Home Theater Forum and Systems banner

1 - 5 of 5 Posts

·
Registered
Joined
·
246 Posts
Discussion Starter #1 (Edited)
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 force it's way 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 and the amount of current used will cause any voltage 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 audio 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.
 

·
Plain ole user
Joined
·
11,121 Posts
Thank you for your attempt to provide useful information. There is some really good info here. We do, however, need to be careful to use terms correctly and to be clear on concepts. Please re-read your post and I am sure you will find several areas that need revision to be more accurate. For instance, when you say "voltage flowing through it" I suspect you meant "current." Voltage does not flow, nor does it leak through insulation. Current flows in proportion to the voltage.

With respect to skin effect, it would appear from what you posted that it is meaningful at audio frequencies. I have never found any research that supports that notion. Have you seen some that gives context to the values in your table?
 

·
Registered
Joined
·
246 Posts
Discussion Starter #3 (Edited)
I'll look at the text again and edit it a bit.

However High Current will not force it's way through thin insulation, You will need High Voltage to do that.

There is a link at the end of the Skin Effect section to a WikiPedia article where I found the information.
Skin effect should not effect High Audio frequencies because there is so little power in those frequencies.
Everytime you double the frequency (go up an octave), you will have half the energy when playing audio.
So when playing music, your amp might have 500 watts in the octave centered on 30 hz
but you will only have 0.5 watt in the top octave centered on 16000 hz.
 

·
Plain ole user
Joined
·
11,121 Posts
Actually, it IS the current that will pass through the insulation with enough voltage. Voltage will exist between the conductor inside the insulator and some external point. Voltage is a characteristic of the conditions of charges that exist in two places. Voltage is the potential for current to flow. The distinction may seem semantic but it is very important. Current flows as a result of voltage between two points and some resistance in between.

You are correct in stating that current does not force itself to go anywhere. The force is the voltage. If there is no flow then there is no current, but it is incorrect to say that vlotage flows.

With respect to skin effect, it is an interesting phenomena, it just is not really relevant to moving signals around in a home theater. The only signals with sufficient current to be affected are amplified audio to speakers and the frequencies are just too low for it to be a factor. At least the theoretical effect seems too low to be meaningful, and I have never seen any empirical evidence to the contrary. Skin effect, it seems, is a convenient marketing tool for those selling wire with lots of strands, but not a factor that justifies such products. Flexibility, yes, skin effect, not that I am aware of.
 

·
Premium Member
Joined
·
5,814 Posts
Another thing that might be helpful to others is... When you run wire in Conduit you need to run individual wires versus running Romex even if the gauge is big enough due to heat.
 
1 - 5 of 5 Posts
Top