To protect my 12 AWG wire from the battery, I need to ensure that its maximum current flow does not become excessive. A measure of a wire's maximum current limit is "ampacity", which is the amount of current a conductor can handle before its temperature exceeds accepted limits. Many online sources (such as UIUC Physics 436 EM Fields & Sources II Lecture Notes) show the following ampacities for chassis wiring (based on the Handbook of Electronic Tables and Formulas for American Wire Gauge), which is a short length in free air and not bundled with others.
AWG gauge | Conductor Diameter Inches |
Conductor Diameter mm |
Ohms per 1000 ft. | Ohms per 1000 m | Maximum Amps for Chassis Wiring |
2 | 0.2576 | 6.54304 | 0.1563 | 0.512664 | 181 |
3 | 0.2294 | 5.82676 | 0.197 | 0.64616 | 158 |
4 | 0.2043 | 5.18922 | 0.2485 | 0.81508 | 135 |
5 | 0.1819 | 4.62026 | 0.3133 | 1.027624 | 118 |
6 | 0.162 | 4.1148 | 0.3951 | 1.295928 | 101 |
7 | 0.1443 | 3.66522 | 0.4982 | 1.634096 | 89 |
8 | 0.1285 | 3.2639 | 0.6282 | 2.060496 | 73 |
9 | 0.1144 | 2.90576 | 0.7921 | 2.598088 | 64 |
10 | 0.1019 | 2.58826 | 0.9989 | 3.276392 | 55 |
11 | 0.0907 | 2.30378 | 1.26 | 4.1328 | 47 |
12 | 0.0808 | 2.05232 | 1.588 | 5.20864 | 41 |
13 | 0.072 | 1.8288 | 2.003 | 6.56984 | 35 |
14 | 0.0641 | 1.62814 | 2.525 | 8.282 | 32 |
15 | 0.0571 | 1.45034 | 3.184 | 10.44352 | 28 |
16 | 0.0508 | 1.29032 | 4.016 | 13.17248 | 22 |
17 | 0.0453 | 1.15062 | 5.064 | 16.60992 | 19 |
18 | 0.0403 | 1.02362 | 6.385 | 20.9428 | 16 |
19 | 0.0359 | 0.91186 | 8.051 | 26.40728 | 14 |
20 | 0.032 | 0.8128 | 10.15 | 33.292 | 11 |
21 | 0.0285 | 0.7239 | 12.8 | 41.984 | 9 |
22 | 0.0253 | 0.64516 | 16.14 | 52.9392 | 7 |
23 | 0.0226 | 0.57404 | 20.36 | 66.7808 | 4.7 |
24 | 0.0201 | 0.51054 | 25.67 | 84.1976 | 3.5 |
25 | 0.0179 | 0.45466 | 32.37 | 106.1736 | 2.7 |
26 | 0.0159 | 0.40386 | 40.81 | 133.8568 | 2.2 |
27 | 0.0142 | 0.36068 | 51.47 | 168.8216 | 1.7 |
28 | 0.0126 | 0.32004 | 64.9 | 212.872 | 1.4 |
29 | 0.0113 | 0.28702 | 81.83 | 268.4024 | 1.2 |
30 | 0.01 | 0.254 | 103.2 | 338.496 | 0.86 |
31 | 0.0089 | 0.22606 | 130.1 | 426.728 | 0.7 |
32 | 0.008 | 0.2032 | 164.1 | 538.248 | 0.53 |
Since the maximum current for 12 AWG is 41 amps, I thought I would safe to replace my fusible link (battery to bulkhead connector) with a 30 amp AGC glass tube fuse. This would allow me to quickly get back on the road (if I remember to bring spare fuses) and avoid calling for a tow. This served me well for many years and many miles until 2017. This fuse holder normally has a plastic cover but in the photo below, the fuse cover is missing.
