In case of 12V/40W/1m, it is 0,42mm^2. In case you have standard-ribbon-cable with 0,08mm^2 each, you will need at least 12 single wires. 6 to the hotend and 6 back.
Ribbon cable conductors are more likely to break from flexing than high-density-strand wiring. An example from the welding industry is valid here. For my EV, I used welding grade wire to connect my batteries, subject to vibration and flexing over many thousands of miles. Scaled down to 24ga wire with hundreds of strands, it is likely to last many hours of use in a 3d printer. By combining power pole connectors, it would also be easily replaced. One can find silicone insulated wires that are not multi-strand, so one must be careful in selecting the right stuff. Multi-strand wire feels limp and excessively flexible, because it tolerates such movement with lower failure rates.
@Fred_U Ditto on the powerpole connectors. They were the most reliable part of my RC race cars. Always used with high flex, silicone insulated wire.
I opt for a lineman splice with shrink tubing.
soldered connections do not perform well in durability testing when considering movement, vibration or other flexing. A mechanical crimped connector, properly done, will outlast a soldered connector in those conditions. For the purposes of this discussion, one connector pair at the power source and one connector pair at the hot end would best be done with crimp-on terminals for the powerpoles, as soldered terminals would eventually fray all of the fine wires within.
Guys, we are talking about 3D-printing. I have year old ribbon-cables on my printer, that never ever give a single f… about this little kind of bending. And if you go with special high-flex-wires, you easily reach more than 10mio full bend-cycles before even comming close to a break. So: let him use his ribboncables. Source: it’s my job.
@Mark_Rehorst those are some nice home made cables ! The requirements for a moving bed are quite a bit higher than for hot ends, so you are right , if that works, it should all work 
@Fred_U I might be too optimistic, but given the very low level of flex for the use case , I am not too paranoid about breakage
@Rene_Jurack Thanks for the inputs !
“12V/40W/1m, it is 0,42mm^2”
those are actually spot on
sorry if I don’t follow , but what is the 0.42mm^2 in this case ?
It’s metric 
it is the area of the cross-section of the needed wire in mm*mm. The “how many times did i pull the wire longer to get it thinner”-AWG-crap needs to go extinct Offtopic: Are americans really THAT happy with all that inch-stuff?
@Rene_Jurack thanks that does make more sense
Also found some good example of ribbon cable use for Repraps, on the Mendel90 no less! (how I missed it is beyond me) http://reprap.org/wiki/Mendel90_Build_Manual
Easy other solution, put one end all wires in a vicegrip, twist all with drill, few short segments shrink tube. Or make 4x4x4 twist as above and brade them like hair cross over cross under…