Here’s my shaver plug. It says 4 volts 900 mA or milliamps. How can a person figure out how many mA is required for say a 36” line at .020” thick.

I’m the furthest thing from a electrical engineer but it just seems like this could be calculated?

Most of the available information is for copper, aluminum, and nichrome.

Use a chart like

this one for copper, which tells you the maximum current capacity and resistance per foot. Your wire is about 24 guage which has a maximum current rating of 577 mA and a resistance of 25.67 ohms per 1000 feet, or about 0.08 ohms for 36 inch piece.

Use Ohm's law. Volts = Amps x Ohms, or Amps = Volts/Ohms

This means you will need Volts = 0.577 amps times 0.08 Ohms, or 0.046 volts. Another way to look at it is 4 volts with 0.08 ohms will result in 50 amps, and lots of smoke from your shaver plug!

But this is for copper wire. I would recommend using Nichrome wire, available on Amazon and eBay. Copper wire will not last long at elevated temperature because of corrosion, but nichrome is designed for that.

You are looking at it backwards. You need to know what temperature you need. I do not know what temperature you need to cut foam, but

this web site has an online calculator to determine the wire size and temperature.

Note that the wire gauge size is the same for nichrome and copper.