also agree with not using the welding cable on a race / performance vehicle. Further, typical battery cable is usually bare copper (= corrosion), and covered in PVC (melts easily and in some cases the insulation can be removed with your thumbnail).
As for the ampacity, it not only varies with gauge size, but also with temperature rating of the insulation, as well as how it is installed (ie; in raceway or free air, bundled with other cables, etc.)
Hopefully the amp rating chart below appears properly. While many other things should be taken into account (ambient temperature, length of run, etc.), this gives a good idea of how using GOOD wire allows you to use a smaller gauge for a given load, and using EXCELLENT wire is even better.
For racing builds or even on street cars, I use a SILICONE RUBBER wire that has a FIBERGLASS BRAID over it and is sealed in an acrylic coating. It is rated for 600v (1,000v Military), and you basically need an angle grinder to destroy the jacket. It is not as flexible, costs more, and it has a larger stranding than welding cable and the strands are TPC (tin-plated copper). Guys that build 4x4's, ATV's, pulling trucks, this is the only way, you just can't break it. I use the same thing in smaller gauge to go down to the starter solenoid, etc. due to the high heat rating and the braided jacket.
Insulation Materials
Polyethylene Polypropylene PVC (Irradiated) Kynar (135° C) Kapton
Neoprene Polyethylene Nylon (Crosslinked)TPE's PTFE
Polyurethane (High Density) Silicone
PVC
Copper Temp. 80° C 90° C 105° C 125° C 200° C
30 AWG 2 3 3 3 4
28 AWG 3 4 4 5 6
26 AWG 4 5 5 6 7
24 AWG 6 7 7 8 10
22 AWG 8 9 10 11 13
20 AWG 10 12 13 14 17
18 AWG 15 17 18 20 24
16 AWG 19 22 24 26 32
14 AWG 27 30 33 40 45
12 AWG 36 40 45 50 55
10 AWG 47 55 58 70 75
8 AWG 65 70 75 90 100
6 AWG 95 100 105 125 135
4 AWG 125 135 145 170 180
2 AWG 170 180 200 225 240
1 AWG 211 265 280
1/0 AWG 245 305 325
2/0 AWG 283 355 370
3/0 AWG 328 410 430
4/0 AWG 380 475 510
Single Conductor in Free Air 30° C Ambient Temperature
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Derating Factors for Bundled Conductors
Bundle # Derating Factor (X Amps)
2 - 5 0.8
6 - 15 0.7
16 - 30 0.5