wiring air compressors

Old 03-24-02, 11:10 PM
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wiring air compressors

I'm trying something new. I have several questions. I'm trying to wire a new 220 VAC single phase QT-5 air compressor. Power is coming from a sub-panel with a 30 amp double pole breaker with 10/3 with ground NM wire. I'm planning to install a double pole 20 amp heavy duty switch rated for 277 VAC to shut the compressor when not needed. Is this common?
2nd question: How is compressor then wired?
I beleive the wires to feed the compressor first enters the pressure switch to L1 and L2, nuetral and ground wire goes to two individual srews on the metal casing of pressure switch.
3rd question: what about the contactor etc. help.
note: the manual pictures the pressure switch should have jumpers between L1 and T1 and T2 and L2; in which it dont. The pressure switch came prewired with wires coming from contactor L1 to pressure switch terminals T2 and L2. Can anybody help?
Thanks in advance.
Old 03-25-02, 08:35 AM
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A dis-connect switch adjacent to the compressor motor is a Code requirement.You don't need the Neutral connection-the White wire. The bare Grounding conductor must be "bonded' to the metal of the dis-connect switch and the metal of the compressor frame.You mention a contactor(motor-starter?).If you have a contactor/starter, L1 to T1 or L2 to T2 in the pressure-switch will complete a circuit between L1 in the contactor and the contactor coil circuit.Where do the motor-leads connect? To a contactor or to the pressure-switch terminals? Good Luck!
Old 03-25-02, 11:17 AM
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30 amp circuit breaker, must be associated with at least a 30 amp disconnect.
Old 03-25-02, 01:23 PM
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If your panel is within sight of that air compressor you may install your branch circuit directly to the pressure switch. As Handy Ron stated you only need two hots and an equipment grounding conductor connecting at the pressure switch. No neutral will be used with this 240 volt motor. The switch you are talking of using is not approved for this type installation and is not necessary if your panel is withing sight of that air comperessor. You may use the breaker to disable the air compressor when not in use to de-energize the branch circuit. If you install a switch to turn off that air compressor that switch must be rated in horse power equal to the horse power rating of your motor being served, and must be rated in amps 1.3 times the full load current rating of the motor it serves. If your air comperssor is not in sight of the panel then I would suggest a 60 amp non fused disconnect rated for use with your 5 horse power rating to serve as your form of disconnect required if the panel is not in sight of the motor. You are not allowed to use a 20 amp rated switch on a 5 horse power motor. You are required to serve that air compressor with a 5 horse power 240 volt single phase motor with an 8 awg copper branch circuit conductor due to the branch circuit conductor being required to have an ampacity of 125% of hte full load current rating of that 5 horse power motor. The maximum breaker size allowed to serve that 8 awg conductor serving that 5 horse power motor would be 70 amp breaker. I suggest a 40 amp breaker to protect that air compressor branch circuit.

Hope this helps

Old 03-27-02, 10:28 PM
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re: wiring air compressors

Thanks to everyone who replied. It was a very good test for me. Everyone is correct. I verified with the store whom sold this brand. The customer wanted the compressor wired on Monday. So I hooked him up. I did let him know its incorrect to use a 20 amp switch for 23 amp compressor. He should have installed a service disconnect fused at 30 amps or unfused rated at 60 amps. Oh well. He didn't want to spend an extra 50 dollars. The problem I was having was with the manual because the pictures shown on wiring the pressure switch was from another model in which It didn't have a contactor. All I had to do was wire the contactor with two hot leads from the switch. I didn't know the white wire or neutral wire is not needed though. Thanks. I knew its an improper switch because of the rated amps. But the Customer didn't care. He also had already run the circuit with 10/3 with ground NM wires. Boy I was pulling out some of my hairs. I'm sure glad I found this site. I still don't remember how. But thanks to God. I have another source of information. Since I do odd jobs I didn't want to give up an opportunity to learn more. I have connected contactors before but not an air compressor. I love this site. I have been doing electrical side jobs since 1986 on residential homes. Some Commercial and Industrial jobs wiring motors and panels. Thanks again everyone. I will visit often. Hopefully I can help giving out information to other persons. Bye.

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