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 DIY Chatroom Home Improvement Forum Real Basic Electrical question re: neutral wire
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07-19-2009, 01:10 AM   #16
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Thanks for all the help...I understand a TON more

 07-19-2009, 10:49 PM #17 DIY Homeowner   Join Date: Jul 2009 Location: Indiana Posts: 169 Rewards Points: 150 This discussion is really helping to clear some things up for me. After reading it another 5 times I think I'll understand how things work for a 120V outlet and why the backwards-wired outlets at my house work! But, I started reading this thread because I don't understand how a 240V circuit does not need a neutral. I spent an hour trying to figure out why a 12-2 wire to my well was not on a single-pole breaker, and then I was really confused when I saw it was on a two-pole breaker. (Same for the electric heater in my bathroom.) I do understand why my dryer and range are wired with neutrals, it is for the 120V circuitry inside them. But I'm still having trouble understanding why a 240V-only circuit does not need a neutral. Is it because the current flows through the one wire, then back through the other wire as a complete sine wave, alternating at 60Hz? Doesn't this normally happen with a 120V circuit? Returning to the 240V circuit, why can we use standard wires? Wouldn't the extra voltage (240V) cause some sort of problem? Along those lines, why does current (amps) decrease? I know the mathematical relationship of Ohm's law, I just don't understand the physical basis why voltage and current are inversely related . (This is where the voltage = pressure, amps = size of pipe analogy falls apart for me.) Thank for your continued help! R.S.T.
 07-20-2009, 01:43 AM #18 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 I'm a little late getting here but if you guys want we can have a discussion and I'll post up some of my diagrams and other graphics to help understand this neutral thing, what causes a breaker to trip and what compromises a 120 volt circuit and a 240 volt circuit. So if the OP and RST are still here you guys let me know and we should have an interesting discussion. I will start a new thread and we can go thru each issue starting with the first two questions asked by hellothere123 in post #1. __________________ " One nice thing about the NEC articles ... you have lots of choices" Stubbie

 07-20-2009, 08:24 AM #19 Member   Join Date: Feb 2008 Location: Atlanta, Ga/Hamilton, Al Posts: 2,487 Rewards Points: 2,350 Yes Stubbie, I figured you would chime in and post one of your diagrams, as I'm too lazy to look one up or draw one myself. And for RST, the 240 V circuit works exactly like the 120 V circuit, the same physics apply. But the reason why we put it on a double pole is because, unlike a 120 V circuit, one of the conductors is not grounded. If we used a single pole breaker, there would still be a hot wire present at the load when it was turned off, and that would be dangerous since the difference in voltage exists to ground, unlike the 120 V circuit where the neutral is at ground voltage. In one half of the AC cycle, current is flowing into the circuit on one conductor and out on the other. This reverses on the other half of the cycle. This is identical to the 120 V circuit, the only real difference is the voltage is twice as high. Why does the current go down as the voltage goes up? Well, power is the product of voltage and current, V x A. So, if we double the voltage, for the same power we only need half the current.
07-20-2009, 11:51 AM   #20
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Quote:
 Is it because the current flows through the one wire, then back through the other wire as a complete sine wave, alternating at 60Hz?
That's pretty much it.

I think of 120 as out on the hot and back on the neutral 60 times a second.

I think of 240 as out on one hot and back on the other, then it swaps, 60 times a second. Power out on phase A, back on B then power out on B, back on A.

Quote:
 Returning to the 240V circuit, why can we use standard wires?
240V simply = two 120's.

Besides, the wire we commonly use is rated for 600 volts.

Quote:
 Along those lines, why does current (amps) decrease?
You are spliiting the load between two phases so the amps per leg is half that of the same item on a 120V circuit.

Lets say that a dual voltage motor 120/240 will draw 6 amps on the single hot leg when wired for 120V.

It will draw 3 amps each on the two hot legs @ 240V

The total power consumption is the same, it is just split up differently.

 07-20-2009, 11:24 PM #21 DIY Homeowner   Join Date: Jul 2009 Location: Indiana Posts: 169 Rewards Points: 150 Thanks, I think I finally get it now. Robert
 07-21-2009, 11:12 AM #22 Licensed Electrician     Join Date: Jan 2009 Location: Rahway, NJ Posts: 168 Rewards Points: 150 Understanding all of this takes a lot of time. It's kinda grounded, but kinda neutral, but it needs to be bonded here, but not there, and here it carries the same amount of current as the "hot" conductor, but here it carries unbalanced current. This is why electricians who k ow what they're doing get paid so well. It's also the reason why an electrician who comes to your house can figure out what the problem is quite quickly. Once you get, you get it, and you'll never forget it. Did you get all that? __________________ Union County, NJ, Licensed Electrician Classic Electric, LLC, Rahway, New Jersey
07-21-2009, 06:07 PM   #23

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Tripping breakers,neutrals,120 volts and 240 volts OH My!!!

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