220v wiring: 3 wire or 4 wire?


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Old 04-21-07, 08:44 PM
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220v wiring: 3 wire or 4 wire?

Hi all,
I am installing a new central air conditioning and I need some help with the wiring. Both out side unit and inside air handler uses 220 volt.
On of the electrical book that I was reading states that the new code requires 4 wires for all new 220 volt installation for ranges. But they I could not find anything for central AC wiring.
Does this new 220 volt wiring requires a 4 wire(2 hot + a neutral conductor + ground) or a three wire(2 hot + one ground) for both wiring?

Thanks in advance
 
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Old 04-21-07, 09:27 PM
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Check your local code!
 
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Old 04-22-07, 07:15 AM
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The code no longer allows grounding a 240 volt appliance through the neutral as has been done in years past. Remember the 3 prong range and dryer cords? Anyway, since condensing units and most/(all?)240 volt air handlers do not require a neutral, 2 hots and a ground are all that is required. If however, a neutral was required (some old 240 volt electric furnaces used a 120v volt blower motor) then four conductors would be required.

It's interesting that small 120v appliances are allowed to be grounded through the neutral.
 
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Old 04-22-07, 08:25 AM
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quoting jim-conner: It's interesting that small 120v appliances are allowed to be grounded through the neutral.

could you explain that a bit? I can't think of any appliances allowed to be grounded via the neutral.

there are some "double insulated" tools that do not reqire a ground and there are some small appliances that are simply not grounded but I can;t think of any that are grounded via the neutral.
 
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Old 04-22-07, 10:55 AM
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a/c heat wiring

The wiring for the heating'cooling systems in a house have nothing to do with wiring for ranges. To find what is needed, check the nameplates on both the indoor and outdoor units. They will indicate the voltage and ampacities required. The outdoor unit will also have some additional information. The voltage for indoor units depends on the type of heat you have. Usually gas heat and oil heat furnaces call for 120 volts while heat pump or electric heat indoor units call for 240 volts. Addiontionally, heat pump and electric heat indoor units will contain heat strips. You will need to know the KW rating of the strips in order to size the wiring/circuit breakers properly. The nameplate on the outdoor unit will say "minimum circuit ampacity", "minimum overcurrent protection" and maximum overcurrent protection" [the overcurent protection is usually specified as fuses or circuit breaker; if you use fuses, make sure they are time delay fuses, if you use circuit breakers make sure it is a HACR breaker which most new circuit breakers are but older ones may not be]. The minimum circuit ampacity is what you will use to select the conductors while the overcurrent protection ratings are used to size the circuit breakers/fuses. You also have to have a disconnecting means at each unit. The installation instuctions for both units should contain information you need. However, the requirements for heat/cool system installations encompass several sections of the NEC, primarily article 440. I did some Internet searching and found a website from an electrical inspectors magazine that discusses about these issues. http://www.iaei.org/subscriber/magazine/99_d/simmons.htm
 
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Old 04-22-07, 11:18 AM
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Regarding grounding through neutral, I have a few table lamps that are! I agree, it's not a good idea but there are lots of things that are not completely safe. Look at portable space heaters (1500 watts/12.5 amps), I have seen many with 18 gauge internal wiring and switches rated for 8 amps. Go figure.
 
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Old 04-22-07, 01:17 PM
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Im betting those lamps are simply not grounded. I suspect the only place the neutral attaches is the shell of the lampholder and it should be isolated from any other metal in the lamp.

If you have an continuity tester, you can unplug the lamp and check continuity from the neutral prong to the shell and then to any other metal part. I bet there is no continuity other than the shell to the neutral prong.
 
 

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