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Single phase 120v from 208 compressor circuit.

kineticmx

Aluminum
Joined
Mar 10, 2014
Location
United States, CT
Hi guys. Question pertaining to a step down transformer, and how you would wire it into an air compressor control for operation in conjunction with the air compressor.

I have a 5hp 208v 3ph Dresser air compressor. I run it off a dedicated 20A circuit(4 wires, 3ph + ground). I run a refrigerated drier after the compressor, which I currently plug in manually. I am also looking to put an auto drain on the compressor to help with condensation accumulating.

My goal is to flip one switch and turn everything on at once. Keep it idiot proof. The motor starter is obviously controlled by the pressure switch, so I could just put a double pole switch in series with the pressure switch.

Long winded, but do I need a transformer to step down from 208-120, because its definitely not up to code to go line - ground for the 110 circuit. Nor do I think its appropriate to pull another wire through the pipe for neutral, since it would be on a three pole breaker.

The easy answer would be run a dedicated circuit for the auxiliaries and put it into the double pole switch, but I dont have any more room in the panel to fit another circuit.

Any input appreciated.


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thermite

Diamond
Hi guys. Question pertaining to a step down transformer, and how you would wire it into an air compressor control for operation in conjunction with the air compressor.

I have a 5hp 208v 3ph Dresser air compressor. I run it off a dedicated 20A circuit(4 wires, 3ph + ground). I run a refrigerated drier after the compressor, which I currently plug in manually. I am also looking to put an auto drain on the compressor to help with condensation accumulating.

My goal is to flip one switch and turn everything on at once. Keep it idiot proof. The motor starter is obviously controlled by the pressure switch, so I could just put a double pole switch in series with the pressure switch.

Long winded, but do I need a transformer to step down from 208-120, because its definitely not up to code to go line - ground for the 110 circuit. Nor do I think its appropriate to pull another wire through the pipe for neutral, since it would be on a three pole breaker.

The easy answer would be run a dedicated circuit for the auxiliaries and put it into the double pole switch, but I dont have any more room in the panel to fit another circuit.

Any input appreciated.


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WTF? "Switch" need not be IN the load-center.

Usually should NOT be. A Circuit-Breaker is only rarely even RATED for use AS a switch if you had somehow not been aware.

If you have five-wire 208 Wye, a(ny) leg to Neutral is OK fro 120. That's why it is a Wye and HAS a Neutral.

If you do NOT? And/or (plan ahead...) might some other time and place, same goods, only have Delta no-Neutral to begin with, not suited to just pulling the missing Neutral?

A 2XX to 1XX transformer is a bog-standard "works with either" answer.

One other option is 2XX controls and actuating coils. They do exist.

I like 24 VAC controls better. MUCH better! So does my bank account.

There'll be "a" transformer, either way.

- One of those can serve 24 VAC controls power to a LOT more than just your compressor & dryer complex. Add-ons. OTHER goods.. "etc."

- 24 VAC actuators are dirt-common, beloved of OSHA in preference to "line Voltage", too.

- Running "Class 2" field wiring for controls in general is easier.. and less of a hazard if/as/WHEN sumthin' happens to get damaged in such a way as to expose conductors to living critters.

And I HAVE five-wire Wye.

I've been getting "Motor Starter" and/or HVAC rated contactors New, Old Stock, major makers.. for around $20-$30 each. Generally with 24 VAC coils.

Go with 24 VAC controls. I did say dirt-common?

One relay/contactor or several. Flip ONE switch wherever you want that switch to be. "Lighted", even.

And go back to work.

Solves more than just the one problem, going forward.
 
Last edited:

corednc

Aluminum
Joined
Sep 3, 2019
Location
East Coast
Have you checked to see if your air dryer can run on 220?

If not, you can pull the neutral and add a subpanel with a 3 pole and a single pole breaker. You will also need a disconnect.
 

martinberryman

Aluminum
Joined
Feb 18, 2011
Location
Vancouver, BC
I like the direction you're headed.
Just a thought - we did similar and set up our auto-drain with a normally open valve. When the air drier is energized (120V single phase) it closes the valve and the tank fills. When we shut the compressor off at night (we have low voltage switches at each exit that can energize / de-energize both circuits) the drain opens and dumps all of the air. If the air drier is functioning properly, there should be very little to no water, but if you've been around refrigerated air driers long enough, then you'll understand the if at the beginning of the sentence. The only downside to the low voltage setup is lockout / tag-out for air pressure has to be done via a local isolation valve at each machine that is hard-plumbed into the air supply, and you need a way to bleed off pressure at the machine (usually an air gun)
 

thermite

Diamond
The only downside to the low voltage setup is lockout / tag-out for air pressure has to be done via a local isolation valve at each machine that is hard-plumbed into the air supply, and you need a way to bleed off pressure at the machine (usually an air gun)

There's no conflict that has anything to do with "low Voltage" controls in that equation.

See HVAC - residential on-up - T-stat vs a compressor outdoors, in the basement, or on the roof - for one of the most universal of examples.

"Controls" are what they say they are. More than one operating station CAN be readily supported. Local/Remote switched or inherent.

As to POWER? "Disconnect" is another animal.

"Rotary" disconnects - with lockout tabs - ELSE fused pull-outs - placed in the same "reach" zone as those mandatory valves, fit ordinary boxes and are cheap enuf, even if weather resistant "outdoor" rated is needed.
 

kineticmx

Aluminum
Joined
Mar 10, 2014
Location
United States, CT
Thermite,
I appreciate the reiteration on not using circuit breaker as a switch. I was unclear in my original post. The pressure switch has an auto/off switch on it, thats how I control it right now. Not flipping the breaker.

The normally open drain valve would work however I think I will opt for a timer switch as sometimes the air system is pressurized for days on end. Needs to be drained during use.

I will look into a transformer otherwise pulling neutral and putting in a small sub panel might be most appropriate.


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thermite

Diamond
I will look into a transformer otherwise pulling neutral and putting in a small sub panel might be most appropriate.

"Both" is good.

Having the Neutral is enduring, and what is expected, anyway, with a Wye distribution system.

Sub-panel load-centers can solve future needs as well as currrent ones.

You came in the door with the desire for a single-point-of-control.
Which is righteous. "Compressor(s) ARMED || OFF" .... even if the actual cycling is done by various sensors.

But once you have a low-Voltage control philosophy, even adding strategically placed 'E-Stop' get easier and cheaper, too.

I like that for RPC & Phase-Perfect as well as the air ... same as I control the heat or air-con from the T-Stat, not by wandering down where the hardware lives.

Well.. the newest Air-con units all came with Wireless / WiFi remotes that even include Artificial-Idiot-App "tell me" interface & internet connectivity!

B****y Hell if I'm connecting THAT S**T though!

Be just like my playful wife to "reach out" and roast or freeze my arse in Virginia .... from her handheld ....in Hong Kong!

Wonder if that's why all the 24 VAC switches, relays, and contactors have gone so cheap to buy lately?
 








 
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