Lets see-

First off, copper wire is maybe the worst thing to use with a variac…or anything else…for cutting foam and such uses. It conducts electricity quite well, so that it’ll only heat up if you put a helluva lot of power through it. It also melts at a relatively low temp and corrodes and burns, so you’d have some real problems there.

But, lets have a look at the basic idea of an electrical hot wire cutter. You have a power source, a switch, a wire that heats up and the various wires bringing mains power ( wall current ) to the power source and from the power source to your hot wire.

Now, on the power source, it’ll have a power rating, probably in watts, of what it can do. Generally it cranks the voltage up and down to give you the variations in power while the watts stays the same. You don’t want to make it run at the high end of what it can do for very long or unpleasant things happen.

How do you avoid that? Select your wire so that it’s gonna heat up to the right temperature ( 400-600°F or around 250-300°C) at a power inside your power source’s ‘comfortable range’ - less than the maximum it’s rated at.

So you don’t want to fall in love with a particular gauge or start thinking that fishing wire ( usually Monel or stainless steel ) will be the answer if it’s a different gauge …cos it is also a different metal, and like copper it’s gonna work very different. You gotta do the math -

watts = amps x volts.

more math here: http://www.wiretron.com/design.html

Watts- what your power source can put out. Amps - what it draws from the wall current. Volts - well, typically a variac will have a dial, and it’ll range from zero to some figure - lets say 32 for an example, yours is almost certainly different.

If, by the way, you only have amps or watts listed on your variac, that equation above will give ya the other one.

So, imagine you have ( making up numbers here for an example ) a 2’ section of wire, a power source that’ll put out 100 watts between zero and 32 volts. What gauge nichrome wire would you want?

Well, look at http://www.wiretron.com/nicrdat.html , a table of wire data. now you can start designing and calculating.

You’re shooting for the right numbers at around 16 volts. 100 watts/16 volts = 6.25 amps. From the table at http://www.wiretron.com/nicrdat.html , you see that 18 gauge wire needs 6.5 amps to get it to 600F… should you use 18 gauge?

No - you have two feet of wire there and those numbers are per foot. Divide by two - you want something that’ll get you up between 400 and 600 F at a little over 3 amps, so ( looking at the table again ) your choices are between 22 and 23 gauge Nichrome wire.

And note I said Nichrome wire - copper, or stainless or Monel wires, will behave differently, so ya gotta have different ( much thinner) wire gauges for those to behave properly. And I don’t have tables for those easily available. The resistance changes at different temperatures, so it’s not especially easy to figure.

Now, I was just making up the numbers above - you want to have a look at the numbers on your own power source to see what wire gauge is right for you. I’ll note that when I put together my setup I had the 23 gauge wire so I had to go transformer shopping to get the right one for the wire length and gauge I had planned. Wound up with the biggest doorbell transformer I could fine, which works okay but not great for a 12" cutting length, but if I go longer I will have to go to lighter wire…

hope that’s of use

doc…