440 foot run #2 Alum. sub-panel question

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  #41  
Old 01-24-08, 05:34 AM
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Mike,

Jon (Winnie) answered most of your questions about the smaller sized neutral wire. I will further clarify, as you still seem to have a question.

In a 120 volt circuit one wire is hot and the other wire is grounded. We call the grounded wire the neutral or the return. In this setup, all the current flows on both wires. The ground wire in a 120 volt circuit is a safety feature.

In a 240 volt circuit there are two hot wires and no neutral. The same current flows on both wires. Each wire acts as a return for the other wire. The ground wire in a 240 volt circuit is a safety feature.

In a combination 120/240 volt circuit there are two hot wires and a neutral. Each hot wire acts as a return for the other hot wire up to the amount of current flowing on itself. The neutral wire acts as the return for the excess current. In other words, If you have say 10 amps flowing on one hot wire and 8 amps on the other hot wire, the neutral wire carries the difference, or 2 amps. The ground wire in a 120/240 volt circuit is a safety feature.

The above applies whether we are talking about a circuit in your house or a feeder from the power company transformer to your house or a feeder from one circuit breaker panel to another panel.

As Jon also stated, smaller wire sizes (such as those for an electric range or an electric dryer), require that all three current carrying wires (2 hots and 1 neutral) be the same size. This is a requirement, even though the neutral carries very much less current than the hot wires. But with larger wire sizes (feeds to sub panels) the neutral is allowed to be smaller. This is because the neutral typically carries much less current. I believe it is allowed because it reduces the cost and makes the overall cable size smaller.

As regards the issue of assembling conduit AFTER the cable installed, I think that you experiment will show nothing. There is no doubt that the glue will soften or even eat through the insulation, but probably only until such time as it dries. My guess is that once it dries it won't have any effect. I think the rule exists more to help ensure that conduit fill requirements are not exceeded.
 
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  #42  
Old 01-24-08, 06:28 AM
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racraft(Bob) im glad you pointed out that Winnie is Jon i dont know why i didnt see his signature and it sounds better if you know the first name. I like learning about this stuff. You note that in a 120/240 circuit there are 2 hots and how can one act as the return if it goes to a seperated section of the panel and doesnt touch anything else? Also so if i have 20 amps on just one leg then the neutral will have 20 amps as well?

as far as PVC goes I am going by what this person said and that it takes time to break down and i think once it dries it wont have any effect and it dries very quick.

Quote "A simple book from Home Depot will tell you to fish the wire regardless of length because the cement can deteriorate the shielding/insulation. You aren't going to be able to sit there and watch it because it is something that happens over time and is speed up with heat. It hardens cracks and eventually breaks."

Mike
 
  #43  
Old 01-24-08, 07:21 AM
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The 240 volts feeding our houses comes from a power company transformer. It is actually generated much further up the line and at much higher voltages, but let's leave that out of the picture. The transformer on the pole or on the lawn or wherever it is near the house is where the power becomes 240 volts. The 240 volt transformer feed is center tapped. This center tap is the neutral. The two hot wires are opposite ends of the transformer.

So while you have two different hot wires in your panel and one neutral in your panel, they are all related in that they all come from the same transformer and the same power company wire.

At our homes we connect the neutral wire to the ground. We do this via one or more of the following (typically): the metal water pipe supplying the home; metal bars in the concrete foundation of the home; or metal bars driven into the ground next to or through the foundation of the home. This ground is for safety and to to provide a reference point for the voltage. It provides a path for excess current which wants to find the earth and it keeps the voltage 120 volts with respect to the ground (which we consider to be zero volts) instead of 120 volts floating.

Note that 3 phase power from the power company is quite different. 3 phase power is not usually run to homes and only run to businesses when they need it or when they are large power users.

The power is alternating, in the form of a sine wave at 60 Hertz (cycles per second). What you see in your house if you use an oscilloscope and look at the two incoming hot lines each with respect to the neutral are two sine waves exactly 180 degrees out of phase. When one hot wire is positive with respect to the neutral wire the other wire is negative with respect to the neutral by the exact same amount.

A voltmeter measuring the alternating current (set to AC) eliminates the positive and negative shows you the value of 120 volts if you measure between the neutral wire and either hot wire.

A voltmeter measuring the alternating current (set to AC) eliminates the positive and negative shows you the value of 240 volts if you measure between the two hot wires.

You need to study electricity to fully understand how the two hot wires together produce 240 volts, and to understand how each hot wire is a return for the other hot wire, and why the neutral wire only carries the difference between the two wires, not the sum and not the lesser the of the two values. I can't do justice to the theory behind it, and I have four plus years of college studying electrical engineering.

A simple explanation is that the two sine waves are added together. Since one is positive when the other is negative, and negative when the other is positive, the sum is zero when they are equal in current. But when they are unequal in current the excess (be it from either side) returns via the neutral.
 
  #44  
Old 01-27-08, 08:36 AM
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well i did some test with pvc cement and #2 aluminum direct burial wire. I tried primer and then cement just like i did the pipe. Couldnt get anything to happen. The cement just builds up on it and then dries quickly. I tried just putting skim coat also and cant see anything happening and i also tried to glue the wire to the pvc pipe on the edge and again could not get it to bond to the side. i left wire sitting against the side of pvc with plenty of cement and i let it dry and cant get it to bond. basically from what i see you cant get the cement to harm the wire at all. I will leave some samples going and see if there will be any long term effects but that cement dries so fast i cant imagine anything happening.

Mike
 
  #45  
Old 02-04-08, 01:20 PM
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checked again and I can scrape pvc cement off the wire with fingernail. It does not bond to it at all and does not eat it for sure. The excess cement on the pvc just builds up and hardens and does not eat it at all. On several tests this is the case. Im glad I didnt consider my installation "Ruined" since its $1000 worth of stuff and its not ruined a bit. I will keep updating this for years and i bet you can all guess the result............no problems at all.

mike
 
  #46  
Old 12-07-14, 05:21 PM
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Update!

No issues after 7 years everything going well as expected. Cement hasn't eaten the wire and it doesn't appear to be ruined. Glad I left it.

Mike
 
  #47  
Old 12-07-14, 09:22 PM
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Thanks for the update. We will consider this thread done.
 
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