Sizing welding conductors with 6000A max current
When a cable dissipates electrical energy, two things limit the temperature rise.

a) energy going into heating the copper
b) energy being lost to the surroundings

Although both matter to some extent, most interesting cases involve very short times, where (a) is the only thing that matters, this is called adiabatic heating. For continuous operation, only (b) matters, this is called isothermal heating.

For a 2 second pulse, you are probably in the adiabatic regime. In this regime, we rate conductors by their \$I^2t\$. If they are safe at some current for a given pulse length, then if we double the current, we double the voltage drop, four times the power means we can only take one quarter the pulse length, for a constant \$I^2t\$.

An insulated cable will be limited by the thermal rating of the insulation. It might also be limited by electromagnetic forces.

To work out the adiabatic rating of the cable, take a unit length of cable, say 1m, and then

a) compute its thermal capacity
b) compute its resistance
c) determine the \$I^2t\$ needed to raise its temperature from ambient to what the insulation will tolerate.