230V air compressor wiring question

LEAKYSEALS951 · 07:03 AM

Hello folks! Craziest thing happened-

A craigslist air compressor somehow landed on my trailer and followed me home yesterday.

The air pressure regulator was previously damaged by PO, destroying the assembly, so I need to rewire a replacement.

Before rewiring, I want to check the previous wiring. I am more familiar with 4 wire 220v applications, but I have read on the internet some compressors use the 3 wire setup, but I want to check before I do this. It is currently wired as two hot lines in and one common as a 3 wire out. The common is grounded into the box/compressor itself.

Mentally, I want to detach the commons from the pressure regulator box, hooking them together, and running a specific ground from the box to the outlet as a four wire. To me (and my limited knowledge), having the common grounded to the unit makes me a potential better ground if my shoes are wet or something and I touch the compressor.

Is this correct, necessary? or should I keep it as is? Thoughts?

Zeke · 04:24 AM

If this is in the US there is no 220v. It's 240. On that rig there will be no current traveling back to the neutral buss in your panel. The most important wire is the ground, or Equipment Grounding Conductor (EGC) and it should be wired to a grounded lug on a plug or the EGC in the box if hard wired. Any white neutral wires are essentially doing nothing.

But any 4-wire 240v appliance will have a separate white neutral (especially one that is using 120v for a timer, ignitor, etc. That set up must have the white go to the neutral buss and the EGC to the ground buss.

If in a subpanel, the neutral buss is isolated from the panel.

Edit: BTW, "common" refers to a wire in HVAC controls. There is no common in line voltage installations. It's hot (line, or lines), neutral and EGC for all single phase residential service.

john70t · 08:48 AM

Quote:

Originally Posted by Zeke

If in a subpanel, the neutral buss is isolated from the panel.

My layman understanding is that the neutral return should be separated from the true ground circuit.

Another component somewhere on that leg could short hot->neutral, with current feeding back into the common neutral of the compressor.
That would energize the outside of the compressor if it's own ground line was not working.

The compressor should be on it's own leg from the box, but that is the idea.

[edit: Zeke and 1990C4S corrected me below for 240v. I did not know 240v welders didn't use a neutral. My drier uses a 4-wire which I thought was hot/hot/ground/neutral.]

Zeke · 09-20-2020, 07:40 AM

Quote:

Originally Posted by john70t

My layman understanding is that the neutral return should be separated from the true ground circuit.

Another circuit somewhere on that leg could short hot->neutral, with current feeding back into the common neutral of the compressor.
That would energize the outside of the compressor if it's own ground line was not working.

The compressor should be on it's own leg from the box, but that is the idea.

That's what I said. And a 240v compressor is to have a dedicated circuit and breaker. There is no other "circuit somewhere" in that compressor circuit. Once the neutral is attached the potential is going back to the panel, period. If the neutral becomes disconnected, that's why the EGC.

His compressor would run fine with only 2 wires, one from each leg in the panel. However, with no ground, he is correct, he may become the ground path.

240v welders use 3 wires and that's up to code. There is no neutral.

LEAKYSEALS951 · 09-20-2020, 08:00 AM

Thanks all. I'm rusty on the terminology, so that helps. I've got a dedicated outlet running from the box, just trying to decide best wiring for new plug.

Time to go throw my back out trying to manhandle this heavy motha-f#$%r!!!!!

Thanks. R

1990C4S · 09-20-2020, 08:00 AM

It looks like you have a 240V motor and probably a 240V contactor, you should not need a neutral.

Black and red to a suitable circuit breaker, green to ground.

LEAKYSEALS951 · 09-20-2020, 11:02 AM

Thanks again for feedback. I had some spare 10 gauge 240v 4 strand wire laying around so I went ahead and made a dedicated neutral and ground. Works like a champ, and I can lick it with impunity.

One Freaking heavy mofo.

dad911 · 09-20-2020, 11:41 AM

What did you connect the neutral to at the compressor? I see just 2 hot legs, red and black. And a ground.

