Disconnect location

[Barbqranch]

I wish to install a hp rated disconnect for one of our air compressors so I can have an auxiliary contact to block one other compressor from running at the same time. It doesn't seem to have an input to block it from running, so I need the auxiliary contacts to force people to turn it off before starting the other. The other compressor has a control input to block operation.
The logical place is directly under the subpanel that feeds it. This would put it not quite 36 inches above the floor. Is that still OK?
Thanks.

 

[MTW]

I would achieve the same functionality with a control relay addition to the one you want to ensure is shut down first.

Add a relay to each to interlock them both, so that neither can be started till the other is at rest.

 

[retirede]

I would achieve the same functionality with a control relay addition to the one you want to ensure is shut down first.

Add a relay to each to interlock them both, so that neither can be started till the other is at rest.

Agree.
If the compressors are the same brand, the manufacturer will likely have recommendations for achieving what you want to do.

 

[LarryFine]

All you need is a normally-closed relay powered by A that interrupts power necessary to start B.

Is there a control circuit, or only a mechanical pressure switch?

 

[augie47]

If, in the end, you do decide to go with the disconnect, there is no minimum height above the floor requirement.
You might keep this Section of 110.26 in mind , however:
Within the height requirements of this section, other equipment that is associated with the electrical installation and is located above or below the electrical equipment shall be permitted to extend not more than 150 mm (6 in.) beyond the front of the electrical equipment.

 

[Buck Parrish]

You could also do it with a double throw switch . The low heights doesn't matter much, see NEC 430 maybe NEC 430.107
Yep watch that 6 inch rule like Augie said.

 

[Barbqranch]

I would achieve the same functionality with a control relay addition to the one you want to ensure is shut down first.

Add a relay to each to interlock them both, so that neither can be started till the other is at rest.

Unfortunately, the one compressor doesn't seem to have a low voltage control input. That is why I want to use the disconnect, and that can communicate w/ the other compressor's control disconnect.

 

[Barbqranch]

One machine has computer controls, and the service person from the mfg. didn't indicate there was an easy way to shut it off.
The other is plain old relay logic, which I can easily block.

 

[LarryFine]

Unfortunately, the one compressor doesn't seem to have a low voltage control input. That is why I want to use the disconnect, and that can communicate w/ the other compressor's control disconnect.

How about adding a contactor in the circuit?

 

[retirede]

One machine has computer controls, and the service person from the mfg. didn't indicate there was an easy way to shut it off.
The other is plain old relay logic, which I can easily block.

Even with solid state controls, I can’t believe there isn’t a hard-wired stop switch (such as an E-stop) that you can wire a contact in series with.

 

[Barbqranch]

How about adding a contactor in the circuit?

That would work, but it will be a rare situation that we want to disable the big compressor. We can't just turn it off, because there are people here not in the maintenance dept. that might turn it back on. I suppose we could lock out the breaker, but I like the idea that there is no way to bypass the block.

Even with solid state controls, I can’t believe there isn’t a hard-wired stop switch (such as an E-stop) that you can wire a contact in series with.

There is an E-stop, but to get to it I need to take out the computer board. There is also a stop button, but it is a soft button, and I really don't want to tear that sort of stuff apart when it is still under warranty. I considered breaking the circuit to the main motor starter, but for the same reason, I don't want to mess w/ the actual compressor. If it had two terminals w/ a jumper for that purpose, I would use them. But it doesn't seem to.

Thanks to all who have made suggestions. That is the real value of this forum.
ps: I will never start a post and then wander away without giving the final resolution. Thanks to all!

 

[MTW]

That would work, but it will be a rare situation that we want to disable the big compressor. We can't just turn it off, because there are people here not in the maintenance dept. that might turn it back on. I suppose we could lock out the breaker, but I like the idea that there is no way to bypass the block.

There is an E-stop, but to get to it I need to take out the computer board. There is also a stop button, but it is a soft button, and I really don't want to tear that sort of stuff apart when it is still under warranty. I considered breaking the circuit to the main motor starter, but for the same reason, I don't want to mess w/ the actual compressor. If it had two terminals w/ a jumper for that purpose, I would use them. But it doesn't seem to.

Thanks to all who have made suggestions. That is the real value of this forum.
ps: I will never start a post and then wander away without giving the final resolution. Thanks to all!

Can You provide a picture of the compressor controls drawing? For both units your working with. With that information we may be better able to help you devise an interlock setup. A photo of the physical layout would be helpful as well.

 

[Meltric_South]

I've had some of my customers use a plug for this function. Unplug one compressor to plug in the other one.

 

[Jraef]

So is there a motor starter on the compressor? There has to be something there turning the motor on and off. If it is a motor starter, you can just add an Auxiliary Contact to the side or top of the contactor and use that as an interlock. If it is a Solid State Soft Starter, it likely has an Aux Contact too. This shouldn't be so difficult as to needing a big disconnect switch.

 

[Barbqranch]

So is there a motor starter on the compressor? There has to be something there turning the motor on and off. If it is a motor starter, you can just add an Auxiliary Contact to the side or top of the contactor and use that as an interlock. If it is a Solid State Soft Starter, it likely has an Aux Contact too. This shouldn't be so difficult as to needing a big disconnect switch.

That is great. I hadn't thought of adding an auxiliary contact to the motor starter. I was starting to look at seeing if I could tap into the low voltage control circuit at some accessible point, but this will be much easier.
I had originally planned to try to lock out the larger compressor when both small ones are wanted, but I can just as easily lock out one of the small ones. My only concern is the total current draw of all three blowing one of the main fuses.