On 10/4/2012, Brian Koch wrote:
"I have two 60 gallon 220v air compressors that I've plumbed together so I now have 120 gallons between the two. They both have there own 220v circuit breaker. They both are complete separate compressors hooked together by the air line, but I want them both to turn on and off at the same time. Is there a way to use just one switch for two motors"?

No problem. Since the two tanks are piped together they will always have identical pressure pressure. In theory this is very simple.

So far each unit has it's own controls. Smaller compressors might use the pressure switch to power the motor directly. Anything larger than 2-HP is likely to have a power contactor, large 2-pole relay (220VAC 2-phase) or 3-pole relay (270-VAC 3-phase) to send line power to the motor, and the pressure switch triggers the contactor. The contactor is just a big relay, so a pressure switch should not have any problem processing signal current to trigger two relays at once.

Open the controls cover on each unit. Find the pressure switches and relays. On the first unit, disconnect the two trigger wires from the relay, and put wire nuts on them to prevent shorting on anything (or remove the wires completely). On the second unit, locate the relay, and find the two connections for the trigger wires there. Run two new wires from unit 2 to unit 1 to connect the trigger terminals of the relays in parallel. Then the pressure switch on unit 2 will trigger both relays at the same time, and both compressors will switch on an off together.

There is a small caution note here. Each compressor has its own circuit breaker for power input. Suppose you switch off the circuit breakers for #1 for servicing, but leave #2 running. When #2 switches on it will send signal power to #2 to trigger the relay on #2. That puts live power on the relay trigger terminals on #1 while you might be working on it. The relay will click, but nothing else happens because you have the power shut off. You could however get a shock from the relay trigger wires.

I don't know what you local electrical codes are like. You can avoid the shock hazard by using low voltage relays triggered by 24-volts (like a door bell transformer). In some places that may still not satisfy local electrical codes. The code might dictate some sort of electrical interlock or cut-off switch between the units to disconnect the relay trigger wires when one unit is powered down for servicing. This may not be any concern if you know how it works and you are the only person ever doing service on these units. But if you even let anyone else touch the electrical controls you might be legally liable for something. Just so you know how it works.