LEAKYSEALS951 · 12:33 PM

Quote:

Originally Posted by dad911

What did you connect the neutral to at the compressor? I see just 2 hot legs, red and black. And a ground.

The red and black (and white) were hots. The greens were the neutral, working as a combined neutral/ground in the pics since they are hooked up to the ground in the switch box. That was my original concern. I don't do enough electric to know if that is done that way factory or not. I didn't know if it was done that way by factory, or if the guy who wired it before me didn't know better. I redid it as a dedicated common and a new dedicated ground.

Of course the pressure switch I put on (used) is bad, so off to order a new pressure switch. It's fun pressurizing an unfamiliar tank wondering if it's going to blow up when it gets up to pressure! .. or if it will cut off.. or if the gauges are correct.. when the pressure blow off valve went, it was time to shut her down!

jrj3rd · 09-20-2020, 12:34 PM

No, the two hots and ground are all you need. No neutral as that is only for getting 120V (which is why the dryer has that leg). Run your two hots and the ground back to the box and be done with it.

LEAKYSEALS951 · 12:47 PM

Quote:

Originally Posted by jrj3rd

No, the two hots and ground are all you need. No neutral as that is only for getting 120V (which is why the dryer has that leg). Run your two hots and the ground back to the box and be done with it.

Yeah, I know.I know.

When I saw it, I was also bit concerned someone have wired it for 120v . With the earlier thread feedback, I figured it was okay, but then since I was replacing the cord anyway, said "screw it" and did it the way I was more familiar with.

Zeke · 09-20-2020, 03:45 PM

Quote:

Originally Posted by LEAKYSEALS951

Yeah, I know.I know.

When I saw it, I was also bit concerned someone have wired it for 120v . With the earlier thread feedback, I figured it was okay, but then since I was replacing the cord anyway, said "screw it" and did it the way I was more familiar with.

If the motor is dual voltage then that could be the case, however, wires need to be changed over in the motor connection box. And it wouldn't have been wired with 2 hots at the pressure switch unless they were switching a neutral. That has been done but the EGC is NEVER switched.

You're good.

dad911 · 05:24 PM

Quote:

Originally Posted by LEAKYSEALS951

The red and black (and white) were hots. The greens were the neutral, working as a combined neutral/ground in the pics since they are hooked up to the ground in the switch box. That was my original concern. I don't do enough electric to know if that is done that way factory or not. I didn't know if it was done that way by factory, or if the guy who wired it before me didn't know better. I redid it as a dedicated common and a new dedicated ground.
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Neutrals are not grounds. Neutral carry current, ground should not. If you connected a new white 'neutral' to the white wire(motor??) below, I believe that is not correct. If you connected it to the ground, it is not necessary.

Quote:

Originally Posted by LEAKYSEALS951

I am a little confused by this pic, and your terminology. Green is always ground. It should attach to metal frame as seen above. I believe the red and black are feeds from a 2 pole breaker. The white and black spade connectors in the pic above go to the motor? If so, and the motor is wired for 240, the white and black are feeds to the motor.

White is not always neutral, it can be used as one leg of a 240v circuit. If I do so, I like to wrap the white wire with red or black tape.

Zeke · 09-21-2020, 04:22 AM

We're gonna beat this to death, aren't we? In a dedicated 240v motor, water heater, welder circuit, etc., a neutral is not needed as it doesn't carry current. Yes, in the above pic the white wire is coming from the motor and attached to the switched side of the PS. Many compressor motors are dual voltage so by moving wires and using the proper PS, they can be run on`120v.

"It is currently wired as two hot lines in and one common as a 3 wire out. The common is grounded into the box/compressor itself."

This statement is false and throwing some off here. There are 4 wires on that PS, nothing more. I said earlier that he could wire it "either way." That is not true, if a neutral contained in a 3 wire cord with ground, or a 3-wire NM-B w/ grd was present it should be wire nutted or taped and tucked into a safe position. It cannot be used as a EGC.

There are only 3 wires necessary to connect that compressor to the wall box and nothing else. End of story.