How to select the size of electric cable
Wire is used to carry current from the battery through the vehi cle to various electrical components: lights, instruments, etc.
In automotive use, the grounded return system is used almost exclusively. In this system, a single wire is run from the battery through switches to the load and the vehicle itself provides the ground or return to the battery.
It is important to note that all electric components have a good ground. In a combination of vehicles (tractortrailer) it is important that a good and adequate size ground wire be pro vided between the two vehicles to carry the current.
Wire has certain resistance to current flow. The smaller the wire, the greater the resistance.
Also, as the amount of current (amps) increases, the resis tance (ohms) increases.
For a 12volt system there are two factors to be considered:
The Wire Size charts shown below are provided for easy selection of wire sizes. These charts, however, neglect losses due to connections or switches. To use either of these charts:
Example
On a 12volt system, operating at an ambient temperature of 125° F, determine the wire size required for a clearance light circuit having eleven lights using No. 97 bulbs
WIRE SIZE CHART 

For A 12Volt System 
For A 6Volt System 
Length of Wire (Most Distant Light) 

10’ 
20’ 
30’ 
40’ 
50’ 
60’ 
70’ 
80’ 
90’ 
100’ 

Load in Amps 
Load in Amps 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
Wire Gauge 
1.0 
0.5 
18 
18 
18 
18 
18 
18 
18 
18 
18 
18 
1.5 
0.75 
18 
18 
18 
18 
18 
18 
18 
18 
18 
18 
2.0 
1.0 
18 
18 
18 
18 
18 
18 
18 
16 
16 
16 
3.0 
1.5 
18 
18 
18 
18 
18 
16 
16 
16 
14 
14 
4.0 
2.0 
18 
18 
18 
16 
16 
16 
14 
14 
14 
12 
5.0 
2.5 
18 
18 
18 
16 
14 
14 
14 
12 
12 
12 
6.0 
3.0 
18 
18 
16 
16 
14 
14 
12 
12 
12 
12 
7.0 
3.5 
18 
18 
16 
14 
14 
12 
12 
12 
12 
10 
8.0 
4.0 
18 
16 
16 
14 
12 
12 
12 
10 
10 
10 
10.0 
5.0 
18 
16 
14 
12 
12 
12 
10 
10 
10 
10 
11.0 
5.5 
18 
16 
14 
12 
12 
10 
10 
10 
10 
8 
12.0 
6.0 
18 
16 
14 
12 
12 
10 
10 
10 
8 
8 
15.0 
7.5 
18 
14 
12 
12 
10 
10 
10 
8 
8 
8 
18.0 
9.0 
16 
14 
12 
10 
10 
8 
8 
8 
8 
8 
20.0 
10.0 
16 
16 
12 
12 
10 
10 
8 
8 
8 
8 
TEMPERATURE CHART 

Cable Size 
Maximum Current Carrying Capacity (For 12 Volts at Listed Temperatures) 

120° F 
125° F 
150° F 

20 Gauge 
15 Amps 
13 Amps 
9 Amps 
18 Gauge 
18 Amps 
15 Amps 
11 Amps 
16 Gauge 
22 Amps 
19 Amps 
14 Amps 
14 Gauge 
27 Amps 
23 Amps 
17 Amps 
12 Gauge 
40 Amps 
32 Amps 
24 Amps 
10 Gauge 
50 Amps 
42 Amps 
31 Amps 