I Think I Need a Relay��.

I built a new shop, and my compressor is on the far wall away from the entrance door. While planning the electrics for the shop, the elec inspector recommended I put a switch by the door so I could break one leg of the 220/240 and turn the compressor on/off whenever I come in or go out.

I ran 10/4 from a dbl 30A in the breaker panel over to the box for the compressor. I also ran 12/2 w/g (his idea) from the same box up thru the ceiling, over to the door, and down to the switch box. The length of the 12/2 is about 40 ft. I installed a 20A switch in the box and connected the black to one lead and the white (with black tape) to the other switch lead.

I�ve been using the 7hp compressor for 6 months now without the switch hooked up, but it was a loose end so I figgered I�d finish it up. I broke one 10/4 hot lead in the compressor box and spliced in the 12/2 leads from the switch by the door. Now I have to cycle the switch several times if I want to start the compressor and I�m afraid I�m going to burn something (the motor) up every time the compressor tries to cycle up.

I�m sure that the 80 ft of 12 ga is what�s killing me. Is there some type of Relay I can mount behind the compressor that can break the 2 hot 10/2 leads and be controlled by the 12/2 switch? I have a separate 120 outlet within 3 feet if I need power. I forgot to mention the walls are finished so running a larger line to the switch is out of the question.

Ideas? Theories? Sorry to be long winded, but I wanted to give you as much info as I could. Cheap is good, but safety rules!

Scott

If you get one with a 24 VAC coil, you can use a standard HVAC transformer to make 24 volts, switch the 24 volts with your wall switch, and let the contactor switch the compressor. You might also find one with a 120 VAC coil, in which case you wouldn't need the transformer.

Check with a local motor supplier, an electrical supply house, or a good HVAC supply company.

Keith

I ran the wire about 30 feet from the main breaker panel to a disconnect in the garage and a wire from the disconnect to a receptacle in the basement that accepts the male plug on the compressor. I can start and stop it from the garage. There's a 20amp breaker at the disconnect and one at the main breaker panel. Its been in use for 36 years. Hal

• Motor Starter Cheap

• Motor Starter Size 1

At that compressor you need two things, One a safety switch/disconnect (and a two pole, DO NOT just open or switch one of the hot legs), Two some sort of motor thermal overload protection. Whats called a Combination Starter is both combined in one single unit.

If you desire some sort of a remote safety shut off then you can add some sort of a shunt trip circuit breaker with like a mushroom head push to kill switch controller at the remote location which opens the breaker if its pushed

Another option to kill the compressor from a remote location is a TWO POLE contactor/relay with the control push to kill switch (similar to above) located by the door

AGAIN use a two pole switch to shut on/off the 240 volt motor DO NOT use a single pole

John T Long retired eolectrical engineer so a lil rusty on this stuff NO WARRANTY

• Motor starter disconnect.

A 10 gauge wire is not enough (or marginal at best) for an actual 7 hp motor which would draw in the neighborhood of 28 amps. I'll bet the data plate on the motor says the FLC amps is about 15 to 17 amps.

With all of that said, as others have noted, if this is a real 7 hp motor it probably should have a motor starter box mounted on it.

The 12 gauge wire, is probably killing this system, if you want some sort of remote shutoff, it needs to be a relay operated, and if you have a motor starter, the circuit could be wired to control the relay, probably by interrupting the pressure switch/starter circuit.

Personally I think a trip across the shop, to turn off the air valve at the tank, and shut off the compressor (you DO have a disconnect at the compressor don't you?) is not too much to ask.

Charles

I don't understand why you ran 10/4. Maybe you mean 10/3, which has three conductors plus a ground. At any rate, 10/2 would be the correct cable for this purpose.

You are right that the 12 gauge wire is killing you. For that long of a run, you probably should have gone with 8 gauge. 10 gauge at the very least for a 30 amp circuit. Whatever you do, get that switch out of the circuit before you wreck your compressor motor or burn down your building.

Last I knew, you can't use 20 amp wire and devices in a 30 amp circuit.

As CH stated, you need to use a pilot circuit(operator, ie; switch, thermostat, etc) to a contactor (load control). I would suggest a DT 40a, 120v coil contactor as they don't cost a couple dollars more but will last longer.

L1&L2 are lines in and T1&T2 are for load switching.

The pilot circuit has to be tied into the main disconnect of the load being controlled. This way the pilot circuit will be disconnected when the compressor is disconnected from the main.

T_Bone