Using the +12v line and the -5v lines will give you a potential difference of 17v. However, the maximum current you will be able to draw will be limited by the -5v line. The -5v line on most computer power supplies is limited to about 1 amp max. So, if you try to use the +12v and -5v line to power your peltier it won't work and may burn out your power supply. The same thing applies to using the -12v line also because the -12v line, just like the -5v line, is limited to about 1 amp. So, your gonna have to use something other than a computer power supply to power this peltier.