Physically determine unknown wire’s gauge

Does it look like this sensor? If so, put the power (maybe try 3 to 5V for starters) across black and white; signal is on red. Even if you apply the maximum force (50kg), you can expect the signal to change by an incredibly small amount. Make sure your voltmeter is up to the task; the sensor sold by SparkFun will vary 1mV per 1V of input, when the max force is applied. Additionally, it'll handle up to 10V of input, so you'll want to get as close to that as you can.

Over 18 inches you would drop about 0.675 volts with 18 gauge wire. 12 gauge would drop about 0.1 volts. You would need to use 6 or 7 gauge to handle 30 A over an extended period.

If you are not using continuous current, but pulses, you could get away with using a smaller gauge wire. However an undersized wire would heat up significantly in just a few seconds. Unless you are sending pulses of less then 1 sec, you will need a large diameter wire.

AS for the insulation, it depends on how the wire is being used and how it is routed. 30 A is a lot of current. If some one can come in contact with it, you need heavy insulation for safety. Especially if it flexes. If it is total enclosed in a non conductive housing, you could use thinner insulation.