How can I work out if a cable can take a momentary current spike?
After doing some research, here's my own answer.

The table here contains two fuse time values for Onderdonk's equation for each guage of wire http://en.wikipedia.org/wiki/American_wire_gauge

Here's the math for calculating the current to fuse a 4AWG cable within 1 second.

Amps = Area*SQRT(LOG(((Tmelt-Tambient)/(234-Tambient))+1)/(Time*33))

Tmelt:    1084.62
Tambient:   10
Time:        1
Area:    41700
---------------------
Ifuse:  = 6341.733   Amps


So taking a conservative 1ms for a lightning strike, 4AWG cable will fuse if the bolt is 200,000A

According to that math, a copper cable of 7700cmils will fuse after 1ms of 37,000A and an average lightning bolt is 20-30,000A in the UK, meaning that 11AWG cable 2.3mm in diameter might survive! [?]