Adding A Switch To The Neutral Line Of A Magnetic Starter

[Doug Walls]

I recently installed a 60 amp sub panel with several dedicated 240 volt circuits in my garage.

One of the dedicated circuits is a 30 amp line for my air compressor!
The compressor is wired to an old AB magnetic starter that has a 120volt coil & uses the old style overload heaters. On these old AB starters (and a lot of newer ones) the overload relay is normally connected to the neutral wire, If the relay is overloaded it breaks the neutral connection to the coil & this results in disconnecting both of the 240 volt power lines.

I know that switching the neutral is a no no & a code violation with normal wiring, But I believe there's a special provision for motor starters that disconnect all power when the neutral wire is disconnected?

Assuming that there is some sort of special provision for motor starters, Would adding a switch to the control circuit of a magnetic starter be allowed?

Basically what I would like to do is replace my garage's main light switch with a double pole switch & connect the compressor's neutral line to the other side of the light switch. This would prevent the compressor from running after I leave the garage for the day, "Providing I remember to turn the lights off" :rolleyes:


Doug

[Bill Dufour]

Not sure why you can't switch the control wiring so the hot is switched instead of neutral. The contactor will switch both hots off so the neutral has no where to go anyway. Ground should never be switched, ever.
Bill D

[Ron Selzer]

My recommendation would be to series the wiring for the existing pressure switch thru a new two pole 24vac relay with a 24 vac ,< 40 va transformer wired to lights. When lights are on the relay is enabled and the air compressor can run. Never good to have multiple circuits thru a switch. Someone will kill power to lights and Not realize power is still on at the switch

[Frank Pratt]

Having the OL relay disconnect the neutral is standard practice and is a matter of internal wiring at the starter. The OL relay is the only contact that should be between the neutral and the coil. All other control devices are to switch the line conductor that connects to the coil.

Switching the neutral to the compressor outlet with a wall switch is quite another. What you would be doing is creating the appearance of a disconnect switch that actually leaves the the line conductor hot. Major hazard.

[Doug Walls]

Not sure why you can't switch the control wiring so the hot is switched instead of neutral.
I guess that could be done, But as "Frank Pratt" stated

Having the OL relay disconnect the neutral is standard practice and is a matter of internal wiring at the starter.

To be able to have the light switch next to the side door of my garage also control the compressor's starter control circuit through the hot lead, I would need to run a separate line through the conduit which is about a 45 foot run.

Doug

[Stan Coryell]

I don't understand where the 120 volt control power comes from. It looks like it's tapped off of L1. I would just refeed the control circuit from the lighting circuit.

[Doug Walls]

Switching the neutral to the compressor outlet with a wall switch is quite another.
Just to clarify things, I'm not intending to switch a neutral line that goes to an outlet!
The main circuit to my compressor is a 30 amp 240 volt line (Red/Black/& Green 10ga. wire) Being the AB Starter has a 120V coil, I also ran a 14ga. White wire for the neutral wire needed for the 120V coil. It's this 14ga. neutral line that goes to the starters overload relay that I was thinking of adding the DPST light switch to.


The OL relay is the only contact that should be between the neutral and the coil. All other control devices are to switch the line conductor that connects to the coil.
Ok it sounds like I may have my pressure switch & coil wired wrong?
I used a old 16ga. power cord Black & White wire to connect both the 120V hot & neutral wires to the pressure switch "Line" side & then I ran a second piece of the power cord from the "Motor" side to the starters coil!

Doug

[Doug Walls]

I don't understand where the 120 volt control power comes from. It looks like it's tapped off of L1.
Yeah the hot line is connected to L1 & goes into a 4amp in-line fuse before heading to the pressure switch.
440309
The neutral wire comes into the overload relay & then goes to the pressure switch.


I would just refeed the control circuit from the lighting circuit.
I thought about that, But wondered if having the control wiring on another circuit would create a code problem!

Doug

[Frank Pratt]

If the compressor is on a 30A circuit, then you should be fusing the control circuit if the wiring extends beyond the actual compressor starter. As it is now, you have 16 ga. wire on a 30A breaker. A short circuit in part of the coil winding (not unheard of) could cause catastrophic overheating of the control wires.

Code rules differ, so check with your AHJ.

[Doug Walls]

If the compressor is on a 30A circuit, then you should be fusing the control circuit if the wiring extends beyond the actual compressor starter.
As mentioned in my previous post, The hot lead for the control circuit coming off of the L1 side of the starter is fused with a 4amp in-line fuse.

Doug

[Doug Walls]

As it is now, you have 16 ga. wire on a 30A breaker. A short circuit in part of the coil winding (not unheard of) could cause catastrophic overheating of the control wires.

I just realized that I posted an older pic. of the starter in my original post!
In that pic. I hadn't installed the fuse holder & 4amp fuse yet.:eek:

Sorry for any confusion!

Doug

[Stan Coryell]

No, problem. The control circuit can come from anywhere. I would just make a note in the starter box.

[Mike Henderson]

I got a DPDT contactor with a 120V coil. I wired the 240 volts to the tool through the contactor and put a sensor on the 120V line. So when I go into the shop, the sensor detects me and turns on the 240V to my compressor. I put a switch across the sensor so that I can bypass the sensor when I want to.

The sensor is one of those that are used to turn lights on when you walk into a room. That way, when you walk out of the shop, the sensor will time out and the voltage will be turned off to your compressor.

A DPST contactor would work but I think most of what you'll find will be DPDT.

Mike

[Rod Sheridan]

Switching the neutral is fine if it doesn’t extend beyond the starter enclosure.

If the starter is factory approved without a control fuse, do not add one as modifying the starter negates the approval for it.....Rod

[Doug Walls]

Switching the neutral is fine if it doesn’t extend beyond the starter enclosure.
Thanks!
That makes things a little easier, And it will work with what I had already planed on doing.


If the starter is factory approved without a control fuse, do not add one as modifying the starter negates the approval for it.....Rod
I'm not sure if I understand this correctly!
Are you saying that the in-line 4amp fuse I added to the coil's feed connection that's tapped at the L1 terminal should be removed?

Doug

[Rod Sheridan]

You can leave it in, modified equipment needs approval in Canada, you may not even need approved equipment where you live....Rod