An alternative solution that is used with some Kossel Mini models, is a simple cardboard covered with aluminum tape (docs).
This solution has the added benefit of actually improving heat-up time, since the reflective surface of the aluminum tape reflects the heat back into the bed, instead of getting absorbed.
It is also a very simple solution, without ...
Your wire should be rated for at least the necessary 11 amps which the MK2a heatbed is supposed to take.
You can check this by measuring the resistivity of your wire:
Always test the heatbed wiring for resistance. Remember, at 10A, 0.1Ohms means 1V voltage drop means 10W dissipated by the wiring!
as taken from http://forums.reprap.org/read.php?392,...
THHN wire is thermoplastic high heat-resistant nylon coated wire.
THWN is thermoplastic heat- and moisture-resistant nylon coated wire.
"T" stands for thermoplastic insulation covering the wire itself.
"H" stands for a heat resistance of the insulation max 167°F.
"HH" stands for a heat resistance, but increased max 194°F.
"W" is for moisture resistant.
Old-style lead/tin solder has a melting range of about 180 to 250 degrees (the transition solid to liquid is spread out). This ought to be fine for the bed (even at the hottest hotspot) but wouldn't be good in the hotend.
Modern electronics lead-free solder varies since there are several different compositions, but is typically a bit higher in melting point ...