# Calculate gauge of wire with voltage and current

I would like to figure out these two things:

• X gauge of wire needed dependent on the current needed, and the length.
• X max length of wire dependent on the current, and the gauge.
• Voltage and gauge are independent. Current and gauge, voltage and insulation. Jul 23 '14 at 0:27
• you want to know Ampacity (current handling capability) Jul 23 '14 at 1:07
• @IgnacioVazquez-Abrams: LoL I knew that. I don't know why I though of putting that there. I think the fact that you can transmit more wattage on a smaller gauge wire with higher voltage tricked me Jul 23 '14 at 1:18
• @placeholder Ampacity does not seem to have correlation to the length of the wire, I looked at ampacity charts, and there were no length units. Jul 23 '14 at 1:23
• Ampacity doesn't depend upon length, voltage drop does. Jul 23 '14 at 1:25

What you care about is called ampacity - Amperage Capacity. It is determined by the the heating limit of the wire carrying the current.

It has no dependancy on length.

IF you have 50 m of wire that is 1 $m\Omega$ the and carries 100 Amps. the power loss along that 50 m is 10 W (0.001 * $100^2$) from $I^2R$. If you now double the wire length, the resistance doubles and the power loss doubles therefore the power loss per unit length stays the same.

What does matter is that the longer wire will drop more voltage across it (ESR - Equivalent series resistance) so you will loose headroom.

The way to look at it is that when you lengthen the wire, it's area also increases so it's ability to handle power also increases (more area to dump heat out of).

• Maybe I should change the question or start a new one because I am now worried about the voltage drop being too high for my appliance, in my case a 12v 50 watt mobile ham radio, which I want to power 25-40 feet ish away from my source of power. Jul 23 '14 at 1:51
• @skyler For such an application, it might make sense to take the wall-socket mains power all the way to the radio unit, convert to DC near the point of use. Jul 23 '14 at 2:02
• Ok, I just have this awesome astron 50A supply and I wanted to supply two things far apart in my house. Maybe I can shrink it down to 15-20 feet or something like that. Jul 23 '14 at 2:32