Wire size considerations
Getting the right wire size is a matter of distance and current. There are two issues to consider. Voltage loss is important as most equipment is rated for best operation at a particular voltage. Power loss in the cable is important because it indicates a risk of overheating cable or connections.
The table shows common wire sizes and the resistance in ohms per thousand feet. The “line” column shows the resistance you’d have in a ten foot cable powering some device. This is for 20′ of line as you need to consider the round trip. Voltage loss and power loss are show for that 10′ cable for 5 amps, 10 amps, and 100 amps. The voltage loss is shown as a percent loss for a 12v circuit.
From Wikipedia on wire gauge: use the resistance table and calculate the voltage loss and power consumed by a ten foot run of wire at 5, 10, and 100 amps. Here’s the result.
|
size |
resistance |
Voltage loss as percent at 12v |
power loss in watts |
|||||
|
AWG |
per k-ft |
line |
5 |
10 |
100 |
5 |
10 |
100 |
|
4/0 |
0.049 |
0.0010 |
0.04% |
0.08% |
0.82% |
0.02 |
0.10 |
9.80 |
|
2/0 |
0.078 |
0.0016 |
0.06% |
0.13% |
1.30% |
0.04 |
0.16 |
15.59 |
|
2 |
0.156 |
0.0031 |
0.13% |
0.26% |
2.61% |
0.08 |
0.31 |
31.26 |
|
4 |
0.249 |
0.0050 |
0.21% |
0.41% |
4.14% |
0.12 |
0.50 |
49.70 |
|
6 |
0.395 |
0.0079 |
0.33% |
0.66% |
6.59% |
0.20 |
0.79 |
79.02 |
|
8 |
0.628 |
0.0126 |
0.52% |
1.05% |
10.47% |
0.31 |
1.26 |
125.64 |
|
10 |
0.999 |
0.0200 |
0.83% |
1.66% |
16.65% |
0.50 |
2.00 |
199.78 |
|
12 |
1.588 |
0.0318 |
1.32% |
2.65% |
26.47% |
0.79 |
3.18 |
317.60 |
|
14 |
2.525 |
0.0505 |
2.10% |
4.21% |
42.08% |
1.26 |
5.05 |
505.00 |
|
18 |
6.385 |
0.1277 |
5.32% |
10.64% |
106.42% |
3.19 |
12.77 |
1277.00 |
|
22 |
16.140 |
0.3228 |
13.45% |
26.90% |
269.00% |
8.07 |
32.28 |
3228.00 |
|
24 |
25.670 |
0.5134 |
21.39% |
42.78% |
427.83% |
12.84 |
51.34 |
5134.00 |
The voltage loss area marked in red is where the drop is more than ten percent at 12v. Note that the power loss columns apply to any voltage.