220 vs 110 Volts

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Old 07-10-02, 07:46 AM
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220 vs 110 Volts

I just need to clear something up, I can't get it out of my mind.
When you have a 20A 110V circuit you can use teorethically use 20A. However, when you have a 20A 220V circuit (double-pole), and you only have 110V circuits (such as if running to a subpanel), then you can have 40A at 110V, right? (20A on each phase, since each breaker in the double-pole is rated for 20A). Or do you still have 20A. It would make sense you can use up to 40A, but then the neutral wire will have 40A going through it if only 110V circuits are used?
 
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Old 07-10-02, 08:39 AM
Gary Tait
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You still have 20A, but at 220V, twice the power, so essentially the equivalent of 110V at 40A
 
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Old 07-10-02, 08:52 AM
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That's what I thought, but the part that still "bothers" me is the neutral wire, since it is still a 20A cable, and it will be shared by both phases, so teorethically, if both phases are using the full 20A, then there will be 40A on the neutral line. Is that true, or am I just making things up?
 
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Old 07-10-02, 09:21 AM
Joe Carpenter
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Your not making things up but it works a little different than we tend to think at first. When one leg is at it's peak the other is at it's low. The neutral carries the difference of the two loads. Example if leg 1 is drawing 20amps and leg 2 is drawing 15amps the neutral would have 5amps on it. Perhaps someone else on the forum will be able to describe it better.
Really, theroeticlly, (don't do it though) if you are running equal loads you would not need a neutral since the neutral would be carrying the difference which in this case would be zero.
 
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Old 07-10-02, 10:18 AM
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Let's say you have two 120-volt circuits with a shared neutral. To make this work, the two hot wires must be on opposite bus bars. Take some examples:
  • Say both circuits are using the full 20 amps (i.e., you have a 20-amp load plugged into each circuit). The first circuit is "pushing" 20 amps worth of electrons through its hot wire, while the second circuit is "sucking" 20 amps worth of electrons through its hot wire. There are no electrons left to flow on the neutral wire. 1/120 of a second later, the first circuit is now doing the "sucking" and the second circuit is doing the "pushing". The two circuits continue to reverse roles like this 120 times a second.
  • Now let's say you unplug the load on the second circuit. While first circuit is still pushing 20-amps worth of electrons, the second circuit is not sucking anything. So those electrons must go somewhere, so 20-amps of electrons are returning to the panel on the neutral wire instead of on the other hot wire. 1/120 of a second later, the first circuit is sucking 20-amps of electrons, and those electrons are flowing out of the panel on the neutral wire.
  • Now a final example. Say you now plug in a 13-amp load on the second circuit. So while the first circuit is pushing 20 amps worth of electrons, the second circuit is sucking 13 amps worth of electrons. Since the second circuit is not sucking down all of the 20 amps being pushed by the first circuit, the remaining 7 amps of electrons return to the panel on the neutral, while the remaining 13 amps of electrons return to the panel on the second circuit's hot.
  • If you were to put both hot wires on the same bus bar, and had a 20-amp load on both circuits, then both circuits would be pushing electrons at the same time. All 40 amps of electrons would need to return to the panel on the neutral. 1/120 of a second later, both circuits would be sucking 20 amps of electrons, and 40 amps of electrons would be flowing out of the panel on the neutral. This would overheat the neutral, and is why you need to make sure that you don't put both hot wires on the same bus bar.
 
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Old 07-10-02, 10:34 AM
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Thank you, now it's very clear.
 
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Old 07-10-02, 10:41 AM
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On a somehow related subject, will another brand breaker fit instead of a Murray's breaker? I looked at a Siemens breaker I had at home last night, and it looks very close. I have an old Murrays' main panel, and replacing it right now is not worth it, so I'll add a subpanel, but the price for a double-pole 50A Murray breaker is "astronomical" ( I can buy a brand new 200A GE or Seymore panel and still have some change left over )
 
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