220V 10/2 vs 10/3 Question


  #1  
Old 03-20-04, 09:35 AM
gregwils
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220V 10/2 vs 10/3 Question

I'm installling a 220V circuit in my attached garage. The circuit will power an older air compressor which has a three prong plug. I have already pulled a 10g 10/3 wire and mounted in a receptical box. I used 10g because the run is about 90 feet and the air compressor is rated at 20 amps. There was a chart in Home Depot that recommended 10g. I purchased a three prong receptical to match the plug.

I'm looking for confirmation on my next steps.

I should cap the white wire on both ends since it will not be in use. The black goes in one side of the double gang breaker and the red goes in the other. The ground attaches like a ground on a 110v circuit.

I'm going to the library to see if I can find a good book to be extra safe, but was looking for specific feedback.

Thanks in advance.
 
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Old 03-20-04, 09:44 AM
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Yes jus t cap the white at both ends {don't cut it, you may need it later}.The ground will go to the box & to the the center prong on the receptacle.
 
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Old 03-20-04, 11:49 AM
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As you probably realize, you do not need the neutral wire (white) for this device. However, someday it may be needed. Leave it the same length as ath other wires and simply place a wire nu over the end. Tuck it out of the way in the panel and in the outlet box.
 
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Old 03-20-04, 01:53 PM
gregwils
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Great. Thank you both very much.

One additional question if I may. I went to the library this afternoon and was reading about the code change in 1996. I don't fully understand it, but it seemed to imply that the ground in a 10/2 is carrying some amount of current back to the panel and not just to trip the breaker. The 10/3 adds the white to return the circuit and the ground is used just to trip the circuit-breaker. Again, I probably screwed the explanation up, but here is my question: Should I investigate whether I can update my compressor to use a four prong plug? Or am I OK with the three prong and capping the white wire on both ends?

Again, thanks.
 
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Old 03-20-04, 02:54 PM
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Originally posted by gregwils
Great. Thank you both very much.

One additional question if I may. I went to the library this afternoon and was reading about the code change in 1996. I don't fully understand it, but it seemed to imply that the ground in a 10/2 is carrying some amount of current back to the panel and not just to trip the breaker. The 10/3 adds the white to return the circuit and the ground is used just to trip the circuit-breaker. Again, I probably screwed the explanation up, but here is my question: Should I investigate whether I can update my compressor to use a four prong plug? Or am I OK with the three prong and capping the white wire on both ends?

Again, thanks.
The change I suspect you are referring to is for ranges and dryer circuits. What is unique about those is that they have 120 volt loads and 240 volt loads in the same appliance. That means that there is a current flowing on the neutral during normal operation. For many years the US NEC allowed the bonding of the metal frame such appliances to the neutral rather than requiring a separate Equipment Grounding Conductor. Since your compressor is only a 240 volt load there is no place to connect a neutral to it so there is no safe conversion that you can undertake.
--
Tom H
 
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Old 03-20-04, 03:50 PM
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As hornet has mentioned, it is dryers and ranges that used to be three wire and are now four wire. Dryers and ranges typically have some 110 volt components, such as lights, clocks, timers, buzzers, etc. These portions of the applicance use 110 volts and need a path for the return current. Straight 220 volts is used for the big job that these appliances perform, heating, and does not require a return, as the return is via the other half of the 220 line.
 
 

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