Basic 240v question...


  #1  
Old 11-17-04, 07:25 PM
tbot
Visiting Guest
Posts: n/a
Basic 240v question...

I've noticed a few posts regarding 240v circuits, and I'm going to have an electrician install a 240v baseboard heater in a bathroom addition, too. So hopefully this simple question is relevant to a few folks...

Does anyone have a simple analogy for how a 240v circuit works? I always use the following explanation (though possibly flawed) for 120v: "juice" comes out of the hot, as if it were water from a faucet... the circuit is completed by the neutral, which acts like a drain returning the power to where it came from.

So how would you explain 240v? Does it have to do with alternating current?
 
  #2  
Old 11-17-04, 08:19 PM
J
Member
Join Date: Jul 2004
Location: Chester, IL
Posts: 285
Likes: 0
Received 0 Likes on 0 Posts
240 is the same, only twice. In your service panel, there are 2 bus bars. Each single pole breaker, vertically, or in the case of 240, each leg of a double pole breaker connect to opposite "phases". If you look at the bus bars in the panel you'll notice that each post top to bottom comes from the opposite side of the main. Anyway, its pretty much the same deal, power in thru hot wireS, and out thru the ground wire, and ultimately back to the POCO.....so they can sell it to you again tomorrow ...what a deal.
 
  #3  
Old 11-17-04, 08:31 PM
M
Member
Join Date: Aug 2004
Location: Bella Vista, AR
Posts: 57
Likes: 0
Received 0 Likes on 0 Posts
Yes, you are on the right track. Lets me explain first off all how 120v works and get into 240v. In almost all residential situations, power comes in via either overhead lines or underground conduits around 7,200 volts. To make this simple, I will deal with single phase only. The power lines have one hot wire (7,200 volts) and the neutral (grounded conductor). At each house, or sometimes a few more, depending on how the power network comes in, there will be a transformer that takes the 7,200 volts coming in and steps it down to the standard 120v/240v we use everyday day in our home. If you have overhead lines coming to your house, you will notice that there is actually 2 black wires and a bare conductor rather than just 1 black conductor. Why is this? Well, the transformer is center tapped, the center is the neutral or ground, which means each leg has 120v on it, and the two legs are 180 deg out of phase... think of it as a sine wave, when one wire has 120v on it, you could say the other wire has -120v on it. Anyways, as you stated, 120v needs a neutral to make a return path. What is really interesting is most people don't understand how Alternating Current really works. To get an understanding, I will talk about Direct Current. DC power sends current from the positive wire to the negative wire and thats the only direction it ever goes, scientists debate whether it goes from positive to negative or vice versa but thats a different subject for a different day. In AC, the current is coming in at 60 Hz, which means it changes directions 60 times a second. Your neutral wire is really grounded, and is held at ground potential, so half the time, the hot wire is sending current to the neutral wire, and the other half of the time, it is pulling current from the neutral, (Ground, Earth). It is a really wierd concept. As for 240v, you do not need a neutral. Since both wires are 180 deg out of phase and their potential is opposite each other, you could say half the time one wire is the neutral, and the other half of the time, the other wire is the neutral. You could picture the flow of current as sort of a vibration, thats probably why it feels the way it does when you get shocked, because it changes directions so often. I hope I explained this clearly enough, if you are interested in how AC actually produced and why it changes direction, I have a link to a great website below. I hope this helps, and for future reference, this might change how you have been taught...... The neutral wire CAN shock the crap out of you.... Especially if you have a load on it. Don't ever treat the neutral wire as a harmless white wire, it can kill you and being shocked by it hurts worse than being shocked by a hot wire.

Best of luck,
Paul
----------------------
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
http://www.newton.dep.anl.gov/askasc...0/phy00430.htm
 
  #4  
Old 11-18-04, 12:57 PM
tbot
Visiting Guest
Posts: n/a
Thanks all for the info... In the most basic terms, it seems to draw the following visual:

DC current: like a bandsaw, constantly running in a circle
120v AC: like a hand saw, pushing back and forth from one end
240v AC: like an old two-person timber saw, with two people sawing in rythm from both ends....

(not to trivialize electrical theory, but the last one helps me see why there is no neutral for 240v)
 
  #5  
Old 11-18-04, 04:04 PM
J
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Likes: 0
Received 1 Like on 1 Post
That's a great analogy. I love it!
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: