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Tool Talk Discussion Forum

Re: Compressor electrical question

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George Marsh

01-04-2013 14:48:04

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If you plan to use a magnetic switch, a contactor, the pressure switch will be used to power up the coil, not the motor. So you will need to find a contactor with a 220v coil. IF you use a 110v coil, then make sure you only run 110v to the pressure switch and wire up the contactor with 220v.

That said, when the pressure switch on my 5 hp compressor went out I couldn't find a switch rated at 5 hp. 3 hp was the largest. So I connected leg 1 of the power in to both sides of the pressure switch and both sides of the out side of the pressure switch which got connected to leg one of the motor. Leg 2 of the power got connected directly to the other leg of the motor. Basically, the pressure switch only turned on and off one side of the 220v. That was 6 years ago. Switch is working just fine.

If anyone has a source of a 5 hp pressure switch, please post. I got my 3 hp switch from grangers.

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01-05-2013 06:15:18

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 Re: Compressor electrical question in reply to George Marsh, 01-04-2013 14:48:04  
George . Why do you delight in taking short cut farmer fixes that work but are not right? I could wire up that compressor using barbed wire and fence insulators and it would work perfectly. Would it be right?

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John T

01-04-2013 15:25:47

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 Re: Compressor electrical question in reply to George Marsh, 01-04-2013 14:48:04  
Yeppers that will "work" (at least motor will stop) but if I had ever tried that when I was an industrial electrical design engineer my boss would have fired me GRRRRRR. We ALWAYS disconnected BOTH HOT LEGS, the maintenance electricians (not to mention NEC) sorta frowned on a hot leg (120 to neutral and case frame) still being present when the motor was supposedly OFF. Of course, if done properly (and as we trained our electricians) the safety switch (in sight and within 50 feet and lockable OFF) was off and locked off BEFORE maintenance...

Take care George, it never hurts to be over safe

John T

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01-04-2013 14:54:11

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 Re: Compressor electrical question in reply to George Marsh, 01-04-2013 14:48:04  
Not code legal as it leaves the motor always energized. Why not just do the job right when at it?

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John T

01-04-2013 19:35:35

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 Re: Compressor electrical question in reply to buickanddeere, 01-04-2013 14:54:11  
EXACTLY old friend, a persons life ought to be worth the price of doing it the safer and code way in my opinion, but its their life at risk not ours if thats how they want to do things???

John T

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George Marsh

01-04-2013 15:39:56

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 Re: Compressor electrical question in reply to buickanddeere, 01-04-2013 14:54:11  
Breaking one leg of 220v is done all the time. I have a 12 year old central air conditioner, Rudd, the contactor only broke one side and the other leg was wired direct. When I replaced the contactor 2 years ago, I replaced it with a 2 pole.

Look at any 220v window air conditioner. They only break one power leg. Current can't flow until the other leg is picked up.

Better tell the people who make air conditioners they are breaking the law.

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John T

01-04-2013 19:27:09

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 Re: Compressor electrical question in reply to George Marsh, 01-04-2013 15:39:56  
Dusty is right, its NOT how the NEC does it nor is it how its done for the best life safety. SURE NOTTTTTTTT HOW IT WAS DONE IN OUR SHOP. Sure do it as you please fine by me and if a manufacturer wants to skimp on safety thats his problem. Like I said it will "work" but its NOT safe nor how the NEC does it or I would ever consider doing it. Its you and your familys life and safety not mine, so do it any which way you desire.......NO WAY I WOULD JUST BREAK ONE LEG AND LEAVE A HOT ENERGIZED LEG IN THE MOTOR YIKESSSSSSSSSSSSSSSSSSSSSSSSSS LOL

God Bless and keep you safe, take care now, its YOUR life that may be saved by taking the safe route remember !!!!!!!!!

John T (A try to be safe kinda guy)

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Dusty MI

01-04-2013 16:17:58

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 Re: Compressor electrical question in reply to George Marsh, 01-04-2013 15:39:56  
Manufactures are allowed to switch only leg within an assembly. BUT it is NOT allowed in the field. Both legs MUST be switched per the National Electrical Code/NEC.


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John T

01-04-2013 19:29:53

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 Re: Compressor electrical question in reply to Dusty MI, 01-04-2013 16:17:58  
Youre sure right and the way you suggest and the way we did it (or be fired lol) could save someones life!!!!!!!!! NO WAY I would leave a hot energized leg to the motor grrrrrrrrrrr

John T

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