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[APD] Yet another ? about DIY heating cables

I know there have been many many questions about diy heating cables posted but 
I still have a question. I have a 12V power supply I was thinking about using 
(although I am not opposed to buying/making a new one). I am aiming to get a 
total of 50watts for a 65 gallon tank. I would like the cable to be roughly 
25' long (obviously this doesn't have to be exact). According to my 
calculations in order to achieve this I need cable that is 2.88 ohms or 0.11 
ohms/ft. If I lengthen the cable to 30' I only need about 0.09 ohms/ft. At the 
moment I don't remember the resistance of copper wire, but I think this puts 
me around 30awg.

Two problems... 

Above is the logic most of the diy documentation has used, however, it is 
completely ignoring that the documented resistance (or even what you get when 
you measure it with your volt-ohm meter) gives you resistance at a specific 
temperature. Especially since these are heating cables, the resistance is 
going to change as the temperature rises. 

If you ignore the change in resistance due to temperature, you still have the 
problem of finding wire that has the correct resistance. I REALLY don't feel 
comfortable putting 30awg wire in a tank full of water... it seems to me it is 
very thin and fragile. I realize that a wet human body still has too much 
resistance to get enough current from 12v to do any real damage, but I still 
like to be careful. I checked into using cable that is designed to be used as 
heating cable (where there is a small heating wire wrapped around a bus within 
a heavily insulated sheathing), but they are designed to use 120v, therefore 
the resistance is too high to work with 12v. 

So two questions...

Does anyone know how much of an impact the change in resistance with change in 
temperature makes?

Does anyone (especially those who have made heating cables themselves) know a 
good wire to use that will give me about 3 ohms total and is safe to be 
submersed in water?

Thanks for the help.

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