220 Circuit Questions

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Old 12-14-02, 12:31 PM
D
Dave Sveden
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220 Circuit Questions

I would like to know a little about 220 volt circuits. First, is there any difference when you say 220 instead of 240? Are we talking about the same thing? When I see a 220 circuit diagrammed in a ďhow-toĒ book, the circuit is run with 2 wire w/ground romex. There are two hots coming off the double pole breaker, (a hot black and a coded for hot white) as well as the ground wire hooked to the receptacle. The ground bus in the picture is labeled ďneutral/groundĒ so it appears to be talking about a bonded neutral bus in a main service panel. To a layman, the ground seems to be no different than a neutral in this particular diagram (other than itís not an insulated white wire). Since there is no true neutral, Iím curious how this circuit operates. Does this situation change at all if you are now working with a sub-panel that has an isolated neutral bus and an equipment ground bus. My only experience up to this point is with a 120/240 dryer circuit which used a neutral as well as the two hots to provide both 120 for the motor/controls etc. and 240 for the heater. This was in my house with the ground and neutral terminating in the main panel on the same bus.
Iím trying to wire some dedicated 220 circuits for woodworking machinery. I have an isolated neutral bus and equipment ground bus in my sub-panel. What is the correct method for wiring a 30 amp/220 circuit. Do I use 10/3 with ground and connect the neutral to the neutral bus, ground to the ground bus? Or do I just run 10/2 with ground and have no neutral at all?
One other reason this has come up is the method that was used to wire my AC unit. My house is wired with conduit. I noticed the run of conduit to my AC unit only has the two hot wires coming off the double breaker running in it. There is no ground wire or neutral. I guess the conduit is supposed to carry the ground? My AC unit works so Iím just curious again. I didnít put it in and itís been this way since before I bought the house. Is this okay?
Thanks,
D.S.
 
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  #2  
Old 12-14-02, 01:09 PM
S
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Yes,,, conduit is often used for the equipment ground. Your 220 wood work equipment will just have a ground wire if it is only a3 prong plug and it should be connected to the equipment ground in a sub panel. Actually it sounds like you have a fair grip on things.
 
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Old 12-14-02, 02:46 PM
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Dave Sveden
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Old 12-14-02, 03:41 PM
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In principle, a 240-volt circuit (aka 220 volt circuit) is the same as a 120-volt circuit. Each circuit uses two insulated conductors plus a non-current carrying grounding wire. Technically, neither uses a neutral.

The three differences are that a 240-volt circuit (1) is higher voltage (duh!), (2) has neither conductor at ground potential, and (3) disconnects both conductors when the breaker trips. Other than that, the two circuit types are the same.

A confusing factor for many people are circuits that provide both 240 volts and 120 volts. These are really neither 120-volt nor 240-volt circuits -- they are a combination. These circuits use a neutral.

Another confusing factor are the 120/240 volt circuits that share one wire for both neutral and ground. It is no longer legal to install these, but you can use ones that already exist -- but for dryers and ranges only.
 
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