Overloading a neutral wire

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  #1  
Old 05-10-02, 10:20 AM
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lanem
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Question Overloading a neutral wire

I am running 2 kitchen sm appliance circuits. I am using 2 20amp breakers, 12g wire, and GFCIs throughout. One thing that I have read about but do not understand is avoiding overloading the neutral wire by being careful about the phase of the circuits. I understand the sin wave shape of AC and assume that alternating phases means to make the two circuits have opposite sin waves, how do I do this? Breakers on different poles? Thanks
 
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Old 05-10-02, 10:23 AM
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lanem
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further info

BTW -- thats 12 gauge 3 wire (2 hots, a neutral, and a ground)
 
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Old 05-10-02, 10:38 AM
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Simply be positively certain that after you have connected the branch-circuit wires to the breaker terminals that there is 220 volts across the Red and Black wires.
 
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Old 05-10-02, 11:32 AM
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lanem
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and what if I don't

Not that I have checked yet -- but what if I don't have 220 volts across the red and black. Can you fill me on the the theory a little?
 
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Old 05-10-02, 11:41 AM
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220 to 240 volts across the red and black simply confirms that you have indeed connected them to opposite phases. If you connect to the same phase, you will have zero volts between them, and your neutral wire will overheat, catch fire and burn your house down.
 
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Old 05-10-02, 11:52 AM
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jlbos83
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A little more theory:
Coming into the house is 240 volts, with the neutral down the middle. So, either hot with respect to the neutral is 120 VAC. A circuit like you have described takes advantage of the fact that the actual flow of electricity on through the neutral will be inthe opposite direction due to either of the hots at the same time. (I don't think that came out well, but think about it.) So, if you use the two different hots with one neutral, the most that can flow in the neutral is the most that can flow in either hot. If you used two of the same phase hot, the flow in the neutral could be twice its rated capacity, which could lead to the consequences that John mentioned. The fiw is easy though, just a matter of having the two circuits hooked to the right breakers.
 
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Old 05-10-02, 11:55 AM
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lanem
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jlbos83 --
Thanks for the theory -- and perhaps I am being a little dense but here is my next questions -- what determines the right breaker. This may be getting a little out of wack, but at this point this is just knowledge for knowledge's sake.
What, in the breaker panel, determines what phase a breaker will be?
 
  #8  
Old 05-10-02, 12:37 PM
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Wgoodrich
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Most residential panels have an alternating phase as you go down the panel circuits in the panel. 1,3,5,7 etc. will be phase 1 and 2,4,6,8 etc. will be phase 2. If you install a double pole breaker in a residential panel you will have 220 volts with the two circuits reading 220 volts between the lugs of the two phases. Just use your voltage tester to make sure you have 220 volts between the two phases between the two lugs of that double pole breaker. We are not talking a half sized breaker but a full sized double pole 220 volt breaker.

If you have two hot conductors fed making 220 volts between the two hot conductors then the first hot conductor will be pushing in one direction while the second hot conductor is pulling in the opposite direction. With the two hot conductors making 220 volts between them you have the two phase counter acting each other making a load of zero on a balanced 220 volt circuit split into two 120 volt circuits. If you had one of these hot conductors pulling 20 amps and the second pulling 10 amps then the two hot conductors would balance each other out on the load of the neutral and the load on the neutral would only be the unbalanced load being the remaining 10 amps being pulled heavier on the one hot conductor than the second hot conductor.

If you created two hot conductors of the same phase that reads 0 volts with a volt meter between the two lugs of two braekers then you would have two hot conductors from the same phase. IF this is true the two hot conductors would not cancel each other because they both would be pushing at the same time and pulling at the same time. Therefore if you had 20 amps on one hot conductor being served by the same white wire that is also serving the second hot conductor of the same phase you would have 30 amps on that white wire causing that white wire to over heat. LIke phases measuring 0 volts between the two lugs of the two breakers add load to a white wire. Opposite phases measuring 220 volts betweeen the two lugs of the two breakers would subtract load canceling each other from applying load on the white wire.

Hope this helps

Wg
 
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Old 05-10-02, 01:17 PM
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Just a clarification of Wg's post. When he says "1,3,5,7 etc.", he is referring to relative position on a side of the box, not to the breaker numbers. Breaker numbers 1, 2, 5, 6, 9, 10 etc. are on one phase and breaker numbers 3, 4, 7, 8, 11, 12, etc. are on the other phase.
 
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Old 05-10-02, 03:12 PM
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Wgoodrich
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John is right in his clarification of my lacking discription. He brought a second warning to mind when I read what he said. A pushmatic ITE panel has all the same phase straight down one side the the second phase goes straight down the other side of the panel.

You really should look at the configuration of the buss bar's two hot phases to make sure how your panel lays out.

Thanks for the clarification John

Wg
 
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Old 05-13-02, 08:18 AM
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lanem
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Thanks for all the info. I do indeed have 220 volts across the red and black hots. No worries here. You guys have really helped ma a lot on this forum, and not just in this thread. Thanks again.
 
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