Renewable Energy Questions/Discussion > Wiring and Code discussion

The Basics of using Wire, Volts-Amps-Insulation

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WooferHound:
I thought I would sit down with you and talk a little bit about wire. I work doing lighting and other electrics in the live entertainment business. So learning about the capability of the wiring has been important throughout my career.

-- Wire Type --
There are 2 basic types of wire
Solid - A single solid conductor that is stiff and installed where the wire won't move
Stranded - Lots of smaller conductors bundled together to make a larger wire, flexible and can be rerolled frequently.

-- Voltage --
Most wire is capable of carrying 600 volts. The thickness of the insulation determines the voltage capability. If the voltage gets too high it will leak out of the insulation and arc to any conductor the wire comes into contact with.

-- Current --
The ability of the wire to carry amps is determined by the diameter of the conductor and the heat resisting ability of the insulation. Here is a chart of the normal capacities of common sizes of wire.

Gauge   Amps
 20   -    5
 18   -    7.5
 16   -   10
 14   -   15
 12   -   20
 10   -   30
   8   -   45
   6   -   60
   4   -   80
   2   - 100

You should always read the writing on the wire to get more accurate ratings.

There are many kinds of wire, you just need to know what you've got and how to use it. If the insulation is able to resist high heat the wire can be rated to carry higher current. A 12ga wire in your house is commonly rated at 20 amps, but if it had high heat insulation it could be rated 40 amps and would need to be installed in a place where it's OK for the wire to get hot.

-- Voltage Drop --
When most of us work with wire we would consider that the wire has Zero Resistance. But wire does have resistance. The smaller size the wire is, the higher the resistance will be. Here is a chart of common wire sizes and the resistance in Ohms Per 1000 feet (305 meters).

Gauge - Ohms Per 1000 feet (305 meters)
  20   - 10.2
  18   -   6.4
  16   -   4.02
  14   -   2.53
  12   -   1.6
  10   -     .1
    8   -     .63
    6   -     .4
    4   -     .249
    2   -     .156
Most normal electrical circuits have 2 conductors of wire to complete the circuit, so you would need to double the resistance in ohms in order to get the true resistance of a length of wire supplying a load.

The resistance in the wire will cause any voltage flowing through it to drop resulting in lower voltage at the end of the wire. The amount the voltage will drop depends on wire size and the amount of current flowing. A smaller or longer wire will lower the voltage. A larger current flow will also cause a voltage drop.
Here are some voltage drop calculators that will allow you to see what the voltage drop will be, after you input the wire Size, Wire Length and the amperage flowing in the wire.
http://www.csgnetwork.com/voltagedropcalc.html
http://www.nooutage.com/vdrop.htm
http://www.southwire.com/support/voltage-drop-calculator.htm

-- Frequency & Skin Effect --
A funny thing about wire is the Skin Effect. As the frequency of the voltage goes up, it stops flowing in the center of the conductor and begins to travel on the outside of the wire, or on the skin. The frequencies that we get from wind & solar power are not high enough to worry about but it's good to know this happens. Here is a list of frequencies and the depth it would travel in a copper wire.

 Freq    Depth
10hz   -  91mm
100hz  -  6mm
1khz   -  2mm
10khz  -  .6mm
100khz -  .1mm
1mhz   -  .06mm
http://en.wikipedia.org/wiki/Skin_effect

Using stranded wire will reduce the problem with Skin Effect because each conductor acts all by it's self.

Wolvenar:
I would love to see more on this topic.
Like a standard wire guide of some sort, like a topic for each wire type, insulation style, where they are used etc.

birdhouse:
woofer nailed it. 

murder can be gotten away with on short runs.  also there's thhn and thn.  thhn has a jacket that is more heat resistant.  90 deg C IIRC. 

i once encountered a 30A 220v hot water heater that was fed with 12/2 NM.  it was about 25' from the panel and ran just fine for 20 years that way.  once the water heater died, i just couldn't justify using the original wiring, so upgraded to #10. 

when i'm working on electrical on my RE system, i almost always go on the thick side even though there are no electrical inspections.  big voltage drops are lost battery amps.  not worth it in the long run, plus it's much safer to use the correct (or bigger) wire size. 

adam

WooferHound:

--- Quote from: birdhouse on January 13, 2012, 12:18:15 am ---i once encountered a 30A 220v hot water heater that was fed with 12/2 NM.  it was about 25' from the panel and ran just fine for 20 years that way.  once the water heater died, i just couldn't justify using the original wiring, so upgraded to #10.

--- End quote ---

Upgrading the wire gauge will put more heat in the water and less heat in the wire.

Wolvenar:
As always, I am not an expert, and this is not how all MSW inverters work

Skin effect is one of the very reasons many Modified Sine Wave inverters work using much lighter hardware.

The oversimplified basic idea of how MSW work is through the use of a pulse width modulation at much higher frequencies than the standard 50-60 HZ. The 50-60hz is then simulated by varying the amplitude of each pulse to grossly replicate  the desired frequency (50-60hz) As power demand increases many of these MSW inverters vary their frequency, pulse width, and amplitudes, but always replicating the desired output frequencies with a complex formula in order to regulate the apparent output voltage.

This skin effect come into play because as the frequency goes up,  electrical conduction generally becomes more efficient ( there is a point where the frequencies get high enough that efficiency boost reverses also).
Cooling needs reduce as efficiency rises, which in turn allows for lighter transformers.

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