant. And as noted previously, no commonly available plugs and receptacles have horsepower ratings exceeding about 3 HP.
Now, if you're wondering why a receptacle/plug which would otherwise be capable of 50A is effectively only "good for" about 20A when used with a motor, the answer can be found in two words: Inrush Current.
