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Old 08-24-2009, 07:43 PM   #1
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Pressure switch ratings


I have become confused, as is common, about air compressor pressure switches and the ratings there on.

The pressure switch I have on my compressor is rated at 10 amps (it's an ordinary Square-D switch like we've all seen a million of). My compressor motor is rated at 15 amps, and does indeed draw 15 amps.

I'm helping my brother fix up an old compressor he got a good deal on, and it has a much larger motor on it, and what appears to be the mangled remains of the same style of switch that's on mine. What am I missing? How can these compressors be sold, or be safe, with motors drawing much more than the pressure switch is rated for?

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Old 08-24-2009, 08:56 PM   #2
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Pressure switch ratings


If the switch has HP ratings stamped on it, go by these, not amps.

It'll have different HP ratings for different voltages and single or 3 phase. If the switch has two poles, both must be used, even if it's 120.

Rob

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Old 08-24-2009, 09:03 PM   #3
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Pressure switch ratings


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Originally Posted by micromind View Post
If the switch has HP ratings stamped on it, go by these, not amps.

It'll have different HP ratings for different voltages and single or 3 phase. If the switch has two poles, both must be used, even if it's 120.
OK I follow that, but the same question applies.

My brother's compressor has a 10 HP motor on it, and even on 3-phase, the switch that's on it is rated for 5 HP or less. What??
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Old 08-24-2009, 09:14 PM   #4
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Pressure switch ratings


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Originally Posted by gp_wa View Post
OK I follow that, but the same question applies.

My brother's compressor has a 10 HP motor on it, and even on 3-phase, the switch that's on it is rated for 5 HP or less. What??
If it was 3 phase, the pressure switch would not start the motor directly, but pull in a starter for the motor.
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Old 08-24-2009, 09:20 PM   #5
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Pressure switch ratings


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Originally Posted by jbfan View Post
If it was 3 phase, the pressure switch would not start the motor directly, but pull in a starter for the motor.
Ah, that may explain it then, this compressor used to be 3-phase before the motor was swapped to a single phase unit. I guess the swap was more involved than I had assumed. Thanks
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Old 08-31-2009, 10:12 PM   #6
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Pressure switch ratings


Aaaand, we're back to it.

This compressor turned out to have a 7.5 HP, 33 amp 240 volt motor on it. A contactor has been acquired that is rated for 7.5 HP. I thought this would be simple, but as usual, I'm not so sure anymore

I whipped up a diagram in Paint. Is the 240 volt diagram any good? It occurs to me that we'd be protecting the circuit through the pressure switch with a 40 amp breaker. That seems wrong to me.

Note that I omitted grounding for simplicity. It's not actually going to be left out, so no yelling is required

[IMG]www.nwlink.com/~glenebob/compressor_wiring.gif[/IMG]
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Old 08-31-2009, 10:43 PM   #7
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Pressure switch ratings


Either one of your drawings will work.

The top one will need a contactor that has a 240 volt coil. It's code-legal (and common practice) to use smaller wire on a control circuit. #14 control wire is code-compliant for up to a 45 amp breaker.

The bottom one will need a contactor that has a 120 volt coil. The wire size will be based on the breaker supplying the control circuit. (#14 for 15 amp, #12 for 20 amp). The neutral must be either white or gray.

If I were installing it, I would use the top drawing. If there's more than one source of power in a control box, there's a much greater chance of getting shocked or blowing something up.

Rob

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