# Current capacity of a conductor?

1. Sep 16, 2014

### Dash-IQ

What determines the current capacity of a conductor? How can I know the maximum current, any conductor can carry safely, over shot or long durations of time.

Example:

A conductor that is 1m long, 0.00025m wide, and about 0.003m thick is used to deliver power continuously in a system. How much power can it deliver continuously without causing any problems(fusing)?
I know there is a difference if its DC or AC, how different? And if the resistance in my example is 0.3Ohms, how does that help finding the current capacity?

2. Sep 16, 2014

### Baluncore

The limiting factor on cable current is thermal. The resistance of the cable rises proportional to absolute temperature, that will give a voltage drop proportional to temperature. The power lost as heat in the cable is W = I2 * R.

At high temperatures the electrical insulation will fail physically or electrically. That will set the maximum temperature permitted.

Electrical or thermal insulation is a problem because it reduces cooling. Free or forced airflow to cool the conductor increases current capacity.

3. Sep 17, 2014

### EverGreen1231

There is a limiting factor for AC in high voltage transmission called the skin effect. It's why our conductors are made of a bundle of small wires instead of one larger one; in some cases a hollow wire can be used. The most common type of cables are those with Aluminum conductors and reinforced in the center by steel.

Here's a short wiki article about the skin effect.
http://en.wikipedia.org/wiki/Skin_effect

There should be a table somewhere in the IEEE guidelines that gives the current ratings for the respective wire sizes. It will be different for AC and DC.
Here's an American gauge guideline for copper wire.
http://www.powerstream.com/Wire_Size.htm

So far as how you'd go about figuring the max. carrying capacity of a wire theoretically, I don't know exactly. I know it would change with material, ambient temperature, proximity to other wires. The main limiting factor will be thermal since resistance in a cable will rise with temperature. (this is fun fact I learned in semi-cond. dev. )

Sorry I couldn't be of more help.
-Tyler.

Last edited: Sep 17, 2014