8000' 460 run revisited.

[copper123]

Ok, I have found out a little more information. This job is for the government and they want to feed one small building. What I have been told is that there is hardly any load for it. For some reason they don't want to install high voltage at any cost. Don't know why, I am sure however they have reasons. I guess they want to run 500 MCM. Its just crazy that they would spend that kind of money to power up a little building that has a few lights and outlets. I just checked the numbers and 500 mcm is like 5lb a foot.
This is what I figured out, 500 MCM at 8000' you could run 30 amps of power at 460.
A couple of questions for you guys:
Transformers are almost 100% efficient right?
So if your transformed this power down from 460 3 wire to a 208/120 4 wire you would still have the 30 amps of power available on the secondary side right? We are still dealing with a VA issue.
Next questions that I can't understand well enough. How do you explain the concept of voltage drop vs amp draw? When we start pulling more amps, our voltage drop changes dramatically. I have always thought that as we we?start having more electrons bumping and pushing through the wire, (amps/current) our resistance to the flow increases. Is this correct. Hince, at 30 amps at 460 volts, we can run 8000' and have 3% drop total. But if we run AT 200 amps we are around 14.9%. More electrons pushing = more resistance to the wire.
Also, how do taps on the transformer work completely? If we are a little low after the run due to resistance of the wire, we can adjust the taps and secondary side to compensate. I know this is true, but how do you adjust for varying amp draw? Your voltage drop is going to change with your load, correct? If i were to put my meter on my load center in my house phase to phase and get 240, would not my meter voltage change considerable when I turned on my range and dryer?
Sorry this is going full circle, but its all sort of tying in to a good learning experience.
Electron theory, 101.

 

[charlie]

Re: 8000' 460 run revisited.

By all means, have the serving electric utility install a small service at the new location and stop worrying about it.

The other way of looking at the problem is to just do it the way they insist on doing it and put a disclaimer in your bid.

Larger transformers are more efficient than small ones but 95% is a good rough figure.

Your concept of voltage drop and taps are essentially correct. You can't adjust for varying current draw. The current draw where the wire is sized for the load and voltage drop will still have voltage drop but it will not be too great. The situation you are setting up would be the same if the conductors were large enough.

 

[oliver100]

Re: 8000' 460 run revisited.

Voltage Primary divided by Voltage Secondary equals Amperage Secondary divided by Amperage Primary.

30*460/208 ~ 66 amps in the secondary.

About voltage drop phenomena - there are plenty of references on the subject.

If you will please tell the government officials not to spend taxpayers money and be more heroic by applying sound engineering practices. Evan with generator or solar power would be less expensive. It is a shame. Probably it is a jail?

 

[physis]

Re: 8000' 460 run revisited.

How do you explain the concept of voltage drop vs amp draw? When we start pulling more amps, our voltage drop changes dramatically. I have always thought that as we we start having more electrons bumping and pushing through the wire, (amps/current) our resistance to the flow increases. Is this correct. Hince, at 30 amps at 460 volts, we can run 8000' and have 3% drop total. But if we run AT 200 amps we are around 14.9%. More electrons pushing = more resistance to the wire.

Are you interested in the math or the physics?

Cause the math is easy.

V=IR

V is the voltage drop for these calculations. You know what R and I is.

Raise I and you can see you'll be raising V with it.

I wont even start on the physics unless you actually care about it.

Edit: Here's a web page with some descrption of the physics. Usually I'm happier with the information from this site but it's a start and you can click keywords to get further info on related topics within the subject. I go through this site for hours sometimes. I can't imagine anyone not having it bookmarked, but I'm wierd though.

[ July 13, 2005, 01:37 AM: Message edited by: physis ]

 

[physis]

Re: 8000' 460 run revisited.

Ditto on this continual flushing of our money down the toilet. They could at least flush some my way.

 

[james wuebker]

Re: 8000' 460 run revisited.

Copper123
Listen to Charlie and install a new small service for that building. You will save a ton of money and look like a star.
Jim

 

[dvmont]

Re: 8000' 460 run revisited.

I'd be really doubtful if the 8000' 480V transmission line would work. Basically, that's what it is - a transmission line. I've never looked at it, but a friend of mine did. He said that he thought the practical limit for a 480V circuit was 1000 feet.

I think the reason for that was the "charging current" of the circuit. Once you close the circuit - even at no load - there will be a current due to the capacitive effects.

I once saw this happen on a motor starter. We tested it out and found out that when the "open" contact, located about 1000' away - was opened, the motor starter held in. The reason was that the leakage current across the conductor capacitance was sufficient to energize the close coil on the starter.

E=IR doesn't work unless you know what all the components of R are.

 

[physis]

Re: 8000' 460 run revisited.

I think the reason for that was the "charging current" of the circuit. Once you close the circuit - even at no load - there will be a current due to the capacitive effects.

That's almost like when some of these guys talk about the electrons falling out of the end of the wire.

I'm curious what values of capacitance you might get. And if the conductors act like a series L and parallel C circuit.

I know what happens with little circuits but it seems like really big circuits like this have other things going on and don't behave the same way.