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.

 

Glass Fuse